Heterozygous RB1 mutation enhanced ATP production in human iPSC-derived retinal organoids.

BACKGROUND: Recent in vitro studies using RB1+/- fibroblasts and MSCs have shown molecular and functional disruptions without the need for biallelic loss of RB1. However, this was not reflected in the recent in vitro studies employing RB1+/- retinal organoids. To gain further insights into the molecular disruptions in the RB1+/- retinal organoids, we performed a high throughput RNA sequencing analysis. METHODS AND RESULTS: iPSCs were generated from RB1+/+ and RB1+/- OAMSCs derived from retinoblastoma patients. RB1+/+ and RB1+/- iPSCs were subjected to a step-wise retinal differentiation protocol. Retinal differentiation was evaluated by Real-time PCR and flow cytometry analysis of the retinal markers. To gain further insights into the molecular differences in RB1+/- retinal organoids, a high throughput RNA sequencing followed by differential gene expression analysis and gene set enrichment analysis (GSEA) was performed. The analysis revealed a shift from the regular metabolic process of glycolysis to oxidative phosphorylation in the RB1+/- retinal organoids. To investigate further, we performed assays to determine the levels of pyruvate, lactate and ATP in the retinal organoids. The results revealed significant increase in ATP and pyruvate levels in RB1+/- retinal organoids of day 120 compared to that of the RB1+/+. The results thus revealed enhanced ATP production in the RB1+/- retinal organoids. CONCLUSION: The study provides novel insights into the metabolic phenotype of heterozygous RB1 mutant suggesting dysregulation of energy metabolism and glycolytic pathways to be first step even before the changes in cellular proliferation or other phenotypic consequences ensue.

The pRb/RBL2-E2F1/4-GCN5 axis regulates cancer stem cell formation and G0 phase entry/exit by paracrine mechanisms.

The lethality, chemoresistance and metastatic characteristics of cancers are associated with phenotypically plastic cancer stem cells (CSCs). How the non-cell autonomous signalling pathways and cell-autonomous transcriptional machinery orchestrate the stem cell-like characteristics of CSCs is still poorly understood. Here we use a quantitative proteomic approach for identifying secreted proteins of CSCs in pancreatic cancer. We uncover that the cell-autonomous E2F1/4-pRb/RBL2 axis balances non-cell-autonomous signalling in healthy ductal cells but becomes deregulated upon KRAS mutation. E2F1 and E2F4 induce whereas pRb/RBL2 reduce WNT ligand expression (e.g. WNT7A, WNT7B, WNT10A, WNT4) thereby regulating self-renewal, chemoresistance and invasiveness of CSCs in both PDAC and breast cancer, and fibroblast proliferation. Screening for epigenetic enzymes identifies GCN5 as a regulator of CSCs that deposits H3K9ac onto WNT promoters and enhancers. Collectively, paracrine signalling pathways are controlled by the E2F-GCN5-RB axis in diverse cancers and this could be a therapeutic target for eliminating CSCs.

RB1 loss induces quiescent state through downregulation of RAS signaling in mammary epithelial cells.

While loss of function (LOF) of retinoblastoma 1 (RB1) tumor suppressor is known to drive initiation of small-cell lung cancer and retinoblastoma, RB1 mutation is rarely observed in breast cancers at their initiation. In this study, we investigated the impact on untransformed mammary epithelial cells given by RB1 LOF. Depletion of RB1 in anon-tumorigenic MCF10A cells induced reversible growth arrest (quiescence) featured by downregulation of multiple cyclins and MYC, upregulation of p27KIP1, and lack of expression of markers which indicate cellular senescence or epithelial-mesenchymal transition (EMT). We observed a similar phenomenon in human mammary epithelial cells (HMEC) as well. Additionally, we found that RB1 depletion attenuated the activity of RAS and the downstream MAPK pathway in an RBL2/p130-dependent manner. The expression of farnesyltransferase ß, which is essential for RAS maturation, was found to be downregulated following RB1 depletion also in an RBL2/p130-dependent manner. These findings unveiled an unexpected mechanism whereby normal mammary epithelial cells resist to tumor initiation upon RB1 LOF.

Retinoblastoma-associated protein is important for TRIM24-mediated activation of the mTOR signaling pathway through DUSP2 action in prostate cancer.

