Dephosphorylation of threonine-821 of the retinoblastoma tumor suppressor protein (Rb) is required for apoptosis induced by UV and Cdk inhibition

In silico enhancer mining reveals SNS-032 and EHMT2 inhibitors as therapeutic candidates in high-grade serous ovarian cancer

BACKGROUND

Epigenomic dysregulation has been linked to solid tumour malignancies, including ovarian cancers. Profiling of re-programmed enhancer locations associated with disease has the potential to improve stratification and thus therapeutic choices. Ovarian cancers are subdivided into histological subtypes that have significant molecular and clinical differences, with high-grade serous carcinoma representing the most common and aggressive subtype.

METHODS

We interrogated the enhancer landscape(s) of normal ovary and subtype-specific ovarian cancer states using publicly available data. With an initial focus on H3K27ac histone mark, we developed a computational pipeline to predict drug compound activity based on epigenomic stratification. Lastly, we substantiated our predictions in vitro using patient-derived clinical samples and cell lines.

RESULTS

Using our in silico approach, we highlighted recurrent and privative enhancer landscapes and identified the differential enrichment of a total of 164 transcription factors involved in 201 protein complexes across the subtypes. We pinpointed SNS-032 and EHMT2 inhibitors BIX-01294 and UNC0646 as therapeutic candidates in high-grade serous carcinoma, as well as probed the efficacy of specific inhibitors in vitro.

CONCLUSION

Here, we report the first attempt to exploit ovarian cancer epigenomic landscapes for drug discovery. This computational pipeline holds enormous potential for translating epigenomic profiling into therapeutic leads.

Proteomic and phosphoproteomic analyses of Jurkat T-cell treated with 2'3' cGAMP reveals various signaling axes impacted by cyclic dinucleotides

Cyclic dinucleotides (CDNs), such as 2'3'-cGAMP, bind to STING to trigger the production of cytokines and interferons, mainly via activation of TBK1. STING activation by CDN also leads to the release and activation of Nuclear Factor Kappa-light-chain-enhancer of activated B cells (NF-κB) via the phosphorylation of Inhibitor of NF-κB (IκB)-alpha (IκBα) by IκB Kinase (IKK). Beyond the canonical TBK1 or IKK phosphorylations, little is known about how CDNs broadly affect the phosphoproteome and/or other signaling axes. To fill this gap, we performed an unbiased proteome and phosphoproteome analysis of Jurkat T-cell treated with 2'3'-cGAMP or vehicle control to identify proteins and phosphorylation sites that are differentially modulated by 2'3'-cGAMP. We uncovered different classes of kinase signatures associated with cell response to 2'3'-cGAMP. 2'3'-cGAMP upregulated Arginase 2 (Arg2) and the antiviral innate immune response receptor RIG-I as well as proteins involved in ISGylation, E3 ISG15-protein ligase HERC5 and ubiquitin-like protein ISG15, while downregulating ubiquitin-conjugating enzyme UBE2C. Kinases that play a role in DNA double strand break repair, apoptosis, and cell cycle regulation were differentially phosphorylated. Overall, this work demonstrates that 2'3'-cGAMP has a much broader effects on global phosphorylation events than currently appreciated, beyond the canonical TBK1/IKK signaling. SIGNIFICANCE: The host cyclic dinucleotide, 2'3'-cGAMP is known to bind to Stimulator of Interferon Genes (STING) to trigger the production of cytokines and interferons in immune cells via STING-TBK1-IRF3 pathway. Beyond the canonical phosphorelay via the STING-TBK1-IRF3 pathway, little is known about how this second messenger broadly affects the global proteome. Using an unbiased phosphoproteomics, this study identifies several kinases and phosphosites that are modulated by cGAMP. The study expands our knowledge about how cGAMP modulates global proteome and also global phosphorylations.

