The Interactive Fly

Zygotically transcribed genes

Oncogenes and Tumor Suppressors

  • Regulation of cell growth by Notch signaling and its differential requirement in normal vs. tumor-forming stem cells in Drosophila
  • Transformed Drosophila cells evade diet-mediated insulin resistance through wingless signaling
  • Bunched and Madm function downstream of Tuberous Sclerosis Complex to regulate the growth of intestinal stem cells in Drosophila
  • The asymmetrically segregating lncRNA cherub is required for transforming stem cells into malignant cells

    Oncogenes and Tumor Supressors

    Regulation of cell growth by Notch signaling and its differential requirement in normal vs. tumor-forming stem cells in Drosophila

    Cancer stem cells (CSCs) are postulated to be a small subset of tumor cells with tumor-initiating ability that shares features with normal tissue-specific stem cells. The origin of CSCs and the mechanisms underlying their genesis are poorly understood, and it is uncertain whether it is possible to obliterate CSCs without inadvertently damaging normal stem cells. This study shows that a functional reduction of eukaryotic translation initiation factor 4E (eIF4E) in Drosophila specifically eliminates CSC-like cells in the brain and ovary without having discernible effects on normal stem cells. Brain CSC-like cells can arise from dedifferentiation of transit-amplifying progenitors upon Notch hyperactivation. eIF4E is up-regulated in these dedifferentiating progenitors, where it forms a feedback regulatory loop with the growth regulator dMyc to promote cell growth, particularly nucleolar growth, and subsequent ectopic neural stem cell (NSC) formation. Cell growth regulation is also a critical component of the mechanism by which Notch signaling regulates the self-renewal of normal NSCs. These findings highlight the importance of Notch-regulated cell growth in stem cell maintenance and reveal a stronger dependence on eIF4E function and cell growth by CSCs, which might be exploited therapeutically (Song, 2011).

    The CSC hypothesis was initially developed based on studies in mammalian systems. Various studies have supported the notion that CSCs share many functional features with normal stem cells, such as signaling molecules, pathways, and mechanisms governing their self-renewal versus differentiation choice. However, the cellular origin of CSCs and the molecular and cellular mechanisms underlying their development or genesis remain poorly understood. It has been proposed that CSCs could arise from (1) an expansion of normal stem cell niches, (2) normal stem cells adapting to different niches, (3) normal stem cells becoming niche-independent, or (4) differentiated progenitor cells gaining stem cell properties. This study has shown that in the Drosophila larval brain, CSCs can arise from the dedifferentiation of transit-amplifying progenitor cells back to a stem cell-like state. Importantly, eIF4E was identified as a critical factor involved in this dedifferentiation process. More significantly, it was shown that reduction of eIF4E function can effectively prevent the formation of CSCs without affecting the development or maintenance of normal stem cells. This particular dependence on eIF4E function by CSCs appears to be a general theme, as reduction of eIF4E function also effectively prevented the formation of CSCs, but not normal GSCs, in the fly ovary. These findings may have important implications for stem cell biology and cancer biology, in terms of both mechanistic understanding and therapeutic intervention (Song, 2011).

    This study also offers mechanistic insights into the cellular processes leading to the dedifferentiation of progenitors back to stem cells. In Drosophila type II NB clones with overactivated N signaling, ribosome biogenesis within ectopic NBs appears to be faster than in normal NBs, as shown by the fact that the ratio of nucleolar to cellular volume of the ectopic NBs is approximately fivefold higher than that of normal NBs. The faster growth rate is accompanied by the up-regulation of dMyc and eIF4E and appears to be essential for transit-amplifying progenitors to undergo complete dedifferentiation back to a stem cell-like state. When the function of cell growth-promoting factors such as eIF4E is attenuated, the faster cell growth of ectopic NBs can no longer be sustained and the dedifferentiation process stalls. As a result, brain tumor formation caused by uncontrolled production of ectopic NBs is suppressed. In contrast, normal NBs, which presumably have relatively lower requirements for cell growth and hence eIF4E function, maintain their stem cell fate and development under similar conditions. Therefore, a potential key to a successful elimination of CSC-induced tumors would be to find the right level of functional reduction in eIF4E, which causes minimal effects on normal stem cells but effectively obliterates CSCs. An ongoing clinical trial with Ribavirin in treating acute myeloid leukemia (AML), a well-characterized CSC-based cancer, demonstrated exciting proof of principle that such a strategy is feasible. The current version of Ribavirin, however, has certain limitations, such as its poor specificity and the high dosage (micromolar range) required for effective treatment. Thus, more specific and effective eIF4E inhibitors are urgently needed. Drug treatment experiments with Ribavirin validated Drosophila NBs as an excellent CSC model for searching further improved drugs. More importantly, the nuclear interaction between eIF4E and Myc unraveled by the biochemical analysis not only provides a new mechanistic explanation for the synergistic effects of eIF4E and Myc in tumorigenesis, but also sheds new light on how to rationally optimize drug design and therapy for treating CSC-based cancer (Song, 2011).