RB transcriptional corepressor 1 (RB) deletion is the most important genomic factor associated with the prognosis of castration-resistant prostate cancer (CRPC) patients receiving androgen receptor (AR) signaling inhibitor therapy. Loss of RB could support prostate cancer cell growth in a hormone-independent manner, but the underlying mechanism by which RB regulates tumor progression extends far beyond the cell cycle pathway. A previous study indicated that RB inactivates AKT signaling but has no effect on mTOR signaling in cancer cells. Here, we found that the S249/T252 site in RB is key to regulating the transcriptional activity of the tumor-promoting factor TRIM24 in CRPC, as identified through FXXXV mapping. The RB/TRIM24 complex functions through DUSP2, which serves as an intermediate bridge, to activate the mTOR pathway and promote prostate cancer progression. Accordingly, we designed RB-linker-proteolysis-targeting chimera (PROTAC) molecules, which decreased TRIM24 protein levels and inactivated the mTOR signaling pathway, thereby inhibiting prostate cancer. Therefore, this study not only elucidates the novel function of RB but also provides a theoretical basis for the development of new drugs for treating prostate cancer.

Immunohistochemical p16 overexpression and Rb loss correlate with high-risk human papillomavirus infection in endocervical adenocarcinomas.

AIMS: p16 is a sensitive surrogate marker for transcriptionally active high-risk human papillomavirus (HR-HPV) infection in endocervical adenocarcinoma (ECA); however, its specificity is not perfect. METHODS AND RESULTS: We examined p16 and Rb expressions by immunohistochemistry (IHC) and the transcriptionally active HR-HPV infection by mRNA in-situ hybridisation (ISH) with histological review in 108 ECA cases. Thirteen adenocarcinomas of endometrial or equivocal origin (six endometrioid and seven serous carcinomas) were compared as the control group. HR-HPV was detected in 83 of 108 ECA cases (77%), including five HPV-associated adenocarcinomas in situ and 78 invasive HPV-associated adenocarcinomas. All 83 HPV-positive cases showed consistent morphology, p16 positivity and partial loss pattern of Rb. Among the 25 cases of HPV-independent adenocarcinoma, four (16%) were positive for p16, and of these four cases, three of 14 (21%) were gastric type adenocarcinomas and one of 10 (10%) was a clear cell type adenocarcinoma. All 25 HPV-independent adenocarcinomas showed preserved expression of Rb irrespective of the p16 status. Similarly, all 13 cases of the control group were negative for HR-HPV with preserved expression of Rb, even though six of 13 (46%) cases were positive for p16. Compared with p16 alone, the combination of p16 overexpression and Rb partial loss pattern showed equally excellent sensitivity (each 100%) and improved specificity (100 versus 73.6%) and positive predictive values (100 versus 89.2%) in the ECA and control groups. Furthermore, HR-HPV infection correlated with better prognosis among invasive ECAs. CONCLUSIONS: The results suggest that the combined use of p16 and Rb IHC could be a reliable method to predict HR-HPV infection in primary ECAs and mimics. This finding may contribute to prognostic prediction and therapeutic strategy.

Functional characterization of D-type cyclins involved in cell division in rice.

BACKGROUND: D-type cyclins (CYCD) regulate the cell cycle G1/S transition and are thus closely involved in cell cycle progression. However, little is known about their functions in rice. RESULTS: We identified 14 CYCD genes in the rice genome and confirmed the presence of characteristic cyclin domains in each. The expression of the OsCYCD genes in different tissues was investigated. Most OsCYCD genes were expressed at least in one of the analyzed tissues, with varying degrees of expression. Ten OsCYCD proteins could interact with both retinoblastoma-related protein (RBR) and A-type cyclin-dependent kinases (CDKA) forming holistic complexes, while OsCYCD3;1, OsCYCD6;1, and OsCYCD7;1 bound only one component, and OsCYCD4;2 bound to neither protein. Interestingly, all OsCYCD genes except OsCYCD7;1, were able to induce tobacco pavement cells to re-enter mitosis with different efficiencies. Transgenic rice plants overexpressing OsCYCD2;2, OsCYCD6;1, and OsCYCD7;1 (which induced cell division in tobacco with high-, low-, and zero-efficiency, respectively) were created. Higher levels of cell division were observed in both the stomatal lineage and epidermal cells of the OsCYCD2;2- and OsCYCD6;1-overexpressing plants, with lower levels seen in OsCYCD7;1-overexpressing plants. CONCLUSIONS: The distinct expression patterns and varying effects on the cell cycle suggest different functions for the various OsCYCD proteins. Our findings will enhance understanding of the CYCD family in rice and provide a preliminary foundation for the future functional verification of these genes.