Translocation of nuclear chromatin distribution to the periphery reflects dephosphorylated threonine-821/826 of the retinoblastoma protein (pRb) in T24 cells treated with Bacillus Calmette-Guérin

The standard treatment for non-muscle-invasive bladder cancer is intravesical Bacillus Calmette-Guérin (BCG) therapy, which is considered the only intravesical therapy that reduces the risk of progression to muscle-invasive cancer. BCG unresponsiveness, in which intravesical BCG therapy is ineffective, has become a problem. It is thus important to evaluate the effectiveness of BCG treatment for patients as soon as possible in order to identify the optimal therapy. Urine cytology is a noninvasive, easy, and cost-effective method that has been used during BCG treatment, but primarily only to determine benign or malignant status; findings concerning the efficacy of BCG treatment based on urine cytology have not been reported. We investigated the relationship between BCG exposure and nuclear an important criterion in urine cytology, i.e., nuclear chromatin patterns. We used three types of cultured cells to evaluate nuclear chromatin patterns and the cell cycle, and we used T24 cells to evaluate the phosphorylation of retinoblastoma protein (pRb) in six-times of BCG exposures. The results revealed that after the second BCG exposure, (i) nuclear chromatin is distributed predominantly at the nuclear periphery and (ii) the dephosphorylation of threonine-821/826 in pRb occurs. This is the first report of a dynamic change in the nuclear chromatin pattern induced by exposure to BCG. Molecular findings also suggested a relationship between this phenomenon and cell-cycle proteins. Although these results are preliminary, they contribute to our understanding of the cytomorphological changes that occur with BCG exposure.

PP2A-B55 and its adapter proteins IER2 and IER5 regulate the activity of RB family proteins and the expression of cell cycle-related genes

The retinoblastoma (RB) tumour suppressor protein regulates cell proliferation, motility, differentiation and apoptosis. The phosphorylation state of RB is modulated by kinases and phosphatases, and RB exhibits phosphorylation-sensitive interactions with E2F family transcription factors. Here, we characterize RB dephosphorylation by protein phosphatase 2A (PP2A). The growth factor-inducible immediate early response (IER) proteins IER2 and IER5 possess an adapter-like function in which IER proteins bind to both PP2A and its target proteins and enhance PP2A activity towards the proteins. IER2 interacts with RB and facilitates dephosphorylation of RB at T821/T826 by PP2A. In IER2 knockdown cells, elevated phosphorylation of RB resulted in reduced binding of RB to the promoters and derepression of cyclin D1 and p21. IER5 binds to both RB and RB-like 1 (p107/RBL1), enhances dephosphorylation of these proteins by PP2A and represses the expression of various cell cycle-related genes. However, IER2-regulated dephosphorylation at T821/T826 is not necessary for the repression function of RB in cell mobility-related gene expression. Our data identify PP2A adapter proteins as critical regulators of RB family proteins and suggest that the phosphorylation status of RB differentially affects gene expression.

Multi-OMICs landscape of SARS-CoV-2-induced host responses in human lung epithelial cells

COVID-19 pandemic continues to remain a global health concern owing to the emergence of newer variants. Several multi-Omics studies have produced extensive evidence on host-pathogen interactions and potential therapeutic targets. Nonetheless, an increased understanding of host signaling networks regulated by post-translational modifications and their ensuing effect on the cellular dynamics is critical to expanding the current knowledge on SARS-CoV-2 infections. Through an unbiased transcriptomics, proteomics, acetylomics, phosphoproteomics, and exometabolome analysis of a lung-derived human cell line, we show that SARS-CoV-2 Norway/Trondheim-S15 strain induces time-dependent alterations in the induction of type I IFN response, activation of DNA damage response, dysregulated Hippo signaling, among others. We identified interplay of phosphorylation and acetylation dynamics on host proteins and its effect on the altered release of metabolites, especially organic acids and ketone bodies. Together, our findings serve as a resource of potential targets that can aid in designing novel host-directed therapeutic strategies.