    The results offer new information on how N signaling helps specify and maintain NSC fate. N signaling regulates stem cell behavior in various tissues of diverse species. However, it remains unclear how differential N signaling determines distinct cell fate within the stem cell hierarchy. This study demonstrates that N signaling maintains Drosophila NSC fate at least in part through promoting cell growth. The following evidence supports that cell growth, but not cell fate, change is the early and primary effect of N signaling inhibition in type II NBs: (1) Pros expression is not immediately turned on in spdo mutant NBs with reduced cell sizes. Instead, it gradually increases during the course of spdo mutant NB divisions. (2) Up-regulation of Pros is not the cause of stem cell fate loss in spdo mutant NBs, as shown by spdo pros double-mutant analysis. (3) Cell growth defects precede the up-regulation of Ase expression in aph-1 (coding for a component of γ secretase) mutant NBs. (4) Promotion of cell growth, and particularly nucleolar growth, by dMyc is sufficient to prevent NB loss caused by N inhibition. At the molecular level, N signaling appears to regulate the transcription of dMyc, which in turn up-regulates the transcription of eIF4E. Such a transcriptional cascade and feedback regulation of dMyc activity by eIF4E may help to sustain and amplify the activity of the Notch-dMyc-eIF4E molecular circuitry. Hence, differential N signaling within the lineage can lead to different cell growth rates, which partially determine differential cell fates. Consistent with this notion, knockdown of both eIF4E and dMyc results in defects of NB cell growth and loss of stem cell fate (Song, 2011).

    While many signaling pathways and molecules have been implicated in the maintenance of stem cell identity, the question of how a stem cell loses its 'stemness' at the cellular level remains poorly understood. A stem cell may lose its stem cell fate by undergoing a symmetric division to yield two daughter cells that are both committed to differentiation or through cell death. Earlier studies provided intriguing hints that cell growth and translational regulation could influence stem cell maintenance in the Drosophila ovary. Detailed clonal analyses of NSCs over multiple time points provides direct evidence that a NSC with impaired N signaling will gradually lose its identity due to a gradual slowing down of cell growth and loss of cell mass. Remarkably, such loss of stem cell fate can be prevented when cell growth is restored by dMyc, but not Rheb, overexpression, demonstrating the functional significance of regulated cell growth, particularly nucleolar growth, in stem cell maintenance. More importantly, this information offers clues on how to specifically eliminate tumor-initiating stem cells. The current studies suggest that a stem cell, normal or malignant, has to reach a certain growth rate in order to acquire and maintain its stemness, presumably because when the stem cell grows below such a threshold, its proliferative capacity becomes too low, whereas the concentration of differentiation-promoting factors becomes too high to be compatible with the maintenance of stem cell fate. Consistent with this notion are the strong correlation between the expression of ribosomal proteins and cellular proliferation as well as the correlation between the reduction of NB sizes and the up-regulation of differentiation-promoting factor Pros or Ase in different developmental contexts (Song, 2011).

    The results also provide new insights into how the evolutionarily conserved tripartite motif and Ncl-1, HT2A, and Lin-41 (TRIM-NHL) domain proteins regulate stem cell homeostasis. The TRIM-NHL protein family, to which Brat and Mei-P26 belong, include evolutionarily conserved stem cell regulators that prevent ectopic stem cell self-renewal by inhibiting Myc. However, the downstream effectors of the TRIM-NHL proteins remain largely unknown. This study has identified eIF4E as such a factor. NB-specific knockdown of eIF4E completely suppresses the drastic brain tumor phenotype caused by loss of Brat. Interestingly, eIF4E knockdown is even more effective than dMyc knockdown in this regard. N signaling and Brat have been proposed to act in parallel in regulating Drosophila type II NB homeostasis. However, at the molecular level, how deregulation of these two rather distinct pathways causes similar brain tumor phenotypes remain largely unknown. The results suggest that these two pathways eventually converge on the dMyc-eIF4E regulatory loop to promote cell growth and stem cell fate. N overactivation and loss of Brat both result in up-regulation of eIF4E and dMyc in transit-amplifying progenitors, accelerating their growth rates and helping them acquire stem cell fate. Consistent with a general role of eIF4E and dMyc in stem cell regulation, it was shown that partial reduction of eIF4E or dMyc function in the Drosophila ovary effectively rescues the ovarian tumor phenotype due to the loss of Mei-P26. The vertebrate member of the TRIM-NHL family, TRIM32, is shown to suppress the stem cell fate of mouse neural progenitor cells, partially through degrading Myc. Whether eIF4E acts as a downstream effector of TRIM32 in balancing stem cell self-renewal versus differentiation in mammalian tissues awaits future investigation (Song, 2011).

