The RB tumor suppressor: a gatekeeper to hormone independence in prostate cancer?

Randomized Phase II Multicenter Trial of Abiraterone Acetate With or Without Cabazitaxel in the Treatment of Metastatic Castration-Resistant Prostate Cancer

PURPOSE

Improving clinical outcomes with novel drug combinations to treat metastatic castration-resistant prostate cancer (mCRPC) is challenging. Preclinical studies showed cabazitaxel had superior antitumor efficacy compared with docetaxel. Gene expression profiling revealed divergent effects of these taxanes in cycling cells. mCRPC are RB deficient rendering them hypersensitive to taxanes. These data suggested that upfront treatment with cabazitaxel with abiraterone may affect therapeutic response. We designed a phase II randomized noncomparative trial of abiraterone acetate/prednisone (AAP) or AAP and cabazitaxel (AAP + C) in men with mCRPC to address this hypothesis.

METHODS

This trial of 81 men with mCRPC determined the radiographic progression-free survival (rPFS), prostate-specific antigen (PSA) progression-free survival, overall objective response, and safety of AAP or AAP + C. Equally allocated patients received AAP followed by switching to cabazitaxel upon radiographic progression (arm 1) or upfront with AAP + C (arm 2). Patients were stratified into high-/low-risk groups by the Halabi nomogram. Real-time assessment of RB status and circulating tumor cell (CTC) analysis to correlate with clinical outcomes was exploratory.

RESULTS

Both treatment arms were well-tolerated. Median rPFS in AAP was 6.4 months (95% CI, 3.8 to 10.6) and median overall survival (OS) 18.3 months (95% CI, 14.4 to 37.6), respectively. Fifty-six percent of patients showed ≥50% decline in PSA. Median rPFS in AAP + C was 14.8 months (95% CI, 10.6 to 16.4), and median OS 24.5 months (95% CI, 20.4 to 35.0). There was a ≥50% decline in PSA in 92.1% of men. Neither RB expression in pretherapy tumor biopsy, CTC, or tissue explants identified those who may benefit from AAP + C.

CONCLUSION

AAP + C was safe with improved rPFS, OS duration, and a higher proportion of PSA declines. This suggests that AAP + C given earlier rather than sequentially may benefit some men. Further work is needed to identify this population.

Looking at Thyroid Cancer from the Tumor-Suppressor Genes Point of View

Thyroid cancer is the most frequent endocrine malignancy and accounts for approximately 1% of all diagnosed cancers. A variety of mechanisms are involved in the transformation of a normal tissue into a malignant one. Loss of tumor-suppressor gene (TSG) function is one of these mechanisms. The normal functions of TSGs include cell proliferation and differentiation control, genomic integrity maintenance, DNA damage repair, and signaling pathway regulation. TSGs are generally classified into three subclasses: (i) gatekeepers that encode proteins involved in cell cycle and apoptosis control; (ii) caretakers that produce proteins implicated in the genomic stability maintenance; and (iii) landscapers that, when mutated, create a suitable environment for malignant cell growth. Several possible mechanisms have been implicated in TSG inactivation. Reviewing the various TSG alteration types detected in thyroid cancers may help researchers to better understand the TSG defects implicated in the development/progression of this cancer type and to find potential targets for prognostic, predictive, diagnostic, and therapeutic purposes. Hence, the main purposes of this review article are to describe the various TSG inactivation mechanisms and alterations in human thyroid cancer, and the current therapeutic options for targeting TSGs in thyroid cancer.