The Four Homeostasis Knights: In Balance upon Post-Translational Modifications

A cancer outcome is a multifactorial event that comes from both exogenous injuries and an endogenous predisposing background. The healthy state is guaranteed by the fine-tuning of genes controlling cell proliferation, differentiation, and development, whose alteration induces cellular behavioral changes finally leading to cancer. The function of proteins in cells and tissues is controlled at both the transcriptional and translational level, and the mechanism allowing them to carry out their functions is not only a matter of level. A major challenge to the cell is to guarantee that proteins are made, folded, assembled and delivered to function properly, like and even more than other proteins when referring to oncogenes and onco-suppressors products. Over genetic, epigenetic, transcriptional, and translational control, protein synthesis depends on additional steps of regulation. Post-translational modifications are reversible and dynamic processes that allow the cell to rapidly modulate protein amounts and function. Among them, ubiquitination and ubiquitin-like modifications modulate the stability and control the activity of most of the proteins that manage cell cycle, immune responses, apoptosis, and senescence. The crosstalk between ubiquitination and ubiquitin-like modifications and post-translational modifications is a keystone to quickly update the activation state of many proteins responsible for the orchestration of cell metabolism. In this light, the correct activity of post-translational machinery is essential to prevent the development of cancer. Here we summarize the main post-translational modifications engaged in controlling the activity of the principal oncogenes and tumor suppressors genes involved in the development of most human cancers.

Post-translational modifications on the retinoblastoma protein

The retinoblastoma protein (pRb) functions as a cell cycle regulator controlling G1 to S phase transition and plays critical roles in tumour suppression. It is frequently inactivated in various tumours. The functions of pRb are tightly regulated, where post-translational modifications (PTMs) play crucial roles, including phosphorylation, ubiquitination, SUMOylation, acetylation and methylation. Most PTMs on pRb are reversible and can be detected in non-cancerous cells, playing an important role in cell cycle regulation, cell survival and differentiation. Conversely, altered PTMs on pRb can give rise to anomalies in cell proliferation and tumourigenesis. In this review, we first summarize recent findings pertinent to how individual PTMs impinge on pRb functions. As many of these PTMs on pRb were published as individual articles, we also provide insights on the coordination, either collaborations and/or competitions, of the same or different types of PTMs on pRb. Having a better understanding of how pRb is post-translationally modulated should pave the way for developing novel and specific therapeutic strategies to treat various human diseases.

Understanding Retinoblastoma Post-Translational Regulation for the Design of Targeted Cancer Therapies

Simple Summary

Rb1 is a regulator of cell cycle progression and genomic stability. This review focuses on post-translational modifications, their effect on Rb1 interactors, and their role in intracellular signaling in the context of cancer development. Finally, we highlight potential approaches to harness these post-translational modifications to design novel effective anticancer therapies.

Abstract

The retinoblastoma protein (Rb1) is a prototypical tumor suppressor protein whose role was described more than 40 years ago. Together with p107 (also known as RBL1) and p130 (also known as RBL2), the Rb1 belongs to a family of structurally and functionally similar proteins that inhibits cell cycle progression. Given the central role of Rb1 in regulating proliferation, its expression or function is altered in most types of cancer. One of the mechanisms underlying Rb-mediated cell cycle inhibition is the binding and repression of E2F transcription factors, and these processes are dependent on Rb1 phosphorylation status. However, recent work shows that Rb1 is a convergent point of many pathways and thus the regulation of its function through post-translational modifications is more complex than initially expected. Moreover, depending on the context, downstream signaling can be both E2F-dependent and -independent. This review seeks to summarize the most recent research on Rb1 function and regulation and discuss potential avenues for the design of novel cancer therapies.