    Transformed Drosophila cells evade diet-mediated insulin resistance through wingless signaling

    Cancer cells demand excessive nutrients to support their proliferation but how cancer cells sense and promote growth in the nutrient favorable conditions remain incompletely understood. Epidemiological studies have indicated that obesity is a risk factor for various types of cancers. Feeding Drosophila a high dietary sugar was previously demonstrated to not only direct metabolic defects including obesity and organismal insulin resistance, but also transform Ras/Src-activated cells into aggressive tumors. This study demonstrates that Ras/Src-activated cells are sensitive to perturbations in the Hippo signaling pathway. Evidence that nutritional cues activate Salt-inducible kinase, leading to Hippo pathway downregulation in Ras/Src-activated cells. The result is Yorkie-dependent increase in Wingless signaling, a key mediator that promotes diet-enhanced Ras/Src-tumorigenesis in an otherwise insulin-resistant environment. Through this mechanism, Ras/Src-activated cells are positioned to efficiently respond to nutritional signals and ensure tumor growth upon nutrient rich condition including obesity (Hirabayashi, 2015).

    The prevalence of obesity is increasing globally. Obesity impacts whole-body homeostasis and is a risk factor for severe health complications including type 2 diabetes and cardiovascular disease. Accumulating epidemiological evidence indicates that obesity also leads to elevated risk of developing several types of cancers. However, the mechanisms that link obesity and cancer remain incompletely understood. Using Drosophila, a whole-animal model system has been developed to study the link between diet-induced obesity and cancer: this model has provided a potential explanation for how obese and insulin resistant animals are at increased risk for tumor progression (Hirabayashi, 2015).

    Drosophila fed a diet containing high levels of sucrose (high dietary sucrose or ‘HDS') developed sugar-dependent metabolic defects including accumulation of fat (obesity), organismal insulin resistance, hyperglycemia, hyperinsulinemia, heart defects and liver (fat body) dysfunctions. Inducing activation of oncogenic Ras and Src together in the Drosophila eye epithelia led to development of small benign tumors within the eye epithelia. Feeding animals HDS transformed Ras/Src-activated cells from benign tumor growths to aggressive tumor overgrowth with tumors spread into other regions of the body (Hirabayashi, 2013). While most tissues of animals fed HDS displayed insulin resistance, Ras/Src-activated tumors retained insulin pathway sensitivity and exhibited an increased ability to import glucose. This is reflected by increased expression of the Insulin Receptor (InR), which was activated through an increase in canonical Wingless (Wg)/dWnt signaling that resulted in evasion of diet-mediated insulin resistance in Ras/Src-activated cells. Conversely, expressing a constitutively active isoform of the Insulin Receptor in Ras/Src-activated cells (InR/Ras/Src) was sufficient to elevate Wg signaling, promoting tumor overgrowth in animals fed a control diet. These results revealed a circuit with a feed-forward mechanism that directs elevated Wg signaling and InR expression specifically in Ras/Src-activated cells. Through this circuit, mitogenic effects of insulin are not only preserved but are enhanced in Ras/Src-activated cells in the presence of organismal insulin resistance (Hirabayashi, 2015).

    These studies provide an outline for a new mechanism by which tumors evade insulin resistance, but several questions remain: (1) how Ras/Src-activated cells sense the organism's increased insulin levels, (2) how nutrient availability is converted into growth signals, and (3) the trigger for increased Wg protein levels, a key mediator that promotes evasion of insulin resistance and enhanced Ras/Src-tumorigenesis consequent to HDS. This study identifies the Hippo pathway effector Yorkie (Yki) as a primary source of increased Wg expression in diet-enhanced Ras/Src-tumors. Ras/Src-activated cells are sensitized to Hippo signaling, and even a mild perturbation in upstream Hippo pathway is sufficient to dominantly promote Ras/Src-tumor growth. Functional evidence is provided that increased insulin signaling promotes Salt-inducible kinases (SIKs) activity in Ras/Src-activated cells, revealing a SIKs-Yki-Wg axis as a key mediator of diet-enhanced Ras/Src-tumorigenesis. Through this pathway, Hippo-sensitized Ras/Src-activated cells are positioned to efficiently respond to insulin signals and promote tumor overgrowth. These mechanisms act as a feed-forward cassette that promotes tumor progression in dietary rich conditions, evading an otherwise insulin resistant state (Hirabayashi, 2015).

    Previously work has demonstrated that Ras/Src-activated cells preserve mitogenic effects of insulin under the systemic insulin resistance induced by HDS-feeding of Drosophila (Hirabayashi, 2013). Evasion of insulin resistance in Ras/Src-activated cells is a consequence of a Wg-dependent increase in InR gene expression (Hirabayashi, 2013). This study identified the Hippo pathway effector Yki as a primary source of the Wnt ortholog Wg in diet-enhanced Ras/Src-tumors. Mechanistically, functional evidence is provided that activation of SIKs promotes Yki-dependent Wg-activation and reveal a SIK-Yki-Wg-InR axis as a key feed-forward signaling pathway that underlies evasion of insulin resistance and promotion of tumor growth in diet-enhanced Ras/Src-tumors (Hirabayashi, 2015).

