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The canonical model of RB-mediated tumour suppression developed over the past 30 years is based on the regulation of E2F transcription factors to restrict cell cycle progression. Several additional functions have been proposed for RB, on the basis of which a non-canonical RB pathway can be described. Mechanistically, the non-canonical RB pathway promotes histone modification and regulates chromosome structure in a manner distinct from cell cycle regulation. These functions have implications for chemotherapy response and resistance to targeted anticancer agents. This Opinion offers a framework to guide future studies of RB in basic and clinical research.

The cyclin D3–CDK6 kinase complex, which is overactive in some cancers, inhibits two key glycolysis enzymes and thereby enhances the levels of antioxidants in cells, promoting tumour cell survival. Cancer cell survival by cyclin D3–CDK6 metabolism Cyclin–CDK complexes are commonly amplified in cancer and promote cell cycle progression. Inhibitors for CDK4/6 are being tested in clinical trials and are thought to work in patients that retain expression of the CDK substrate RB1. Here, the authors describe an additional pro-survival role of one cyclin–CDK complex, D3–CDK6, which controls cellular metabolism. When hyperactivated in cancer cells, the complex phosphorylates and inactivates two glycolysis enzymes. This redirects glycolytic intermediates to the pentose phosphate and serine pathways, providing enhanced antioxidant capacity. CDK4/6 inhibitors can induce apoptosis by increasing the oxidative stress in tumour cells expressing high levels of D3–CDK6 complexes. The findings suggest that, in addition to RB1, markers such as levels of D3–CDK6 complexes could be useful for identifying patients likely to respond to CDK4/6 inhibitors. D-type cyclins (D1, D2 and D3) and their associated cyclin-dependent kinases (CDK4 and CDK6) are components of the core cell cycle machinery that drives cell proliferation 1 , 2 . Inhibitors of CDK4 and CDK6 are currently being tested in clinical trials for patients with several cancer types, with promising results 2 . Here, using human cancer cells and patient-derived xenografts in mice, we show that the cyclin D3–CDK6 kinase phosphorylates and inhibits the catalytic activity of two key enzymes in the glycolytic pathway, 6-phosphofructokinase and pyruvate kinase M2. This re-directs the glycolytic intermediates into the pentose phosphate (PPP) and serine pathways. Inhibition of cyclin D3–CDK6 in tumour cells reduces flow through the PPP and serine pathways, thereby depleting the antioxidants NADPH and glutathione. This, in turn, increases the levels of reactive oxygen species and causes apoptosis of tumour cells. The pro-survival function of cyclin D-associated kinase operates in tumours expressing high levels of cyclin D3–CDK6 complexes. We propose that measuring the levels of cyclin D3–CDK6 in human cancers might help to identify tumour subsets that undergo cell death and tumour regression upon inhibition of CDK4 and CDK6. Cyclin D3–CDK6, through its ability to link cell cycle and cell metabolism, represents a particularly powerful oncoprotein that affects cancer cells at several levels, and this property can be exploited for anti-cancer therapy.

Key Points Studies of pRB and E2F function in flies and worms share some important similarities that provide insight into the core activities of these proteins and show how a common regulatory module can control various biological functions in different organisms. In Drosophila melanogaster , RBF and dE2F proteins are key regulators of cell proliferation. E2F-protein control results from the functional interplay between activator and repressor complexes. dE2F and RBF proteins have also been linked to the control of re-replication and function in the cellular responses to DNA damage. Mutation of the Caenorhabditis elegans pRB and E2F genes cause developmental defects with limited changes in cell proliferation. C. elegans pRB and E2F complexes repress the expression of developmentally controlled genes and have an important role in maintaining distinct patterns of gene expression in the germline and soma. Genetic studies in C. elegans place pRB and E2F into a large set of genes with a shared function in the formation of the vulva (synMuv B genes). Despite the apparently different uses of pRB and E2F proteins in flies and worms, these proteins share a common molecular mechanism of action that is also preserved in mammalian cells. In each species, pRB and E2F proteins form stable complexes (dREAM/MMB complexes) with orthologues of synMuv B genes. dREAM/MMB complexes function as molecular switches and provide patterns of gene repression that are stable, but also readily reversible. These complexes probably affect some aspect of chromatin structure, but precisely how the switch works is not yet known. The retinoblastoma tumour suppressor pRB, the E2F transcription factor and related proteins are conserved in many species of plants and animals. Genetic and biochemical experiments now show that pRB and E2F proteins of worms, flies and mammals share similar core activities and regulatory mechanisms. Proteins that are related to the retinoblastoma tumour suppressor pRB and the E2F transcription factor are conserved in many species of plants and animals. The mammalian orthologues of pRB and E2F are best known for their roles in cell proliferation, but it has become clear that they affect many biological processes. Here we describe the functions of pRB-related proteins and E2F proteins that have emerged from genetic and biochemical experiments in Caenorhabditis elegans and Drosophila melanogaster . The similarities that have been observed between worms, flies and mammals provide insight into the core activities of pRB and E2F proteins and show how a common regulatory module can control various biological functions in different organisms.