Analysis of E2F1 single-nucleotide polymorphisms reveals deleterious non-synonymous substitutions that disrupt E2F1-RB protein interaction in cancer.

Cancer is a medical condition that is caused by the abnormal growth and division of cells, leading to the formation of tumors. The E2F1 and RB pathways are critical in regulating cell cycle, and their dysregulation can contribute to the development of cancer. In this study, we analyzed experimentally reported SNPs in E2F1 and assessed their effects on the binding affinity with RB. Out of 46, nine mutations were predicted as deleterious, and further analysis revealed four highly destabilizing mutations (L206W, R232C, I254T, A267T) that significantly altered the protein structure. Molecular docking of wild-type and mutant E2F1 with RB revealed a docking score of -242 kcal/mol for wild-type, while the mutant complexes had scores ranging from -217 to -220 kcal/mol. Molecular simulation analysis revealed variations in the dynamics features of both mutant and wild-type complexes due to the acquired mutations. Furthermore, the total binding free energy for the wild-type E2F1-RB complex was -64.89 kcal/mol, while those of the L206W, R232C, I254T, and A267T E2F1-RB mutants were -45.90 kcal/mol, -53.52 kcal/mol, -55.67 kcal/mol, and -61.22 kcal/mol, respectively. Our study is the first to extensively analyze E2F1 gene mutations and identifies candidate mutations for further validation and potential targeting for cancer therapeutics.

Effect of Vitamin D on Retinoblastoma Protein in Prediabetic Individuals.

About 50.8 million people were diagnosed with diabetes in 2011; the count has increased by 10 million in the last five years. Type-1 diabetes could occur at any age, but predominantly in children and young adults. The risk of developing type II diabetes mellitus in the offspring of parents with DM II is 40% if one parent has DM II and approaches 70% if both parents have DM II. The process of developing diabetes from normal glucose tolerance is continuous, with insulin resistance being the first stage. As prediabetes progresses slowly to DM II, it may take approximately 15-20 years for an individual to become diabetic. This progression can be prevented or delayed by taking some precautions and making some lifestyle amendments, e.g., reducing weight by 5-7% of total body weight if obese, etc. Retinoblastoma protein is one of the pocket proteins that act as crucial gatekeepers during the G1/S transition in the cell cycle. A loss or defect in single- cell cycle activators (especially CDK4 and CDK6) leads to cell failure. In diabetic or stress conditions, p53 becomes a transcription factor, resulting in the transactivation of CKIs, which leads to cell cycle arrest, cell senescence, or cell apoptosis. Vitamin D affects insulin sensitivity by increasing insulin receptors or the sensitivity of insulin receptors to insulin. It also affects peroxisome proliferator-activated receptors (PPAR) and extracellular calcium. These influence both insulin resistance and secretion mechanisms, undertaking the pathogenesis of type II diabetes. The study confines a marked decrement in the levels of random and fasting blood glucose levels upon regular vitamin D intake, along with a significant elevation of retinoblastoma protein levels in the circulatory system. The most critical risk factor for the occurrence of the condition came out to be family history, showing that patients with first-degree relatives with diabetes are more susceptible. Factors such as physical inactivity or comorbid conditions further aggravate the risk of developing the disease. The increase in pRB levels caused by vitamin D therapy in prediabetic patients directly influences blood glucose levels. pRB is supposed to play a role in maintaining blood sugar levels. The results of this study could be used for further studies to evaluate the role of vitamin D and pRB in regeneration therapy for beta cells in prediabetics.

Patterns in the tapestry of chromatin-bound RB.

The retinoblastoma protein (RB)-mediated regulation of E2F is a component of a highly conserved cell cycle machine. However, RB's tumor suppressor activity, like RB's requirement in animal development, is tissue-specific, context-specific, and sometimes appears uncoupled from cell proliferation. Detailed new information about RB's genomic distribution provides a new perspective on the complexity of RB function, suggesting that some of its functional specificity results from context-specific RB association with chromatin. Here we summarize recent evidence showing that RB targets different types of chromatin regulatory elements at different cell cycle stages. RB controls traditional RB/E2F targets prior to S-phase, but, when cells proliferate, RB redistributes to cell type-specific chromatin loci. We discuss the broad implications of the new data for RB research.