    In animals fed a control diet, at most a mild increase was observed in Yki reporter activity within ras1G12V;csk-/- cells. A previous report indicates that activation of oncogenic Ras (ras1G12V) led to slight activation of Yki in eye tissue. Activation of Src through over-expression of the Drosophila Src ortholog Src64B has been shown to induce autonomous and non-autonomous activation of Yki. In contrast, inducing activation of Src through loss of csk (csk-/-) failed to elevate diap1 expression. The results indicate that activation of Yki is an emergent property of activating Ras plus Src (ras1G12V;csk-/-). However, this level of Yki-activation was not sufficient to promote stable tumor growth of Ras/Src-activated cells in the context of a control diet: Ras/Src-activated cells were progressively eliminated from the eye tissue (Hirabayashi, 2013). It was, however, sufficient to sensitize Ras/Src-activated cells to upstream Hippo pathway signals: loss of a genetic copy of ex-which was not sufficient to promote growth by itself-dominantly promoted tumor growth of Ras/Src-activated cells even in animals fed a control diet. These data provide compelling evidence that Ras/Src-transformed cells are sensitive to upstream Hippo signals (Hirabayashi, 2015).

    SIK was recently demonstrated to phosphorylate Sav at Serine-413, resulting in dissociation of the Hippo complex and activation of Yki (Wehr, 2013). SIKs are required for diet-enhanced Ras/Src-tumor growth in HDS. Conversely, expression of a constitutively activated isoform of SIK was sufficient to promote Ras/Src-tumor overgrowth even in a control diet. Mammalian SIKs are regulated by glucose and by insulin signaling. However, a recent report indicated that glucagon but not insulin regulates SIK2 activity in the liver. The current data demonstrate that increased insulin signaling is sufficient to promote SIK activity through Akt in Ras/Src-activated cells. It is concluded that SIKs couple nutrient (insulin) availability to Yki-mediated evasion of insulin resistance and tumor growth, ensuring Ras/Src-tumor growth under nutrient favorable conditions (Hirabayashi, 2015).

    The results place SIKs as key sensors of nutrient and energy availability in Ras/Src-tumors through increased insulin signaling and, hence, increased glucose availability. SIK activity promotes Ras/Src-activated cells to efficiently respond to upstream Hippo signals, ensuring tumor overgrowth in organisms that are otherwise insulin resistant. One interesting question is whether this mechanism is relevant beyond the context of an obesity-cancer connection: both Ras and Src have pleiotropic effects on developmental processes including survival, proliferation, morphogenesis, differentiation, and invasion, and these mechanisms may facilitate these processes under nutrient favorable conditions. From a treatment perspective the current data highlight SIKs as potential therapeutic targets. Limiting SIK activity through compounds such as HG-9-91-01 may break the connection between oncogenes and diet, targeting key aspects of tumor progression that are enhanced in obese individuals (Hirabayashi, 2015).

    Bunched and Madm function downstream of Tuberous Sclerosis Complex to regulate the growth of intestinal stem cells in Drosophila

    The Drosophila adult midgut contains intestinal stem cells that support homeostasis and repair. This study shows that the leucine zipper protein Bunched and the adaptor protein MLF1-adaptor molecule (Madm) are novel regulators of intestinal stem cells. MARCM mutant clonal analysis and cell type specific RNAi revealed that Bunched and Madm were required within intestinal stem cells for proliferation. Transgenic expression of a tagged Bunched showed a cytoplasmic localization in midgut precursors, and the addition of a nuclear localization signal to Bunched reduced its function to cooperate with Madm to increase intestinal stem cell proliferation. Furthermore, the elevated cell growth and 4EBP phosphorylation phenotypes induced by loss of Tuberous Sclerosis Complex or overexpression of Rheb were suppressed by the loss of Bunched or Madm. Therefore, while the mammalian homolog of Bunched, TSC-22, is able to regulate transcription and suppress cancer cell proliferation, these data suggest the model that Bunched and Madm functionally interact with the TOR pathway in the cytoplasm to regulate the growth and subsequent division of intestinal stem cells (Nie, 2015).

    Homeostasis and regeneration of an adult tissue is normally supported by resident stem cells. Elucidation of the mechanisms that regulate stem cell-mediated homeostasis is important for the development of therapeutics for various diseases. The intestine with fast cell turnover rate supported by actively proliferating stem cells is a robust system to study tissue homeostasis. In the mouse intestine, two interconverting intestinal stem cell (ISC) populations marked by Bmi1 and Lgr5 located near the crypt base can replenish cells of various lineages along the crypt-villus axis Furthermore, recent data suggest that Lgr5+ cells are the main stem cell population and that immediate progeny destined for the secretory lineage can revert to Lgr5+ stem cells under certain conditions [6, 7]. Together, the results suggest previously unexpected plasticity in stem cell maintenance and differentiation in the adult mammalian intestine (Nie, 2015).