The MiT/TFE family of transcription factors is found to coordinate constitutive activation of autophagy and lysosome biogenesis to drive the metabolic programming and malignant growth of pancreatic cancer. Cellular stress and autophagy linked in cancer Various cancers including pancreatic ductal adenocarcinoma (PDA) are known to depend on high levels of autophagy, the highly conserved self-degradative process required in normal cells for nutrient scavenging and quality control activities. Here Rushika Perera et al . describe a previously unknown link between cellular stress and autophagy leading to altered cell metabolism in pancreatic cancer. They show that aberrant expression and constitutive activation of the MiT/TFE family transcription factors mediates metabolic reprogramming through greatly enhanced autophagy–lysosomal function in human PDA specimens and cell lines. These findings identify lysosome regulation as a focus for nutrient utilization and energy homeostasis in cancer cells. Activation of cellular stress response pathways to maintain metabolic homeostasis is emerging as a critical growth and survival mechanism in many cancers 1 . The pathogenesis of pancreatic ductal adenocarcinoma (PDA) requires high levels of autophagy 2 , 3 , 4 , a conserved self-degradative process 5 . However, the regulatory circuits that activate autophagy and reprogram PDA cell metabolism are unknown. Here we show that autophagy induction in PDA occurs as part of a broader transcriptional program that coordinates activation of lysosome biogenesis and function, and nutrient scavenging, mediated by the MiT/TFE family of transcription factors. In human PDA cells, the MiT/TFE proteins 6 —MITF, TFE3 and TFEB—are decoupled from regulatory mechanisms that control their cytoplasmic retention. Increased nuclear import in turn drives the expression of a coherent network of genes that induce high levels of lysosomal catabolic function essential for PDA growth. Unbiased global metabolite profiling reveals that MiT/TFE-dependent autophagy–lysosome activation is specifically required to maintain intracellular amino acid pools. These results identify the MiT/TFE proteins as master regulators of metabolic reprogramming in pancreatic cancer and demonstrate that transcriptional activation of clearance pathways converging on the lysosome is a novel hallmark of aggressive malignancy.

The Hippo pathway regulates proliferation and survival in Drosophila and mammals, although shared transcriptional targets of their effectors have not been identified. Mammalian YAP controls expression of the EGFR ligand amphiregulin to regulate epithelial-to-mesenchymal transition in mammary epithelial cells, and the EGFR pathway genetically interacts with Yorkie in Drosophila. The Hippo signalling pathway regulates cellular proliferation and survival, thus has profound effects on normal cell fate and tumorigenesis 1 , 2 , 3 . The pivotal effector of this pathway is YAP (yes-associated protein), a transcriptional co-activator amplified in mouse and human cancers, where it promotes epithelial to mesenchymal transition (EMT) and malignant transformation 4 , 5 , 6 , 7 , 8 , 9 , 10 . So far, studies of YAP target genes have focused on cell-autonomous mediators; here we show that YAP-expressing MCF10A breast epithelial cells enhance the proliferation of neighbouring untransfected cells, implicating a non-cell-autonomous mechanism. We identify the gene for the epidermal growth factor receptor (EGFR) ligand amphiregulin (AREG) as a transcriptional target of YAP, whose induction contributes to YAP-mediated cell proliferation and migration, but not EMT. Knockdown of AREG or addition of an EGFR kinase inhibitor abrogates the proliferative effects of YAP expression. Suppression of the negative YAP regulators LATS1 and 2 (large tumour suppressor 1 and 2) is sufficient to induce AREG expression, consistent with physiological regulation of AREG by the Hippo pathway. Genetic interaction between the Drosophila YAP orthologue Yorkie and Egfr signalling components supports the link between these two highly conserved signalling pathways. Thus, YAP-dependent secretion of AREG indicates that activation of EGFR signalling is an important non-cell-autonomous effector of the Hippo pathway, which has implications for the regulation of both physiological and malignant cell proliferation.

Conservation of major signaling pathways between humans and flies has made Drosophila a useful model organism for cancer research. Our understanding of the mechanisms regulating cell growth, differentiation and development has been considerably advanced by studies in Drosophila. Several recent high profile studies have examined the processes constraining the metastatic growth of tumor cells in fruit fly models. Cell invasion can be studied in the context of an in vivo setting in flies, enabling the genetic requirements of the microenvironment of tumor cells undergoing metastasis to be analyzed. This Perspective discusses the strengths and limitations of Drosophila models of cancer invasion and the unique tools that have enabled these studies. It also highlights several recent reports that together make a strong case for Drosophila as a system with the potential for both testing novel concepts in tumor progression and cell invasion, and for uncovering players in metastasis.