Multiomics Analysis of Spatially Distinct Stromal Cells Reveals Tumor-Induced O-Glycosylation of the CDK4-pRB Axis in Fibroblasts at the Invasive Tumor Edge

The invasive leading edge represents a potential gateway for tumor metastasis. The role of fibroblasts from the tumor edge in promoting cancer invasion and metastasis has not been comprehensively elucidated. We hypothesize that cross-talk between tumor and stromal cells within the tumor microenvironment results in activation of key biological pathways depending on their position in the tumor (edge vs. core). Here we highlight phenotypic differences between tumor-adjacent-fibroblasts (TAF) from the invasive edge and tumor core fibroblasts from the tumor core, established from human lung adenocarcinomas. A multiomics approach that includes genomics, proteomics, and O-glycoproteomics was used to characterize cross-talk between TAFs and cancer cells. These analyses showed that O-glycosylation, an essential posttranslational modification resulting from sugar metabolism, alters key biological pathways including the cyclin-dependent kinase 4 (CDK4) and phosphorylated retinoblastoma protein axis in the stroma and indirectly modulates proinvasive features of cancer cells. In summary, the O-glycoproteome represents a new consideration for important biological processes involved in tumor-stroma cross-talk and a potential avenue to improve the anticancer efficacy of CDK4 inhibitors.

SIGNIFICANCE

A multiomics analysis of spatially distinct fibroblasts establishes the importance of the stromal O-glycoproteome in tumor-stroma interactions at the leading edge and provides potential strategies to improve cancer treatment. See related commentary by De Wever, p. 537.

Protein phosphatase 1 in tumorigenesis: is it worth a closer look?

Cancer cells take advantage of signaling cascades to meet their requirements for sustained growth and survival. Cell signaling is tightly controlled by reversible protein phosphorylation mechanisms, which require the counterbalanced action of protein kinases and protein phosphatases. Imbalances on this system are associated with cancer development and progression. Protein phosphatase 1 (PP1) is one of the most relevant protein phosphatases in eukaryotic cells. Despite the widely recognized involvement of PP1 in key biological processes, both in health and disease, its relevance in cancer has been largely neglected. Here, we provide compelling evidence that support major roles for PP1 in tumorigenesis.

Fadraciclib (CYC065), a novel CDK inhibitor, targets key pro-survival and oncogenic pathways in cancer

Cyclin-dependent kinases (CDKs) contribute to the cancer hallmarks of uncontrolled proliferation and increased survival. As a result, over the last two decades substantial efforts have been directed towards identification and development of pharmaceutical CDK inhibitors. Insights into the biological consequences of CDK inhibition in specific tumor types have led to the successful development of CDK4/6 inhibitors as treatments for certain types of breast cancer. More recently, a new generation of pharmaceutical inhibitors of CDK enzymes that regulate the transcription of key oncogenic and pro-survival proteins, including CDK9, have entered clinical development. Here, we provide the first disclosure of the chemical structure of fadraciclib (CYC065), a CDK inhibitor and clinical candidate designed by further optimization from the aminopurine scaffold of seliciclib. We describe its synthesis and mechanistic characterization. Fadraciclib exhibits improved potency and selectivity for CDK2 and CDK9 compared to seliciclib, and also displays high selectivity across the kinome. We show that the mechanism of action of fadraciclib is consistent with potent inhibition of CDK9-mediated transcription, decreasing levels of RNA polymerase II C-terminal domain serine 2 phosphorylation, the pro-survival protein Myeloid Cell Leukemia 1 (MCL1) and MYC oncoprotein, and inducing rapid apoptosis in cancer cells. This cellular potency and mechanism of action translate to promising anti-cancer activity in human leukemia mouse xenograft models. Studies of leukemia cell line sensitivity identify mixed lineage leukemia (MLL) gene status and the level of B-cell lymphoma 2 (BCL2) family proteins as potential markers for selection of patients with greater sensitivity to fadraciclib. We show that the combination of fadraciclib with BCL2 inhibitors, including venetoclax, is synergistic in leukemic cell models, as predicted from simultaneous inhibition of MCL1 and BCL2 pro-survival pathways. Fadraciclib preclinical pharmacology data support its therapeutic potential in CDK9- or CDK2-dependent cancers and as a rational combination with BCL2 inhibitors in hematological malignancies. Fadraciclib is currently in Phase 1 clinical studies in patients with advanced solid tumors (NCT02552953) and also in combination with venetoclax in patients with relapsed or refractory chronic lymphocytic leukemia (CLL) (NCT03739554) and relapsed refractory acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) (NCT04017546).