Novel insights in cell cycle dysregulation during prostate cancer progression

Prostate cancer (CaP) remains the second leading cause of cancer deaths in Western men. These deaths occur because metastatic CaP acquires resistance to available treatments. The novel and functionally diverse treatment options that have been introduced in the clinic over the past decade each eventually induce resistance for which the molecular basis is diverse. Both initiation and progression of CaP have been associated with enhanced cell proliferation and cell cycle dysregulation. A better understanding of the specific pro-proliferative molecular shifts that control cell division and proliferation during CaP progression may ultimately overcome treatment resistance. Here, we examine literature for support of this possibility. We start by reviewing recently renewed insights in prostate cell types and their proliferative and oncogenic potential. We then provide an overview of the basic knowledge on the molecular machinery in charge of cell cycle progression and its regulation by well-recognized drivers of CaP progression such as androgen receptor and retinoblastoma protein. In this respect, we pay particular attention to interactions and reciprocal interplay between cell cycle regulators and androgen receptor. Somatic alterations that impact the cell cycle-associated and -regulated genes encoding p53, PTEN and MYC during progression from treatment-naïve, to castration-recurrent, and in some cases, neuroendocrine CaP are discussed. We considered also non-genomic events that impact cell cycle determinants, including transcriptional, epigenetic and micro-environmental switches that occur during CaP progression. Finally, we evaluate the therapeutic potential of cell cycle regulators and address challenges and limitations in the approaches modulating their action for CaP treatment.

The SOX4/miR-17-92/RB1 Axis Promotes Prostate Cancer Progression

Although androgen-deprivation treatment (ADT) is the main treatment for advanced prostate cancer (PCa), it eventually fails. This failure invariably leads to castration-resistant prostate cancer (CRPC) and the development of the neuroendocrine (NE) phenotype. The molecular basis for PCa progression remains unclear. Previously, we and others have demonstrated that the sex-determining region Y-box 4 (SOX4) gene, a critical developmental transcription factor, is overexpressed and associated with poor prognosis in PCa patients. In this study, we show that SOX4 expression is associated with PCa progression and the development of the NE phenotype in androgen deprivation conditions. High-throughput microRNA profiling and bioinformatics analyses suggest that SOX4 may target the miR-17-92 cluster. SOX4 transcriptionally upregulates miR-17-92 cluster expression in PCa cells. SOX4-induced PCa cell proliferation, migration, and invasion are also mediated by miR-17-92 cluster members. Furthermore, RB1 is a target gene of miR-17-92 cluster. We found that SOX4 downregulates RB1 protein expression by upregulating the miR-17-92 expression. In addition, SOX4-knockdown restrains NE phenotype and PCa cell proliferation. Clinically, the overexpression of miR-17-92 members is shown to be positively correlated with SOX4 expression in PCa patients, whereas RB1 expression is negatively correlated with SOX4 expression in patients with the aggressive PCa phenotype. Collectively, we propose a novel model of a SOX4/miR-17-92/RB1 axis that may exist to promote PCa progression.

Novel RB1-Loss Transcriptomic Signature Is Associated with Poor Clinical Outcomes across Cancer Types

PURPOSE

Rb-pathway disruption is of great clinical interest, as it has been shown to predict outcomes in multiple cancers. We sought to develop a transcriptomic signature for detecting biallelic RB1 loss (RBS) that could be used to assess the clinical implications of RB1 loss on a pan-cancer scale.

EXPERIMENTAL DESIGN

We utilized data from the Cancer Cell Line Encyclopedia (N = 995) to develop the first pan-cancer transcriptomic signature for predicting biallelic RB1 loss (RBS). Model accuracy was validated using The Cancer Genome Atlas (TCGA) Pan-Cancer dataset (N = 11,007). RBS was then used to assess the clinical relevance of biallelic RB1 loss in TCGA Pan-Cancer and in an additional metastatic castration-resistant prostate cancer (mCRPC) cohort.

RESULTS

RBS outperformed the leading existing signature for detecting RB1 biallelic loss across all cancer types in TCGA Pan-Cancer (AUC, 0.89 vs. 0.66). High RBS (RB1 biallelic loss) was associated with promoter hypermethylation (P = 0.008) and gene body hypomethylation (P = 0.002), suggesting RBS could detect epigenetic gene silencing. TCGA Pan-Cancer clinical analyses revealed that high RBS was associated with short progression-free (P < 0.00001), overall (P = 0.0004), and disease-specific (P < 0.00001) survival. On multivariable analyses, high RBS was predictive of shorter progression-free survival in TCGA Pan-Cancer (P = 0.03) and of shorter overall survival in mCRPC (P = 0.004) independently of the number of DNA alterations in RB1.