CDK4/6 Inhibition With Lerociclib is a Potential Therapeutic Strategy for the Treatment of Pediatric Sarcomas.

BACKGROUND: Sarcomas are a heterogenous collection of bone and soft tissue tumors. The heterogeneity of these tumors makes it difficult to standardize treatment. CDK 4/6 inhibitors are a family of targeted agents which limit cell cycle progression and have been shown to be upregulated in sarcomas. In the current preclinical study, we evaluated the effects of lerociclib, a CDK4/6 inhibitor, on pediatric sarcomas in vitro and in 3D bioprinted tumors. METHODS: The effects of lerociclib on viability, proliferation, cell cycle, motility, and stemness were assessed in established sarcoma cell lines, U-2 OS and MG-63, as well as sarcoma patient-derived xenografts (PDXs). 3D printed biotumors of each of the U-2 OS, MG-63, and COA79 cells were utilized to study the effects of lerociclib on tumor growth ex vivo. RESULTS: CDK 4/6, as well as the intermediaries retinoblastoma protein (Rb) and phosphorylated Rb were identified as targets in the four sarcoma cell lines. Lerociclib treatment induced cell cycle arrest, decreased proliferation, motility, and stemness of sarcoma cells. Treatment with lerociclib decreased sarcoma cell viability in both traditional 2D culture as well as 3D bioprinted microtumors. CONCLUSIONS: Inhibition of CDK 4/6 activity with lerociclib was efficacious in traditional 2D sarcoma cell culture as well as in 3D bioprints. Lerociclib holds promise and warrants further investigation as a novel therapeutic strategy for management of these heterogenous groups of tumors.

Targeting the E2F1/Rb/HDAC1 axis with the small molecule HR488B effectively inhibits colorectal cancer growth.

Colorectal cancer (CRC), the third most common cancer worldwide, remains highly lethal as the disease only becomes symptomatic at an advanced stage. Growing evidence suggests that histone deacetylases (HDACs), a group of epigenetic enzymes overexpressed in precancerous lesions of CRC, may represent promising molecular targets for CRC treatment. Histone deacetylase inhibitors (HDACis) have gradually become powerful anti-cancer agents targeting epigenetic modulation and have been widely used in the clinical treatment of hematologic malignancies, while only few studies on the benefit of HDACis in the treatment of CRC. In the present study, we designed a series of small-molecule Thiazole-based HDACis, among which HR488B bound to HDAC1 with a high affinity and exerted effective anti-CRC activity both in vitro and in vivo. Moreover, we revealed that HR488B specifically suppressed the growth of CRC cells by inducing cell cycle G0/G1 arrest and apoptosis via causing mitochondrial dysfunction, reactive oxygen species (ROS) generation, and DNA damage accumulation. Importantly, we noticed that HR488B significantly decreased the expression of the E2F transcription factor 1 (E2F1), which was crucial for the inhibitory effect of HR488B on CRC. Mechanistically, HR488B obviously decreased the phosphorylation level of the retinoblastoma protein (Rb), and subsequently prevented the release of E2F1 from the E2F1/Rb/HDAC1 complex, which ultimately suppressed the growth of CRC cells. Overall, our study suggests that HR488B, a novel and efficient HDAC1 inhibitor, may be a potential candidate for CRC therapy in the future. Furthermore, targeting the E2F1/Rb/HDAC1 axis with HR488B provides a promising therapeutic avenue for CRC.

Molecular Docking and In Vivo Biological Studies of Sodium Salt of 3-(4-Methyl-2-oxo-2-H-quinoline-7-yloxy)-3-phenylacrylic Acid As Anticancer Agent.