Translating RB1 predictive value in clinical cancer therapy: Are we there yet?

The retinoblastoma RB1 gene has been identified in the 80s as the first tumor suppressor. RB1 loss of function, as well alterations in its pathway, occur in most human cancers and often have prognostic value. RB1 has a key role in restraining cell cycle entry and, along with its family members, regulates a myriad of cellular processes and affects cell response to a variety of stimuli, ultimately determining cell fate. Consistently, RB1 status is a crucial determinant of the cell response to antitumoral therapies, impacting on the outcome of both traditional and modern anti-cancer strategies, including precision medicine approaches, such as kinase inhibitors, and immunotherapy. Despite many efforts however, the predictive value of RB1 status in the clinical practice is still underused, mainly owing to the complexity of RB1 function, to differences depending on the cellular context and on the therapeutic strategies, and, not-lastly, to technical issues. Here, we provide an overview of studies analyzing the role of RB1 in response to conventional cytotoxic and cytostatic therapeutic agents in different cancer types, including hormone dependent ones. We also review RB1 predictive value in the response to the last generation CDK4/6 inhibitors, other kinase inhibitors, and immunotherapy and discuss new emerging non-canonical roles of RB1 that could impact on the response to antitumoral treatments.

A Code of Mono-phosphorylation Modulates the Function of RB

Hyper-phosphorylation of RB controls its interaction with E2F and inhibits its tumor suppressor properties. However, during G1 active RB can be mono-phosphorylated on any one of 14 CDK phosphorylation sites. Here, we used quantitative proteomics to profile protein complexes formed by each mono-phosphorylated RB isoform (mP-RB) and identified the associated transcriptional outputs. The results show that the 14 sites of mono-phosphorylation co-ordinate RB's interactions and confer functional specificity. All 14 mP-RBs interact with E2F/DP proteins, but they provide different shades of E2F regulation. RB mono-phosphorylation at S811, for example, alters RB transcriptional activity by promoting its association with NuRD complexes. The greatest functional differences between mP-RBs are evident beyond the cell cycle machinery. RB mono-phosphorylation at S811 or T826 stimulates the expression of oxidative phosphorylation genes, increasing cellular oxygen consumption. These results indicate that RB activation signals are integrated in a phosphorylation code that determines the diversity of RB activity.

Hyper-phosphorylation of Rb S249 together with CDK5R2/p39 overexpression are associated with impaired cell adhesion and epithelial-to-mesenchymal transition: Implications as a potential lung cancer grading and staging biomarker

Prediction of lung cancer metastasis relies on post-resection assessment of tumor histology, which is a severe limitation since only a minority of lung cancer patients are diagnosed with resectable disease. Therefore, characterization of metastasis-predicting biomarkers in pre-resection small biopsy specimens is urgently needed. Here we report a biomarker consisting of the phosphorylation of the retinoblastoma protein (Rb) on serine 249 combined with elevated p39 expression. This biomarker correlates with epithelial-to-mesenchymal transition traits in non-small cell lung carcinoma (NSCLC) cells. Immunohistochemistry staining of NSCLC tumor microarrays showed that strong phospho-Rb S249 staining positively correlated with tumor grade specifically in the squamous cell carcinoma (SCC) subtype. Strong immunoreactivity for p39 positively correlated with tumor stage, lymph node invasion, and distant metastases, also in SCC. Linear regression analyses showed that the combined scoring for phospho-Rb S249, p39 and E-cadherin in SCC is even more accurate at predicting tumor staging, relative to each score individually. We propose that combined immunohistochemistry staining of NSCLC samples for Rb phosphorylation on S249, p39, and E-cadherin protein expression could aid in the assessment of tumor staging and metastatic potential when tested in small primary tumor biopsies. The intense staining for phospho-Rb S249 that we observed in high grade SCC could also aid in the precise sub-classification of poorly differentiated SCCs.