    In the adult Drosophila midgut, which is equivalent to the mammalian stomach and small intestine, ISCs are distributed evenly along the basal side of the monolayered epithelium to support repair. The maintenance and regulation of Drosophila midgut ISCs depend on both intrinsic and extrinsic factors. When a midgut ISC divides, it generates a renewed ISC and an enteroblast (EB) that ceases to divide and starts to differentiate. The ISC-EB asymmetry is established by the Delta-Notch signaling, with Delta in the renewed ISC activating Notch signaling in the newly formed neighboring EB . Growth factors such as Wingless/ Wnt, insulin-like peptides, Decapentaplegic/BMP, Hedgehog and ligands for the EGF receptor and JAK-STAT pathways are secreted from surrounding cells and constitute the niche signals that regulate both ISC division and EB differentiation. ISC-intrinsic factors including Myc, Target of Rapamycin (TOR) and Tuberous Sclerosis Complex act to coordinate the growth and division of ISCs. Furthermore, chromatin modifiers such as Osa, Brahma and Scrawny function within ISCs to regulate Delta expression or ISC proliferation (Nie, 2015).

    This study reports the identification of the leucine zipper protein Bunched (Bun) and the adaptor protein myeloid leukemia factor 1 adaptor molecule (Madm) as intrinsic factors for ISC proliferation. A single bun genomic locus generates multiple predicted transcripts that encode 4 long isoforms, BunA, F, G and P, and 5 short isoforms, BunB, C, D, E, H and O. The first identified mammalian homolog of Bun is TGF-β1 stimulated clone-22 (TSC-22). In the mouse genome four different TSC- 22 domain genes also encode multiple short and long isoforms. All isoforms of Bun and TSC-22 contain an approximately 200 amino acids C-terminal domain where the conserved TSC-box and leucine zippers are located. The originally identified TSC-22 is a short isoform and various assays suggest that it suppresses cancer cell proliferation and may function as a transcriptional regulator. Meanwhile, in Drosophila, the long Bun isoforms positively regulate growth, while the short isoforms may antagonize the function of long isoforms. Transgenic fly assays also demonstrate that the long TSC-22 can rescue the bun mutant phenotypes, whereas short isoforms cannot. These results suggest an alternative model that the long Bun isoforms positively regulate proliferation, while the short isoforms may dimerize with and inhibit the functions of long isoforms (Nie, 2015).

    Madm also can promote growth. The long isoform BunA binds to Madm via a conserved motif located in the N- terminus that is not present in the short Bun isoforms. The molecular function of this novel BunA- Madm complex, nonetheless, remains to be elucidated. The results in this report demonstrate that Bun and Madm modulate the Tuberous Sclerosis Complex-target of Rapamycin (TOR)-eIF4E binding protein (4EBP) pathway to regulate the growth and division of ISCs in the adult midgut (Nie, 2015).

    This report shows that Bun and Madm are intrinsically required for ISC growth and division. The results suggest a model that Bun and Madm form a complex in the cytoplasm to promote cellular growth and proliferation. The evidence that support this model includes the observation that transgenic expressed Bun localizes in the cytoplasm of midgut precursor cells, similar to the results from transfection in S2 cells and immune-staining in eye discs. Bun physically and functionally interacts with Madm, which has also been proposed as a cytoplasmic adaptor protein. Adding a nuclear localization signal to Bun reduced the growth promoting ability of Bun. Although there is a possibility this signal peptide changes the functionality in an unpredicted way, the interpretation is favored that Bun normally acts in the cytoplasm and with Madm to regulate the proliferation of ISCs. This is in contrast to mammalian TSC-22, which was reported to function in the nucleus (Nie, 2015).

    The results seem to contradict a previous publication reporting that TSC-22 arrests proliferation during human colon epithelial cell differentiation. However, this apparent contradiction is resolved when the growing evidence for distinct functions for large and small Bun/ TSC-22 isoforms is considered. The Bun/TSC-22 proteins have short and long isoforms that contain the conserved TSC-box and leucine zippers in the C-terminal domain. The prototypical TSC-22 protein, TSC22D1-001, may act as a transcriptional regulator and repress cancer cell proliferation, particularly for blood lineages. Another recent model suggests that in Drosophila the long Bun isoforms interact with Madm and have a growth promoting activity, which is inhibited by the short Bun isoforms. Similarly, the long isoform, TSC22D1-002, enhances proliferation in mouse mammary glands, whereas the short isoform promotes apoptosis. Unpublished result that transgenic expression of BunB also has lower function than BunA in fly intestinal progenitor cells is consistent with this model where large isoforms have a distinct function, namely in growth promotion (Nie, 2015).

    Loss of either Bun or Madm can potently suppress all the growth stimulation by multiple pathways in the midgut as shown in this report. These results are intrepeted to indicate that Bun and Madm do not act specifically in one of the signaling pathways tested but instead function in a fundamental process required for cell growth, such as protein synthesis or protein turnover. It is therefore speculated that Bun and Madm may regulate the TOR pathway. In support of this idea, it was shown that bunRNAi or MadmRNAi efficiently suppresses the Tuberous Sclerosis Complex 2RNAi-induced cell growth and p4EBP phenotypes. A recent study of genetic suppression of TOR complex 1-S6K function in S2 cells also suggests that Bun and Madm can interact with this pathway. Furthermore, proteomic analyses of Bun and Madm interacting proteins in S2 cells have shown interactions with ribosomal proteins and translation initiation factors. Therefore, a model is proposed that Bun and Madm function in the Tuberous Sclerosis Complex-TOR- 4EBP pathway to regulate protein synthesis in ISCs for their growth, which is a prerequisite for ISC proliferation. Suppression of Tuberous Sclerosis Complex mutant cell growth phenotype by bun or Madm RNAi was substantial but not complete. Earlier papers demonstrated that Bun also interacts with Notch and EGF pathway in ovary follicle cells. Therefore by definition Bun and Madm are neither 100% essential nor restricted to the TOR pathway. The genetic data suggest that Bun and Madm work downstream of Tuberous Sclerosis Complex and upstream of 4EBP, but they could also work in parallel to the TOR pathway components (Nie, 2015).