CONCLUSIONS

Our study provides the first validated tool to assess RB1 biallelic loss across cancer types based on gene expression. RBS can be useful for analyzing datasets with or without DNA-sequencing results to investigate the emerging prognostic and treatment implications of Rb-pathway disruption.See related commentary by Choudhury and Beltran, p. 4199.

The retinoblastoma protein regulates hypoxia-inducible genetic programs, tumor cell invasiveness and neuroendocrine differentiation in prostate cancer cells

Loss of tumor suppressor proteins, such as the retinoblastoma protein (Rb), results in tumor progression and metastasis. Metastasis is facilitated by low oxygen availability within the tumor that is detected by hypoxia inducible factors (HIFs). The HIF1 complex, HIF1α and dimerization partner the aryl hydrocarbon receptor nuclear translocator (ARNT), is the master regulator of the hypoxic response. Previously, we demonstrated that Rb represses the transcriptional response to hypoxia by virtue of its association with HIF1. In this report, we further characterized the role Rb plays in mediating hypoxia-regulated genetic programs by stably ablating Rb expression with retrovirally-introduced short hairpin RNA in LNCaP and 22Rv1 human prostate cancer cells. DNA microarray analysis revealed that loss of Rb in conjunction with hypoxia leads to aberrant expression of hypoxia-regulated genetic programs that increase cell invasion and promote neuroendocrine differentiation. For the first time, we have established a direct link between hypoxic tumor environments, Rb inactivation and progression to late stage metastatic neuroendocrine prostate cancer. Understanding the molecular pathways responsible for progression of benign prostate tumors to metastasized and lethal forms will aid in the development of more effective prostate cancer therapies.

Differential blood-based diagnosis between benign prostatic hyperplasia and prostate cancer: miRNA as source for biomarkers independent of PSA level, Gleason score, or TNM status

Since the benefit of prostate-specific antigen (PSA) screening remains controversial, new non-invasive biomarkers for prostate carcinoma (PCa) are still required. There is evidence that microRNAs (miRNAs) in whole peripheral blood can separate patients with localized prostate cancer from healthy individuals. However, the potential of blood-based miRNAs for the differential diagnosis of PCa and benign prostatic hyperplasia (BPH) has not been tested. We compared the miRNome from blood of PCa and BPH patients and further investigated the influence of the tumor volume, tumor-node-metastasis (TNM) classification, Gleason score, pretreatment risk status, and the pretreatment PSA value on the miRNA pattern. By microarray approach, we identified seven miRNAs that were significantly deregulated in PCa patients compared to BPH patients. Using quantitative real time PCR (qRT-PCR), we confirmed downregulation of hsa-miR-221* (now hsa-miR-221-5p) and hsa-miR-708* (now hsa-miR-708-3p) in PCa compared to BPH. Clinical parameters like PSA level, Gleason score, or TNM status seem to have only limited impact on the overall abundance of miRNAs in patients' blood, suggesting a no influence of these factors on the expression of deregulated miRNAs.

Preclinical Characterization of G1T28: A Novel CDK4/6 Inhibitor for Reduction of Chemotherapy-Induced Myelosuppression

Chemotherapy-induced myelosuppression continues to represent the major dose-limiting toxicity of cytotoxic chemotherapy, which can be manifested as neutropenia, lymphopenia, anemia, and thrombocytopenia. As such, myelosuppression is the source of many of the adverse side effects of cancer treatment including infection, sepsis, bleeding, and fatigue, thus resulting in the need for hospitalizations, hematopoietic growth factor support, and transfusions (red blood cells and/or platelets). Moreover, clinical concerns raised by myelosuppression commonly lead to chemotherapy dose reductions, therefore limiting therapeutic dose intensity, and reducing the antitumor effectiveness of the treatment. Currently, the only course of treatment for myelosuppression is growth factor support which is suboptimal. These treatments are lineage specific, do not protect the bone marrow from the chemotherapy-inducing cytotoxic effects, and the safety and toxicity of each agent is extremely specific. Here, we describe the preclinical development of G1T28, a novel potent and selective CDK4/6 inhibitor that transiently and reversibly regulates the proliferation of murine and canine bone marrow hematopoietic stem and progenitor cells and provides multilineage protection from the hematologic toxicity of chemotherapy. Furthermore, G1T28 does not decrease the efficacy of cytotoxic chemotherapy on RB1-deficient tumors. G1T28 is currently in clinical development for the reduction of chemotherapy-induced myelosuppression in first- and second-line treatment of small-cell lung cancer. Mol Cancer Ther; 15(5); 783-93. ©2016 AACR.