A System-wide Approach to Monitor Responses to Synergistic BRAF and EGFR Inhibition in Colorectal Cancer Cells

Intrinsic and/or acquired resistance represents one of the great challenges in targeted cancer therapy. A deeper understanding of the molecular biology of cancer has resulted in more efficient strategies, where one or multiple drugs are adopted in novel therapies to tackle resistance. This beneficial effect of using combination treatments has also been observed in colorectal cancer patients harboring the BRAF(V600E) mutation, whereby dual inhibition of BRAF(V600E) and EGFR increases antitumor activity. Notwithstanding this success, it is not clear whether this combination treatment is the only or most effective treatment to block intrinsic resistance to BRAF inhibitors. Here, we investigate molecular responses upon single and multi-target treatments, over time, using BRAF(V600E) mutant colorectal cancer cells as a model system. Through integration of transcriptomic, proteomic and phosphoproteomics data we obtain a comprehensive overview, revealing both known and novel responses. We primarily observe widespread up-regulation of receptor tyrosine kinases and metabolic pathways upon BRAF inhibition. These findings point to mechanisms by which the drug-treated cells switch energy sources and enter a quiescent-like state as a defensive response, while additionally compensating for the MAPK pathway inhibition.

Detection of Retinoblastoma Protein Phosphorylation by Immunoblot Analysis

The retinoblastoma tumor suppressor protein (pRb) is a preeminent tumor suppressor that acts as a cell cycle repressor, specifically as an inhibitor of the G1-S transition of the cell cycle . pRb is a phosphoprotein whose function is repressed by extensive phosphorylation in several key residues, and therefore, pRb's phosphorylation status has become a surrogate for pRb activity. In particular, hyperphosphorylation of pRb has been associated with pathological states such as cancer, and therefore, assessing pRb's phosphorylation status is increasingly gaining diagnostic and prognostic value, may be used to inform therapeutic decisions, and is also an important tool for the cancer biologists seeking an understanding of the molecular etiology of cancer. In this chapter, we discuss an immunoblot protocol to detect pRb phosphorylation in two residues, serine 612 and threonine 821, in protein extracts from cancer cells.

Dephosphorylation of the Retinoblastoma protein (Rb) inhibits cancer cell EMT via Zeb

The tumor suppressor Retinoblastoma (Rb) protein is highly phosphorylated in cancer cells largely due to the overexpression of cyclins or the loss of expression of cyclin dependent kinase inhibitors (cdki). Hyperphosphorylation of Rb promotes proliferation, and plays a role in the regulation of apoptosis. Recently, inhibition of cyclin dependent activity toward Rb has been identified as a strategy that has shown clinical efficacy. We utilized a method to induce phosphatase activity toward Rb in cells by shRNA silencing of PNUTS (Phosphatase Nuclear Targeting Subunit) that regulates PP1-mediated dephosphorylation of Rb. In this study, the effect of Rb dephosphorylation on the epithelial to mesenchymal transition (EMT) was determined. The EMT transition is observed in cancer cells that have acquired invasive characteristics. In breast cancer cells grown in 3D Matrigel cultures, MCF7 cells undergo apoptosis in response to Rb dephosphorylation, whereas MDA-MB-231 and Hs578T cells exhibit a reduction in the EMT. Cells devoid of phosphorylated Rb (nontransformed MCF10A and Rb-null MDA-MB-468) lacked any response to PNUTS depletion, showing the effect is Rb-dependent. In addition, these studies showed that Rb dephosphorylation in 3D Matrigel cultures of highly invasive HT1080 cells led to the inhibition of the EMT. Furthermore we observed association between dephosphorylated Rb with ZEB1, a zinc-finger E-box-binding transcription factor that regulates expression of E- and N-cadherins. Finally Rb dephosphorylation led to inhibition of ZEB1 transcriptional activity, this data supports the notion that Rb dephosphorylation modulates the EMT. These studies suggest targeting Rb phosphorylation in mesenchymal cancer cells may decrease invasiveness.