    ISCs with loss of Tuberous Sclerosis Complex function have substantial cell size increase. Meanwhile, the Bun/ Madm overexpression caused increased ISC division but not cell hypertrophy. Both loss of Tuberous Sclerosis Complex and overexpression of Bun/Madm should promote cell growth but the phenotypes at the end are different. It is speculated that the reason is the Bun/Madm overexpressing ISCs are still capable of mitosis, while the Tuberous Sclerosis Complex mutant ISCs do not divide anymore thereby resulting in the very big cells. In Bun and Madm overexpressing mid- guts, the p-H3+ and GFP+ cell count showed a significant increase, indicating increased mitosis. Therefore, an explanation is that Bun and Madm overexpression may increase cell size/cell growth, but when they grow to certain size they divide, resulting in rather normal cell size (Nie, 2015). The knockout of the Madm mammalian homolog, NRBP1, can cause accumulation of the short isoform TSC22D2. Up-regulation of Madm/NRBP1 has been associated with poor clinical outcome and increased growth of prostate cancer. Further analysis based on this model may reveal whether high ratio of long Bun/TSC22 isoforms over short isoforms may associate with high Madm activity and poor clinical outcomes (Nie, 2015).

    The asymmetrically segregating lncRNA cherub is required for transforming stem cells into malignant cells

    Tumor cells display features that are not found in healthy cells. How they become immortal and how their specific features can be exploited to combat tumorigenesis are key questions in tumor biology.This study describes the long non-coding RNA cherub (long non-coding RNA:CR43283) that is critically required for the development of brain tumors in Drosophila but is dispensable for normal development. In mitotic Drosophila neural stem cells, cherub localizes to the cell periphery and segregates into the differentiating daughter cell. During tumorigenesis, de-differentiation of cherub-high cells leads to the formation of tumorigenic stem cells that accumulate abnormally high cherub levels. cherub establishes a molecular link between the RNA-binding proteins Staufen and Syncrip. As Syncrip is part of the molecular machinery specifying temporal identity in neural stem cells, it is proposed that tumor cells proliferate indefinitely, because cherub accumulation no longer allows them to complete their temporal neurogenesis program (Landskron, 2018).

    Throughout the animal kingdom, stem cells supply tissues with specialized cells. They can do this because they have the unique ability to both replicate themselves (an ability termed self-renewal) and to simultaneously generate other daughter cells with a more restricted developmental potential. Besides their role in tissue homeostasis, stem cells have also been linked to tumor formation. They can turn into so-called tumor stem cells that sustain tumor growth indefinitely. The mechanisms that endow tumor stem cells with indefinite proliferation potential are not fully understood (Landskron, 2018).

    Most Drosophila brain tumors originate from the so-called type II neuroblasts (NBIIs). NBIIs divide asymmetrically into a larger cell that retains NB characteristics and a smaller intermediate neural progenitor (INP). Newly formed immature INPs (iINPs) go through a defined set of maturation steps to become transit-amplifying mature INPs (mINPs). After this, a mINP undergoes 3-6 divisions generating one mINP and one ganglion mother cell (GMC) that in turn divides into two terminally differentiating neurons or glial cells (Landskron, 2018).

    During each NBII division, a set of cell fate determinants is segregated into the INP. Among those are the Notch inhibitor Numb and the TRIM-NHL protein Brain tumor (Brat). Loss of these cell fate determinants leads to the generation of ectopic NB-like cells at the expense of differentiated brain cells. Formation of malignant brain tumors has also been observed upon the depletion of downstream factors that normally maintain the INP fate (Landskron, 2018).

    These features make Drosophila a model for the stepwise acquisition of tumor stem cell properties. When numb or brat are inactivated, the smaller NBII progeny fails to establish an INP fate and initially enters a long transient cell cycle arrest. Only after this lag period, the smaller cell regrows to a NB-sized cell that has acquired tumor stem cell properties and that it is therefore refered to as tumor neuroblast (tNB). NBIIs and ectopic tNBs are indistinguishable in terms of markers. Both cell populations are characterized by the expression of self-renewal genes and lack differentiation markers, but nevertheless behave differently. Shortly after entering pupal stages, NBs decrease their cell volumes successively with each NB division before they exit the cell cycle and differentiate. However, tNBs do not shrink during metamorphosis and continue to proliferate even in the adult fly brain. Moreover, in contrast to wild-type brains, the resulting tumor brains can be serially transplanted into host flies for years, indicating the immortality of these tumors (Landskron, 2018).