Molecular pathways and targets in prostate cancer

Prostate cancer co-opts a unique set of cellular pathways in its initiation and progression. The heterogeneity of prostate cancers is evident at earlier stages, and has led to rigorous efforts to stratify the localized prostate cancers, so that progression to advanced stages could be predicted based upon salient features of the early disease. The deregulated androgen receptor signaling is undeniably most important in the progression of the majority of prostate tumors. It is perhaps because of the primacy of the androgen receptor governed transcriptional program in prostate epithelium cells that once this program is corrupted, the consequences of the ensuing changes in activity are pleotropic and could contribute to malignancy in multiple ways. Following localized surgical and radiation therapies, 20-40% of patients will relapse and progress, and will be treated with androgen deprivation therapies. The successful development of the new agents that inhibit androgen signaling has changed the progression free survival in hormone resistant disease, but this has not changed the almost ubiquitous development of truly resistant phenotypes in advanced prostate cancer. This review summarizes the current understanding of the molecular pathways involved in localized and metastatic prostate cancer, with an emphasis on the clinical implications of the new knowledge.

Mass-array screening of frequent mutations in cancers reveals RB1 alterations in aggressive adrenocortical carcinomas

CONTEXT

Adrenocortical carcinoma (ACC) is a rare disease with a poor overall outcome. Transcriptome analysis identified two groups of ACCs with different prognosis. In aggressive ACCs, somatic mutations of the tumor suppressor gene TP53 and the proto-oncogene β-catenin are detected in 50% of cases. For the remaining aggressive ACCs and for the group with a better prognosis, molecular alterations are unknown.

OBJECTIVE

To identify new molecular actors driving adrenal tumorigenesis.

EXPERIMENTAL DESIGN

Analysis by mass array of 374 mutations among 32 common oncogenes or tumor suppressor genes was performed on the tumoral DNA of 26 ACCs, using Sequenom OncoCarta Panels.

RESULTS

Four mutations were identified, two previously known β-catenin mutations and one alteration in two other genes: JAK3 and retinoblastoma gene (RB1). The JAK3 alteration was found in leukocyte DNA and therefore considered as a polymorphism and not a somatic event. The full RB1 tumor suppressor gene was subsequently sequenced in a cohort of 49 ACCs (26 ACCs from the 'OncoCarta cohort' and 23 other ACCs): three somatic mutations were identified, all in the poor-outcome ACC group. By immunohistochemistry, a loss of the retinoblastoma protein (pRb) was found exclusively in aggressive ACCs in 27% of cases (seven out of 26), three of them with an inactivating RB1 mutation. Among the seven pRb-negative ACCs, five had an allele loss at the RB1 locus.

CONCLUSIONS

Parallel analysis of somatic mutations among known cancer genes allowed us to identify RB1 as a new actor in aggressive ACCs. These results suggest a prognostic significance of pRb expression loss in ACCs.

Molecular alterations and emerging targets in castration resistant prostate cancer

Prostate cancer is the most common malignancy in Western Europe, of which approximately 10-20% presents with advanced or metastatic disease. Initial response with androgen deprivation therapy is almost universal, but progression to castration resistant prostate cancer (CRPC), an incurable disease, occurs in approximately 2-3 years. In recent years, the novel taxane cabazitaxel, the hormonal agents abiraterone and enzalutamide, the immunotherapeutic agent sipuleucel-T and the radiopharmaceutical radium-223 have been shown to prolong survival in large randomised trials, thus widely increasing the therapeutic armamentarium against the disease. Despite these advances, the median survival in the first-line setting of metastatic castration-resistant prostate cancer (mCRPC) is still up to 25 months and in the post-docetaxel setting is about 15-18 months. There is an urgent need for the development of biomarkers of treatment response, and for a deeper understanding of tumour heterogeneity and the molecular biology underlying the disease. In this review, we attempt to provide insight into the novel molecular targets showing promise in clinical trials.

