RB depletion is required for the continuous growth of tumors initiated by loss of RB.

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.

The Long-Lost Ligase: CRL4AMBRA1 Regulates the Stability of D-Type Cyclins.

D-type cyclins (cyclin D1, D2, and D3, together cyclin D) are central drivers of the cell division cycle and well-described proto-oncoproteins. Rapid turnover of cyclin D is critical for its regulation, but the underlying mechanism has remained a matter of debate. Recently, AMBRA1 was identified as the major regulator of the stability of all three D-type cyclins. AMBRA1 serves as the substrate receptor for one of ∼40 CUL4-RING E3 ubiquitin ligase (CRL4) complexes to mediate the polyubiquitylation and subsequent degradation of cyclin D. Consequently, AMBRA1 regulates cell proliferation to impact tumor growth and the cellular response to cell cycle-targeted cancer therapies. Here we discuss the findings that implicate AMBRA1 as a core member of the cell cycle machinery.

The AMBRA1 E3 ligase adaptor regulates the stability of cyclin D.

The initiation of cell division integrates a large number of intra- and extracellular inputs. D-type cyclins (hereafter, cyclin D) couple these inputs to the initiation of DNA replication1. Increased levels of cyclin D promote cell division by activating cyclin-dependent kinases 4 and 6 (hereafter, CDK4/6), which in turn phosphorylate and inactivate the retinoblastoma tumour suppressor. Accordingly, increased levels and activity of cyclin D-CDK4/6 complexes are strongly linked to unchecked cell proliferation and cancer2,3. However, the mechanisms that regulate levels of cyclin D are incompletely understood4,5. Here we show that autophagy and beclin 1 regulator 1 (AMBRA1) is the main regulator of the degradation of cyclin D. We identified AMBRA1 in a genome-wide screen to investigate the genetic basis of  the response to CDK4/6 inhibition. Loss of AMBRA1 results in high levels of cyclin D in cells and in mice, which promotes proliferation and decreases sensitivity to CDK4/6 inhibition. Mechanistically, AMBRA1 mediates ubiquitylation and proteasomal degradation of cyclin D as a substrate receptor for the cullin 4 E3 ligase complex. Loss of AMBRA1 enhances the growth of lung adenocarcinoma in a mouse model, and low levels of AMBRA1 correlate with worse survival in patients with lung adenocarcinoma. Thus, AMBRA1 regulates cellular levels of cyclin D, and contributes to cancer development and the response of cancer cells to CDK4/6 inhibitors.

RB1 Deletion in Retinoblastoma Protein Pathway-Disrupted Cells Results in DNA Damage and Cancer Progression.

Proliferative control in cancer cells is frequently disrupted by mutations in the retinoblastoma protein (RB) pathway. Intriguingly, RB1 mutations can arise late in tumorigenesis in cancer cells whose RB pathway is already compromised by another mutation. In this study, we present evidence for increased DNA damage and instability in cancer cells with RB pathway defects when RB1 mutations are induced. We generated isogenic RB1 mutant genotypes with CRISPR/Cas9 in a number of cell lines. Cells with even one mutant copy of RB1 have increased basal levels of DNA damage and increased mitotic errors. Elevated levels of reactive oxygen species as well as impaired homologous recombination repair underlie this DNA damage. When xenografted into immunocompromised mice, RB1 mutant cells exhibit an elevated propensity to seed new tumors in recipient lungs. This study offers evidence that late-arising RB1 mutations can facilitate genome instability and cancer progression that are beyond the preexisting proliferative control deficit.

Intertumoral Heterogeneity in SCLC Is Influenced by the Cell Type of Origin.

The extent to which early events shape tumor evolution is largely uncharacterized, even though a better understanding of these early events may help identify key vulnerabilities in advanced tumors. Here, using genetically defined mouse models of small cell lung cancer (SCLC), we uncovered distinct metastatic programs attributable to the cell type of origin. In one model, tumors gain metastatic ability through amplification of the transcription factor NFIB and a widespread increase in chromatin accessibility, whereas in the other model, tumors become metastatic in the absence of NFIB-driven chromatin alterations. Gene-expression and chromatin accessibility analyses identify distinct mechanisms as well as markers predictive of metastatic progression in both groups. Underlying the difference between the two programs was the cell type of origin of the tumors, with NFIB-independent metastases arising from mature neuroendocrine cells. Our findings underscore the importance of the identity of cell type of origin in influencing tumor evolution and metastatic mechanisms.Significance: We show that SCLC can arise from different cell types of origin, which profoundly influences the eventual genetic and epigenetic changes that enable metastatic progression. Understanding intertumoral heterogeneity in SCLC, and across cancer types, may illuminate mechanisms of tumor progression and uncover how the cell type of origin affects tumor evolution. Cancer Discov; 8(10); 1316-31. ©2018 AACR. See related commentary by Pozo et al., p. 1216 This article is highlighted in the In This Issue feature, p. 1195.

Beyond the Cell Cycle: Enhancing the Immune Surveillance of Tumors Via CDK4/6 Inhibition.

Inhibitors of the cyclin-dependent kinases 4 and 6 (CDK4/6) were originally designed to block proliferation and cell cycle progression of cancer cells in which the activity of these kinases is dysregulated. CDK4/6 inhibitors have already been FDA approved for the treatment of estrogen receptor (ER)-positive breast cancer and are being tested in numerous other cancer types. However, several recent studies have identified novel effects of CDK4/6 inhibitors on tumor growth, most notably an indirect effect resulting from the activation of immune surveillance. This Perspective discusses these recent observations, including the effects that CDK4/6 inhibitors may have on immune cells themselves. It is likely that CDK4/6 inhibitors will have a broader impact than their expected induction of cell cycle arrest in the treatment of human cancers. Mol Cancer Res; 16(10); 1454-7. ©2018 AACR.

α-Ketoglutarate Accelerates the Initial Differentiation of Primed Human Pluripotent Stem Cells.

Pluripotent stem cells (PSCs) can self-renew or differentiate from naive or more differentiated, primed, pluripotent states established by specific culture conditions. Increased intracellular α-ketoglutarate (αKG) was shown to favor self-renewal in naive mouse embryonic stem cells (mESCs). The effect of αKG or αKG/succinate levels on differentiation from primed human PSCs (hPSCs) or mouse epiblast stem cells (EpiSCs) remains unknown. We examined primed hPSCs and EpiSCs and show that increased αKG or αKG-to-succinate ratios accelerate, and elevated succinate levels delay, primed PSC differentiation. αKG has been shown to inhibit the mitochondrial ATP synthase and to regulate epigenome-modifying dioxygenase enzymes. Mitochondrial uncoupling did not impede αKG-accelerated primed PSC differentiation. Instead, αKG induced, and succinate impaired, global histone and DNA demethylation in primed PSCs. The data support αKG promotion of self-renewal or differentiation depending on the pluripotent state.