The E2F transcription factors play a critical role in controlling cell fate. In Drosophila , the inactivation of E2F in either muscle or fat body results in lethality, suggesting an essential function for E2F in these tissues. However, the cellular and organismal consequences of inactivating E2F in these tissues are not fully understood. Here, we show that the E2F loss exerts both tissue-intrinsic and systemic effects. The proteomic profiling of E2F-deficient muscle and fat body revealed that E2F regulates carbohydrate metabolism, a conclusion further supported by metabolomic profiling. Intriguingly, animals with E2F-deficient fat body had a lower level of circulating trehalose and reduced storage of fat. Strikingly, a sugar supplement was sufficient to restore both trehalose and fat levels, and subsequently rescued animal lethality. Collectively, our data highlight the unexpected complexity of E2F mutant phenotype, which is a result of combining both tissue-specific and systemic changes that contribute to animal development.

Retinoblastoma protein and E2-promoter binding factor (E2F) family members are important regulators of G1-S phase progression. Deregulated E2F also sensitizes cells to apoptosis, but this aspect of E2F function is poorly understood. Studies of E2F-induced apoptosis have mostly been carried out in tissue culture cells, and the analysis of the factors that are important for this process has been restricted to the testing of a few candidate genes. Using Drosophila as a model system, we have generated tools that allow genetic modifiers of E2F-dependent apoptosis to be identified in vivo and developed assays that allow effects on E2F-induced apoptosis to be studied in cultured cells. Genetic interactions show that dE2F1-dependent apoptosis in vivo involves dArk/Apaf1 apoptosome-dependent activation of both initiator and effector caspases and is sensitive to levels of Drosophila inhibitor of apoptosis-1 (dIAP1). Using these approaches, we report the surprising finding that apoptosis inhibitor-5/antiapoptosis clone-11 (Api5/Aac11) is a critical determinant of dE2F1-induced apoptosis in vivo and in vitro. This functional interaction occurs in multiple tissues, is specific to E2F-induced apoptosis, and is conserved from flies to humans. Interestingly, Api5/Aac11 acts downstream of E2F and suppresses E2F-dependent apoptosis without generally blocking E2F-dependent transcription. Api5/Aac11 expression is often upregulated in tumor cells, particularly in metastatic cells. We find that depletion of Api5 is tumor cell lethal. The strong genetic interaction between E2F and Api5/Aac11 suggests that elevated levels of Api5 may be selected during tumorigenesis to allow cells with deregulated E2F activity to survive under suboptimal conditions. Therefore, inhibition of Api5 function might offer a possible mechanism for antitumor exploitation.

The RB family is usually associated with the regulation of the G1/S transition and cell cycle entry. Recent data have shown a role for the RB family in regulating S phase and mitosis, which has implications for the genomic stability of tumour cells in which the RB family is inactivated. RB, a well known tumour suppressor that functions in the control of cell cycle progression and proliferation, has recently been shown to have additional functions in the maintenance of genomic stability, such that inactivation of RB family proteins promotes chromosome instability (CIN) and aneuploidy. Several studies have provided potential explanations for these phenomena that occur following RB loss, and they suggest that this new function of RB may contribute to its role in tumour suppression.

Previously, we discovered a conserved interaction between RB proteins and the Condensin II protein CAP-D3 that is important for ensuring uniform chromatin condensation during mitotic prophase. The Drosophila melanogaster homologs RBF1 and dCAP-D3 co-localize on non-dividing polytene chromatin, suggesting the existence of a shared, non-mitotic role for these two proteins. Here, we show that the absence of RBF1 and dCAP-D3 alters the expression of many of the same genes in larvae and adult flies. Strikingly, most of the genes affected by the loss of RBF1 and dCAP-D3 are not classic cell cycle genes but are developmentally regulated genes with tissue-specific functions and these genes tend to be located in gene clusters. Our data reveal that RBF1 and dCAP-D3 are needed in fat body cells to activate transcription of clusters of antimicrobial peptide (AMP) genes. AMPs are important for innate immunity, and loss of either dCAP-D3 or RBF1 regulation results in a decrease in the ability to clear bacteria. Interestingly, in the adult fat body, RBF1 and dCAP-D3 bind to regions flanking an AMP gene cluster both prior to and following bacterial infection. These results describe a novel, non-mitotic role for the RBF1 and dCAP-D3 proteins in activation of the Drosophila immune system and suggest dCAP-D3 has an important role at specific subsets of RBF1-dependent genes.