Mitochondrial dysfunction in cancer

A mechanistic understanding of how mitochondrial dysfunction contributes to cell growth and tumorigenesis is emerging beyond Warburg as an area of research that is under-explored in terms of its significance for clinical management of cancer. Work discussed in this review focuses less on the Warburg effect and more on mitochondria and how dysfunctional mitochondria modulate cell cycle, gene expression, metabolism, cell viability, and other established aspects of cell growth and stress responses. There is increasing evidence that key oncogenes and tumor suppressors modulate mitochondrial dynamics through important signaling pathways and that mitochondrial mass and function vary between tumors and individuals but the significance of these events for cancer are not fully appreciated. We explore the interplay between key molecules involved in mitochondrial fission and fusion and in apoptosis, as well as in mitophagy, biogenesis, and spatial dynamics of mitochondria and consider how these distinct mechanisms are coordinated in response to physiological stresses such as hypoxia and nutrient deprivation. Importantly, we examine how deregulation of these processes in cancer has knock on effects for cell proliferation and growth. We define major forms of mitochondrial dysfunction and address the extent to which the functional consequences of such dysfunction can be determined and exploited for cancer diagnosis and treatment.

Estimating the order of mutations during tumorigenesis from tumor genome sequencing data

MOTIVATION

Tumors are thought to develop and evolve through a sequence of genetic and epigenetic somatic alterations to progenitor cells. Early stages of human tumorigenesis are hidden from view. Here, we develop a method for inferring some aspects of the order of mutational events during tumorigenesis based on genome sequencing data for a set of tumors. This method does not assume that the sequence of driver alterations is the same for each tumor, but enables the degree of similarity or difference in the sequence to be evaluated.

RESULTS

To evaluate the new method, we applied it to colon cancer tumor sequencing data and the results are consistent with the multi-step tumorigenesis model previously developed based on comparing stages of cancer. We then applied the new method to DNA sequencing data for a set of lung cancers. The model may be a useful tool for better understanding the process of tumorigenesis.

AVAILABILITY

The software is available at: http://linus.nci.nih.gov/Data/YounA/OrderMutation.zip.

p53, Stem Cells, and Reprogramming: Tumor Suppression beyond Guarding the Genome

p53 is well recognized as a potent tumor suppressor. In its classic role, p53 responds to genotoxic insults by inducing cell cycle exit or programmed cell death to limit the propagation of cells with corrupted genomes. p53 is also implicated in a variety of other cellular processes in which its involvement is less well understood including self-renewal, differentiation, and reprogramming. These activities represent an emerging area of intense interest for cancer biologists, as they provide potential mechanistic links between p53 loss and the stem cell-like cellular plasticity that has been suggested to contribute to tumor cell heterogeneity and to drive tumor progression. Despite accumulating evidence linking p53 loss to stem-like phenotypes in cancer, it is not yet understood how p53 contributes to acquisition of "stemness" at the molecular level. Whether and how stem-like cells confer survival advantages to propagate the tumor also remain to be resolved. Furthermore, although it seems reasonable that the combination of p53 deficiency and the stem-like state could contribute to the genesis of cancers that are refractory to treatment, direct linkages and mechanistic underpinnings remain under investigation. Here, we discuss recent findings supporting the connection between p53 loss and the emergence of tumor cells bearing functional and molecular similarities to stem cells. We address several potential molecular and cellular mechanisms that may contribute to this link, and we discuss implications of these findings for the way we think about cancer progression.