    Similarly, mammalian homologues of numb and brat have been shown to inhibit tumor growth. Furthermore, the human brat homologue TRIM3 is depleted in 24% of gliomas and NUMB protein levels are markedly reduced in 55% of breast tumor cases. Therefore, results obtained in these Drosophila tumor models are highly relevant (Landskron, 2018).

    This study used the Drosophila brat tumor model to investigate how tNBs differ from their physiological counterparts, the NBIIs. The results indicate that progression towards a malignant state is an intrinsic process in brat tNBs that does not correlate with stepwise acquisition of DNA alterations. Transcriptome profiling of larval NBs identified the previously uncharacterized long non-coding (lnc) RNA cherub as crucial for tumorigenesis, but largely dispensable for NB development. The data show that cherub is the first identified lncRNA to be asymmetrically segregated during mitosis into INPs, where the initial high cherub levels decrease with time. Upon the loss of brat, the smaller cherub-high cell reverts into an ectopic tNBs resulting in tumors with high cortical cherub. Molecularly, cherub facilitates the binding between the RNA-binding protein Staufen and the late temporal identity factor Syp and consequently tethers Syp to the plasma membrane. Depleting cherub in brat tNBs leads to the release of Syp from the cortex into the cytoplasm and represses tumor growth. These data provide insight into how defects in asymmetric cell division can contribute to the acquisition of tumorigenic traits without the need of DNA alterations (Landskron, 2018).

    It is commonly assumed that cancer cells become malignant and gain replicative immortality by acquiring genetic lesions. Surprisingly, however, the current data indicate that brat tumors do not require additional genetic lesions for the transition to an immortal state. This is not a general feature of Drosophila tumors as genomic instability alone can induce tumors in Drosophila epithelial cells and intestinal stem cells. However, the current results are supported by previous experiments demonstrating that defects in genome integrity do not contribute to primary tumor formation in NBs. Similarly, tumors induced by loss of epigenetic regulators in Drosophila wing discs do not display genome instability. In addition, the short time it takes from the inactivation of brat to the formation of a fully penetrant tumor phenotype would most likely be insufficient for the acquisition of tumor-promoting DNA alterations. More likely, the enormous self-renewal capacity and fast cell cycle of Drosophila NBs requires only minor alterations for the adoption of malignant growth. Interestingly, epigenetic tumorigenesis has been described before in humans, where childhood brain tumors only harbor an extremely low mutation rate and very few recurrent DNA alterations. Comparable observations have been made for leukemia. The current results might help to understand mechanisms of epigenetic tumor formation, which are currently unclear in humans (Landskron, 2018).

    cherub is the first lncRNA described to segregate asymmetrically during mitosis. Once cherub is allocated through binding to the RNA-binding protein Staufen into the cytoplasm of INPs, its levels decrease over time. The results show that the inability to segregate cherub into differentiating cells leads to its accumulation in tNBs. The increasing amount of tumor transcriptome data indicates that a vast number of lncRNAs show increased expression levels in various tumor types. Intriguingly, the mammalian homologue of cherub's binding partner Staufen has been also described to asymmetrically localize RNA in dividing neural stem cells. Hence, besides transcriptional upregulation, asymmetric distribution of lncRNAs between sibling cells might play a role in the accumulation of such RNAs in mammalian tumors (Landskron, 2018).

    The data suggest a functional connection between cherub and proteins involved in temporal neural stem cell patterning. This study that tNBs retain the early temporal identity factor Imp even during late larval stages. However, IGF-II mRNA-binding protein (Imp) expression in brat mutants is heterogeneous and only a subpopulation of tNBs maintains young identity (Landskron, 2018).

    Tumor heterogeneity has also been described for pros tumors, where only a subset of tNBs maintains expression of the early temporal factors Imp and Chinmo. Interestingly, it is this subpopulation that drives tumor growth in prospero tumors (Landskron, 2018).

    Consistent with this, genetic experiments show that 'rejuvenating' tNBs enhances tumor growth and consequently increases the survival of tumor bearing flies, whereas 'aging' tNBs identity has the opposite effect. Although mammalian counterparts of Imp have not yet being shown to act as temporal identity genes, their upregulated expression has been implied in various cancer types. Therefore, temporal patterning of NBs has an essential role in brain tumor propagation in Drosophila (Landskron, 2018).

    The subset of tNBs that retain early identity in tumors is lost in a cherub mutant background. This suggests that cherub itself might regulate temporal identity. In NBs and tNBs cherub regulates Syp localization by facilitating the binding of Syp to Staufen and thus recruiting it to the cell cortex. In tumors depleted of cherub, Syp localizes mainly to the cytoplasm and is no longer observed at the cortex. As the removal of Syp in tNBs leads to enhanced tumor growth and early lethality, those data suggest that cherub could control temporal NB identity by regulating the subcellular localization of Syp(Landskron, 2018).