Quinoline derivatives possess several therapeutic properties. Aim: studying the anticancer effect of 3-(4-methyl-2-oxo-2-H-quinoline-7-yloxy)-3-phenylacrylic acid's sodium solution on the Ehrlich ascites carcinoma (EAC). Median lethal dose (LD50) and dose response curve was determined for sodium salt solution of 3-(4-methyl-2-oxo-2-H-quinoline-7-yloxy)-3-phenylacrylic acid, then diving a group of one hundred Swiss albino mice, which are all females, into five groups: group 1: (negative control) where intraperitoneally injected with saline into mice for 10 successive days; group 2 (positive control), also namely (EAC-bearing group): where the EAC cells were intraperitoneally injected into mice (2.5 × 106 cells/mouse) only one time on the first day; group 3 which is defined as the (therapeutic group) where the Na+ salt of the synthetic compound was injected into the peritoneum of the mice (2.5 mg/kg) the very first day after the injection of the EAC, then the compound was injected every two days for a period of 10 days; group 4 which is the (preventive group) where the sodium salt of the synthetic compound (2.5 mg/kg) was injected in the peritoneum of the mice the day before the injection of the EAC, then the compound was successively injected every day for a period of ten days; and group 5 which is the (drug group) in which mice were repeatedly injected) in their peritoneum with the sodium salt of the synthetic compound (2.5 mg/kg on a daily basis over a period of ten days. On the eleventh day of the trial, EAC cells were harvested from each mouse in a heparinized saline, in addition to blood samples, liver and kidney tissues which are also collected. Molecular docking showed that compound's sodium salt was docked into (PDB: 2R7G) and (PDB: 2R3I), which are the retinoblastoma protein receptor and the cyclin D-1 receptor respectively. Compared to those in the positive control group, mice in both the therapeutic and preventive groups, has shown a significant decrease in MDA, cyclin D-1 levels in the tissues of both liver and kidney tissues, in addition to the serum ALT, AST, CK-MB, and LDH activities, and the serum urea and creatinine concentration. However, mice in the formerly mentioned groups, both therapeutic and preventive groups, have shown an increase in the serum albumin, total protein, retinoblastoma protein in both liver and kidney tissues as well as the total antioxidant capacity, when compared to mice in the positive control group. It is worth mentioning that histopathological findings have confirmed that. Sodium salt of 3-(4-methyl-2-oxo-2H-quinoline-7-yloxy)-3-phenylacrylic acid showed potential in vivo anticancer and antioxidant effects against Ehrlich ascites carcinoma cells; (EAC cells).

Pharmacological induction of translational readthrough of nonsense mutations in the retinoblastoma (RB1) gene.

The retinoblastoma protein (Rb) is encoded by the RB1 tumor suppressor gene. Inactivation of RB1 by inherited or somatic mutation occurs in retinoblastoma and various other types of tumors. A significant fraction (25.9%) of somatic RB1 mutations are nonsense substitutions leading to a premature termination codon (PTC) in the RB1 coding sequence and expression of truncated inactive Rb protein. Here we show that aminoglycoside G418, a known translational readthrough inducer, can induce full-length Rb protein in SW1783 astrocytoma cells with endogenous R579X nonsense mutant RB1 as well as in MDA-MB-436 breast carcinoma cells transiently transfected with R251X, R320X, R579X or Q702X nonsense mutant RB1 cDNA. Readthrough was associated with increased RB1 mRNA levels in nonsense mutant RB1 cells. Induction of full-length Rb protein was potentiated by the cereblon E3 ligase modulator CC-90009. These results suggest that pharmacological induction of translational readthrough could be a feasible strategy for therapeutic targeting of tumors with nonsense mutant RB1.

Genetic alterations that deregulate RB and PDGFRA signaling pathways drive tumor progression in IDH2-mutant astrocytoma.

In IDH-mutant astrocytoma, IDH2 mutation is quite rare and biological mechanisms underlying tumor progression in IDH2-mutant astrocytoma remain elusive. Here, we report a unique case of IDH2 mutant astrocytoma, CNS WHO grade 3 that developed tumor progression. We performed a comprehensive genomic and epigenomic analysis for primary and recurrent tumors and found that both tumors harbored recurrent IDH2R172K and TP53R248W mutation with CDKN2A/B hemizygous deletion. We also found amplifications of CDK4 and MDM2 with PDGFRA gain in the recurrent tumor and upregulated protein expressions of these genes. We further developed, for the first time, a xenograft mouse model of IDH2R172K and TP53R248W mutant astrocytoma from the recurrent tumor, but not from the primary tumor. Consistent with parent recurrent tumor cells, amplifications of CDK4 and MDM2 and PDGFRA gain were found, while CDKN2A/B was identified as homozygous deletion in the xenografts, qualifying for integrated diagnosis of astrocytoma, IDH2-mutant, CNS WHO grade 4. Cell viability assay found that CDK4/6 inhibitor and PDGFR inhibitor potently decreased cell viability in recurrent tumor cells, as compared to primary tumor cells. These findings suggest that gene alterations that activate retinoblastoma (RB) signaling pathways and PDGFR may drive tumor progression and xenograft formation in IDH2-mutant astrocytoma, which is equivalent to progressive IDH1-mutant astrocytoma. Also, our findings suggest that these genomic alterations may represent therapeutic targets in IDH2-mutant astrocytoma.