Time to stratify? The retinoblastoma protein in castrate-resistant prostate cancer

It is generally held that the retinoblastoma (RB) tumor suppressor functions in multiple tissues to protect against tumor development. However, preclinical studies and analysis of tumor samples of early disease did not support an important role of RB loss in the origin of prostate cancer. By contrast, recent observations in the clinical setting and subsequent modeling of RB function indicate that the tumor suppressor has specialized roles in controlling androgen receptor expression in prostate cancer, and primarily functions to prevent progression to the castration-resistant stage of disease. Furthermore, preclinical models have now shown that loss of RB expression or functional activity decreases the effectiveness of hormone therapy, yet seems to increase sensitivity to a subset of chemotherapeutic agents. Here, the current state of knowledge regarding the implications of RB loss for prostate cancer progression will be reviewed, and potential opportunities for developing RB as a metric to predict therapeutic response will be considered.

Outsmarting androgen receptor: creative approaches for targeting aberrant androgen signaling in advanced prostate cancer

Prostatic adenocarcinomas are reliant on androgen receptor (AR) activity for survival and progression. Therefore, first-line therapeutic intervention for disseminated disease entails the use of AR-directed therapeutics, achieved through androgen deprivation and direct AR antagonists. While initially effective, recurrent, 'castrate-resistant' prostate cancers arise, for which there is no durable means of treatment. An abundance of clinical study and preclinical modeling has led to the revelation that restored AR activity is a major driver of therapeutic failure and castrate-resistant prostate cancer development. The mechanisms underpinning AR reactivation have been identified, providing the foundation for a new era of drug discovery and rapid translation into the clinic. As will be reviewed in this article, these creative new ways of suppressing AR show early promise.

Activation of NAG-1 via JNK signaling revealed an isochaihulactone-triggered cell death in human LNCaP prostate cancer cells

Background

We explored the mechanisms of cell death induced by isochaihulactone treatment in LNCaP cells.

Methods

LNCaP cells were treated with isochaihulactone and growth inhibition was assessed. Cell cycle profiles after isochaihulactone treatment were determined by flow cytometry. Expression levels of cell cycle regulatory proteins, caspase 9, caspase 3, and PARP were determined after isochaihulactone treatment. Signaling pathway was verified by inhibitors pre-treatment. Expression levels of early growth response gene 1 (EGR-1) and nonsteroidal anti-inflammatory drug-activated gene 1 (NAG-1) were determined to investigate their role in LNCaP cell death. NAG-1 expression was knocked down by si-NAG-1 siRNA transfection. Rate of cell death and proliferation were obtained by MTT assay.

Results

Isochaihulactone caused cell cycle arrest at G2/M phase in LNCaP cells, which was correlated with an increase of p53 and p21 levels and downregulation of the checkpoint proteins cdc25c, cyclin B1, and cdc2. Bcl-2 phosphorylation and caspase activation were also observed. Isochaihulactone induced phosphorylation of c-Jun-N-terminal kinase (JNK), and JNK inhibitor partially reduced isochaihulactone-induced cell death. Isochaihulactone also induced the expressions of EGR-1 and NAG-1. Expression of NAG-1 was reduced by JNK inhibitor, and knocking down of NAG-1 inhibited isochaihulactone-induced cell death.

Conclusions

Isochaihulactone apparently induces G2/M cell cycle arrest via downregulation of cyclin B1 and cdc2, and induces cellular death by upregulation of NAG-1 via JNK activation in LNCaP cells.

Epidemiology and Rb1 gene of retinoblastoma

Retinoblastoma (Rb) is the most common eye cancer in children and it can be inherited. Rb is quite rare and originators from the neural retina with a significant genetic component in etiology, which occurs in approximately 1 in every 20 0000 births. In children with the heritable genetic form of Rb, there is a mutation on chromosome 13, called the retinoblastoma 1 (Rb1) gene. Early diagnosis and intervention is critical to the successful treatment of the Rb. The Rb1 gene is the first cloned tumor suppressor gene. As a negative regulator of the cell cycle, Rb1 gene could maintain a balance between cell growth and development through binding to transcription factors and regulating the expression of genes involved in cell proliferation and differentiation. Thus, it is involved in cell cycle, cell senescence, growth arrest, apoptosis and differentiation. We summarized the recent advances on the epidemiology and Rb1 gene of Rb in this review.