    How could cherub regulate the function of Syp? The RNA-binding protein Syp is a translational regulator and has been suggested to control mRNA stability. As mammalian SYNCRIP/hnRNP Q interacts with a lncRNA that suppresses translation, cherub might regulate Syp to inhibit or promote the translation of a subset of target mRNAs. In particular, in NBs Syp acts at two stages in NBs during development: Firstly, approximately 60 hr after larval hatching it represses early temporal NB factors, like Imp. Secondly, at the end of the NB lifespan Syp promotes levels of the differentiation factor prospero to facilitate the NB's final cell cycle exit. As cherub depletion in brat tumors leads to decreased tumor growth, it is possible that cherub inhibits the Syp-dependent repression of the early factor Imp, which this study shows to be required for optimal tumor growth. However, cherub mutant NBIIs do not show altered timing or expression of Imp during development. In accordance, brat tumors show high cortical cherub levels, but only a subset of NBs expresses Imp. Rather than rendering Syp completely inactive, it is suggested that cherub decreases the ability of Syp to promote factors important to restrict NB proliferation. As prospero is not expressed in NBIIs, it remains to be investigated which Syp targets are affected by cherub (Landskron, 2018).

    Remarkably, cherub mutants are viable, fertile and do not affect NBII lineages. Neurons generated by NBIIs predominantly integrate into the adult brain structure termed central complex, which is important for locomotor activity. As cherub mutants show normal geotaxis, function of the lncRNA seems dispensable for NBIIs to generate their neural descendants (Landskron, 2018).

    Nevertheless, the conserved secondary RNA structures of cherub and its conserved expression pattern in other Drosophila species suggest that it has a functional role. There are several possibilities why no phenotype is observed upon the loss of cherub. In wild-type flies cherub might confer robustness. A similar scenario was observed in embryonic NBs, in which Staufen segregates prospero mRNA into GMCs. The failure to segregate prospero mRNA does not result in a phenotype, but it enhances the hypomorphic prospero GMC phenotype. Thus segregation of prospero mRNA serves as support for Prospero protein to induce a GMC fate. Similarly, cherub could act as a backup to reliably establish correct Syp levels in NBIIs and in INPs. Alternatively, cherub might fine-tune the temporal patterning by regulating the cytoplasmic pool of Syp in the NBs. Increasing Syp levels have been suggested to determine distinct temporal windows, in which different INPs and ultimately neurons with various morphologies are sequentially born. Therefore, it cannot be excluded that changes in Syp levels lead to subtle alterations in the number of certain neuron classes produced by NBIIs that only reveal themselves in pathological conditions like tumorigenesis (Landskron, 2018).

    This study illustrates how a lncRNA can control the subcellular localization of temporal factors. In addition to temporal NB identity, Syp regulates synaptic transmission and maternal RNA localization. While cherub is not expressed in ovaries or adult heads, Staufen has been implicated in these processes, suggesting that other RNAs might act similarly to cherub. Interestingly, the mammalian Syp homolog hnRNP Q binds the noncoding RNA BC200, whose upregulation is used as a biomarker in ovarian, esophageal, breast and brain cancer. In the future, it will be interesting to investigate whether the mechanism identified in Drosophila is involved in mammalian tumorigenesis as well (Landskron, 2018).

    References

    Hirabayashi, S., Baranski, T. J. and Cagan, R. L. (2013). Transformed Drosophila cells evade diet-mediated insulin resistance through wingless signaling. Cell 154: 664-675. PubMed ID: 23911328

    Hirabayashi, S. and Cagan, R. L. (2015). Salt-inducible kinases mediate nutrient-sensing to link dietary sugar and tumorigenesis in Drosophila. Elife 4. PubMed ID: 26573956

    Landskron, L., Steinmann, V., Bonnay, F., Burkard, T. R., Steinmann, J., Reichardt, I., Harzer, H., Laurenson, A. S., Reichert, H. and Knoblich, J. A. (2018). The asymmetrically segregating lncRNA cherub is required for transforming stem cells into malignant cells. Elife 7. PubMed ID: 29580384

    Nie, Y., Li, Q., Amcheslavsky, A., Duhart, J. C., Veraksa, A., Stocker, H., Raftery, L. A. and Ip, Y. T. (2015). Bunched and Madm function downstream of Tuberous Sclerosis Complex to regulate the growth of intestinal stem cells in Drosophila. Stem Cell Rev 11: 813-825. PubMed ID: 26323255

    Song, Y. and Lu, B. (2011). Regulation of cell growth by Notch signaling and its differential requirement in normal vs. tumor-forming stem cells in Drosophila. Genes Dev. 25(24): 2644-58. PubMed Citation: 22190460

    Wehr, M. C., Holder, M. V., Gailite, I., Saunders, R. E., Maile, T. M., Ciirdaeva, E., Instrell, R., Jiang, M., Howell, M., Rossner, M. J. and Tapon, N. (2013). Salt-inducible kinases regulate growth through the Hippo signalling pathway in Drosophila. Nat Cell Biol 15: 61-71. PubMed ID: 23263283



    Zygotically transcribed genes

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