RB1 dual role in proliferation and apoptosis: cell fate control and implications for cancer therapy

Inactivation of the retinoblastoma (RB1) tumor suppressor is one of the most frequent and early recognized molecular hallmarks of cancer. RB1, although mainly studied for its role in the regulation of cell cycle, emerged as a key regulator of many biological processes. Among these, RB1 has been implicated in the regulation of apoptosis, the alteration of which underlies both cancer development and resistance to therapy. RB1 role in apoptosis, however, is still controversial because, depending on the context, the apoptotic cues, and its own status, RB1 can act either by inhibiting or promoting apoptosis. Moreover, the mechanisms whereby RB1 controls both proliferation and apoptosis in a coordinated manner are only now beginning to be unraveled. Here, by reviewing the main studies assessing the effect of RB1 status and modulation on these processes, we provide an overview of the possible underlying molecular mechanisms whereby RB1, and its family members, dictate cell fate in various contexts. We also describe the current antitumoral strategies aimed at the use of RB1 as predictive, prognostic and therapeutic target in cancer. A thorough understanding of RB1 function in controlling cell fate determination is crucial for a successful translation of RB1 status assessment in the clinical setting.

Phosphorylation of the Retinoblastoma protein (Rb) on serine-807 is required for association with Bax

The recent finding that the Retinoblastoma protein (Rb) is able to regulate apoptosis in a non-transcriptional manner directly at the mitochondria by interaction with the pro-apoptotic protein Bax prompted this investigation of the complex formed between Rb and Bax. Because the function of Rb in the cellular processes of proliferation, apoptosis, senescence and differentiation is regulated by phosphorylation we endeavored to elucidate the phosphorylation status of Rb with respect to its association with Bax and its role in apoptosis. In this study we found that Rb phosphorylated on at least 4 C-terminal phosphorylation sites (S608, S795, S807/S811, and T821) is present at the mitochondria under non-stressed cellular conditions. An in vitro binding assay showed that Bax binds to Rb phosphorylated at S807/S811 in 3 cancer cell types. Physiologically relevant association between Bax and Rb phosphorylated on S807/S811 was demonstrated by reciprocal co-immunoprecipitation experiments using antibodies specific for Rb phosphorylated on S807/S811 and Bax. Mutant Rb proteins expressed in Rb-null C33A cells showed that phosphorylation of S807 of Rb promotes association with Bax and that mimicking phosphorylation at S807 of Rb can block the induction of apoptosis due to PNUTS downregulation. Finally using siRNA to activate phosphatase activity in MCF7 cells, Rb is dephosphorylated at several sites including S807/S811, dissociates from Bax and apoptosis is triggered. These studies show that phosphorylation of Rb regulates its association with Bax and its role in apoptosis.

The retinoblastoma tumor suppressor modulates DNA repair and radioresponsiveness

PURPOSE

Perturbations in the retinoblastoma pathway are over-represented in advanced prostate cancer; retinoblastoma loss promotes bypass of first-line hormone therapy. Conversely, preliminary studies suggested that retinoblastoma-deficient tumors may become sensitized to a subset of DNA-damaging agents. Here, the molecular and in vivo consequence of retinoblastoma status was analyzed in models of clinical relevance.