Non-canonical pathway for Rb inactivation and external signaling coordinate cell-cycle entry without CDK4/6 activity.

Cyclin-dependent kinases 4 and 6 (CDK4/6) are critical for initiating cell proliferation by inactivating the retinoblastoma (Rb) protein. However, mammalian cells can bypass CDK4/6 for Rb inactivation. Here we show a non-canonical pathway for Rb inactivation and its interplay with external signals. We find that the non-phosphorylated Rb protein in quiescent cells is intrinsically unstable, offering an alternative mechanism for initiating E2F activity. Nevertheless, this pathway incompletely induces Rb-protein loss, resulting in minimal E2F activity. To trigger cell proliferation, upregulation of mitogenic signaling is required for stabilizing c-Myc, thereby augmenting E2F activity. Concurrently, stress signaling promotes Cip/Kip levels, competitively regulating cell proliferation with mitogenic signaling. In cancer, driver mutations elevate c-Myc levels, facilitating adaptation to CDK4/6 inhibitors. Differentiated cells, despite Rb-protein loss, maintain quiescence through the modulation of c-Myc and Cip/Kip levels. Our findings provide mechanistic insights into an alternative model of cell-cycle entry and the maintenance of quiescence.

Combined inactivation of RB and Hippo converts differentiating Drosophila photoreceptors into eye progenitor cells through derepression of homothorax.

The retinoblastoma (RB) and Hippo pathways interact to regulate cell proliferation and differentiation. However, the mechanism of interaction is not fully understood. Drosophila photoreceptors with inactivated RB and Hippo pathways specify normally but fail to maintain their neuronal identity and dedifferentiate. We performed single-cell RNA sequencing to elucidate the cause of dedifferentiation and to determine the fate of these cells. We find that dedifferentiated cells adopt a progenitor-like fate due to inappropriate activation of the retinal differentiation suppressor homothorax (hth) by Yki/Sd. This results in the activation of a distinct Yki/Hth transcriptional program, driving photoreceptor dedifferentiation. We show that Rbf physically interacts with Yki and, together with the GAGA factor, inhibits the hth expression. Thus, RB and Hippo pathways cooperate to maintain photoreceptor differentiation by preventing inappropriate expression of hth in differentiating photoreceptors. Our work highlights the importance of both RB and Hippo pathway activities for maintaining the state of terminal differentiation.

Sequential activation of E2F via Rb degradation and c-Myc drives resistance to CDK4/6 inhibitors in breast cancer.

Cyclin-dependent kinase 4 and 6 inhibitors (CDK4/6i) are key therapeutic agents in the management of metastatic hormone-receptor-positive breast cancer. However, the emergence of drug resistance limits their long-term efficacy. Here, we show that breast cancer cells develop CDK4/6i resistance via a sequential two-step process of E2F activation. This process entails retinoblastoma (Rb)-protein degradation, followed by c-Myc-mediated amplification of E2F transcriptional activity. CDK4/6i treatment halts cell proliferation in an Rb-dependent manner but dramatically reduces Rb-protein levels. However, this reduction in Rb levels insufficiently induces E2F activity. To develop CDK4/6i resistance, upregulation or activating mutations in mitogenic or hormone signaling are required to stabilize c-Myc levels, thereby augmenting E2F activity. Our analysis of pre-treatment tumor samples reveals a strong correlation between c-Myc levels, rather than Rb levels, and poor therapeutic outcomes after CDK4/6i treatment. Moreover, we propose that proteasome inhibitors can potentially reverse CDK4/6i resistance by restoring Rb levels.

STRA8-RB interaction is required for timely entry of meiosis in mouse female germ cells.