EXPERIMENTAL DESIGN

Experimental work was performed with multiple isogenic prostate cancer cell lines (hormone sensitive: LNCaP and LAPC4 cells and hormone resistant C42, 22Rv1 cells; stable knockdown of retinoblastoma using shRNA). Multiple mechanisms were interrogated including cell cycle, apoptosis, and DNA damage repair. Transcriptome analysis was performed, validated, and mechanisms discerned. Cell survival was measured using clonogenic cell survival assay and in vivo analysis was performed in nude mice with human derived tumor xenografts.

RESULTS

Loss of retinoblastoma enhanced the radioresponsiveness of both hormone-sensitive and castrate-resistant prostate cancer. Hypersensitivity to ionizing radiation was not mediated by cell cycle or p53. Retinoblastoma loss led to alteration in DNA damage repair and activation of the NF-κB pathway and subsequent cellular apoptosis through PLK3. In vivo xenografts of retinoblastoma-deficient tumors exhibited diminished tumor mass, lower PSA kinetics, and decreased tumor growth after treatment with ionizing radiation (P < 0.05).

CONCLUSIONS

Loss of retinoblastoma confers increased radiosensitivity in prostate cancer. This hypersensitization was mediated by alterations in apoptotic signaling. Combined, these not only provide insight into the molecular consequence of retinoblastoma loss, but also credential retinoblastoma status as a putative biomarker for predicting response to radiotherapy.

Inhibition of CK2 enhances UV-triggered apoptotic cell death in lung cancer cell lines

Lung cancer is a high-grade malignancy with poor 5 year-survival rates that remains incurable with current therapies. Different cellular stresses, including antitumor agents, ionizing radiation and ultraviolet (UV) light, can induce apoptosis and activate signaling pathways. UV has multiple effects on tumor cells, including DNA damage, and increases the expression of some genes involved in tumor cell apoptosis and DNA repair. It has been reported that UV can also activate casein kinase 2 (CK2). CK2, a Ser/Thr protein kinase, has been reported to be frequently overexpressed in various types of human cancer, including lung cancer, and is associated with tumor development. Thus, combination of UV and CK2 inhibitors may be a new strategy for the treatment of lung cancer. Our results demonstrated that inhibition of CK2a through CK2 siRNA or a CK2 inhibitor [(4,5,6,7-tetrabromobenzotriazole (TBB)] enhances the decrease in cell viability of lung cancer cells (A549 and H2030) induced by UV. Western blot analysis demonstrated that the combination increased the expression of apoptotic protein markers cytochrome c and the cleavage of poly ADP-ribose polymerase (PARP) and caspase-3. Furthermore, our results indicated that UV decreased the expression of the tumor suppressor protein PML through activation of CK2. Inhibition of CK2 by CK2 siRNA and TBB can recover the reduction of PML induced by UV. Collectively, these results demonstrate the significant apoptosis of lung cancer cells induced by combination treatment of the CK2 inhibitor and UV radiation. CK2 enhanced cell apoptosis by UV radiation may due, at least partly, to recover the expression of PML. These findings warrant the clinical testing of CK2 inhibitors which, when used in conjunction with DNA-damaging agents such as radiation, may be an effective cancer therapeutic strategy.

Deciphering the retinoblastoma protein phosphorylation code

Multisite phosphorylation modulates the function of regulatory proteins with complex signaling properties and outputs. The retinoblastoma tumor suppressor protein (Rb) is inactivated by cyclin-dependent kinase (Cdk) phosphorylation in normal and cancer cell cycles, so understanding the molecular mechanisms and effects of Rb phosphorylation is imperative. Rb functions in diverse processes regulating proliferation, and it has been speculated that multisite phosphorylation might act as a code in which discrete phosphorylations control specific activities. The idea of an Rb phosphorylation code is evaluated here in light of recent studies of Rb structure and function. Rb inactivation is discussed with an emphasis on how multisite phosphorylation changes Rb structure and associations with protein partners.