Meiosis is differently regulated in males and females. In females, germ cells initiate meiosis within a limited time period in the fetal ovary and undergo a prolonged meiotic arrest until puberty. However, how meiosis initiation is coordinated with the cell cycle to coincide with S phase remains elusive. Here, we demonstrate that STRA8 binds to RB via the LXCXE motif. Mutation of the RB-binding site of STRA8 in female mice delays meiotic entry, which consequently delays progression of meiotic prophase and leads to precocious depletion of the oocyte pool. Single-cell RNA-sequencing analysis reveals that the STRA8-RB interaction is required for S phase entry and meiotic gene activation, ensuring precise timing of meiosis initiation in oocytes. Strikingly, the results suggest STRA8 could sequester RB from E2F during pre-meiotic G1/S transition. This study highlights the gene regulatory mechanisms underlying the female-specific mode of meiotic initiation in mice.

CSE1L is a negative regulator of the RB-DREAM pathway in p53 wild-type NSCLC and can be targeted using an HDAC1/2 inhibitor.

P53 represses transcription by activating p21 expression and promoting formation of RB1-E2F1 and RBL1/RBL2-DREAM transcription repressor complexes. The DREAM complex is composed of DP1, RB-family proteins RBL1 or RBL2 (p107/p130), E2F4/5, and MuvB. We recently reported RBL2-DREAM contributes to improved therapy responses in p53 wild-type NSCLC cells and improved outcomes in NSCLC patients whose tumors express wild-type p53. In the current study we identified CSE1L as a novel inhibitor of the RBL2-DREAM pathway and target to activate RBL2-DREAM in NSCLC cells. CSE1L is an oncoprotein that maintains repression of genes that can be reactivated by HDAC inhibitors. Mocetinostat is a HDAC inhibitor in clinical trials with selectivity against HDACs 1 and 2. Knockdown of CSE1L in NSCLC cells or treatment with mocetinostat increased p21, activated RB1 and RBL2, repressed DREAM target genes, and induced toxicity in a manner that required wild-type p53. Lastly, we found high levels of CSE1L and specific DREAM-target genes are candidate markers to identify p53 wild-type NSCLCs most responsive to mocetinostat. Thus, we identified CSE1L as a critical negative regulator of the RB-DREAM pathway in p53 wild-type NSCLC that can be indirectly targeted with HDAC1/2 inhibitors (mocetinostat) in current clinical trials. High expression of CSE1L and DREAM target genes could serve as a biomarker to identify p53 wild-type NSCLCs most responsive to this HDAC1/2 inhibitor.

Metastatic Competency and Tumor Spheroid Formation Are Independent Cell States Governed by RB in Lung Adenocarcinoma.

Inactivation of the retinoblastoma (RB) tumor suppressor in lung adenocarcinoma is associated with the rapid acquisition of metastatic ability and the loss of lung cell lineage commitment. We previously showed that restoration of RB in advanced lung adenocarcinomas in the mouse was correlated with a decreased frequency of lineage decommitted tumors and overt metastases. To identify a causal relationship for RB and its role in reprogramming lineage commitment and reducing metastatic competency in lung adenocarcinoma, we developed multiple tumor spheroid forming lines where RB restoration could be achieved after characterization of the degree of each spheroid's lineage commitment and metastatic ability. Surprisingly, we discovered that RB inactivation dramatically promoted tumor spheroid forming potential in tumors that arise in the KrasLSL-G12D/+; p53flox/flox lung adenocarcinoma model. However, RB reactivation had no effect on the maintenance of tumor spheroid lines once established. In addition, we show that RB-deficient tumor spheroid lines are not uniformly metastatically competent but are equally likely to be nonmetastatic. Interestingly, unlike tumor spheroid maintenance, RB restoration could functionally revert metastatic tumor spheroids to a nonmetastatic cell state. Thus, strategies to reinstate RB pathway activity in lung cancer may reverse metastatic ability and have therapeutic potential. Finally, the acquisition of tumor spheroid forming potential reflects underlying cell state plasticity, which is often predictive of, or even conflated with metastatic ability. Our data support that each is a discrete cell state restricted by RB and question the suitability of tumor spheroid models for their predictive potential of advanced metastatic tumor cell states. SIGNIFICANCE: Members of the RB pathway are frequently mutated in lung adenocarcinoma. We show that RB regulates cell state plasticity, tumor spheroid formation, and metastatic competency. Our data indicate that these are independent states where spheroid formation is distinct from metastatic competency. Thus, we caution against conflating spheroid formation and other signs of cell state plasticity with advanced metastatic cell states. Nevertheless, our work supports clinical strategies to reactivate RB pathways.