Characterizing the Therapeutic Potential of a Potent BET Degrader in Merkel Cell Carcinoma

Merkel cell carcinoma: updates in tumor biology, emerging therapies, and preclinical models

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma thought to arise via either viral (Merkel cell polyomavirus) or ultraviolet-associated pathways. Surgery and radiotherapy have historically been mainstays of management, and immunotherapy has improved outcomes for advanced disease. However, there remains a lack of effective therapy for those patients who fail to respond to these established approaches, underscoring a critical need to better understand MCC biology for more effective prognosis and treatment. Here, we review the fundamental aspects of MCC biology and the recent advances which have had profound impact on management. The first genetically-engineered mouse models for MCC tumorigenesis provide opportunities to understand the potential MCC cell of origin and may prove useful for preclinical investigation of novel therapeutics. The MCC cell of origin debate has also been advanced by recent observations of MCC arising in association with a clonally related hair follicle tumor or squamous cell carcinoma in situ. These studies also suggested a role for epigenetics in the origin of MCC, highlighting a potential utility for this therapeutic avenue in MCC. These and other therapeutic targets form the basis for a wealth of ongoing clinical trials to improve MCC management. Here, we review these recent advances in the context of the existing literature and implications for future investigations.

Therapeutic Potential of 5'-Methylschweinfurthin G in Merkel Cell Polyomavirus-Positive Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare but aggressive form of skin cancer predominantly caused by the human Merkel cell polyomavirus (MCPyV). Treatment for MCC includes excision and radiotherapy of local disease, and chemotherapy or immunotherapy for metastatic disease. The schweinfurthin family of natural compounds previously displayed potent and selective growth inhibitory activity against the NCI-60 panel of human-derived cancer cell lines. Here, we investigated the impact of schweinfurthin on human MCC cell lines. Treatment with the schweinfurthin analog, 5'-methylschweinfurth G (MeSG also known as TTI-3114), impaired metabolic activity through induction of an apoptotic pathway. MeSG also selectively inhibited PI3K/AKT and MAPK/ERK pathways in the MCPyV-positive MCC cell line, MS-1. Interestingly, expression of the MCPyV small T (sT) oncogene selectively sensitizes mouse embryonic fibroblasts to MeSG. These results suggest that the schweinfurthin family of compounds display promising potential as a novel therapeutic option for virus-induced MCCs.

Viral Status Predicts the Patterns of Genome Methylation and Decitabine Response in Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma that is classified as Merkel cell polyomavirus-positive (virus positive [VP]) or Merkel cell polyomavirus-negative (virus negative [VN]). Epigenetic changes, such as DNA methylation, can alter gene expression and influence cancer progression. However, patterns of DNA methylation and the therapeutic efficacy of hypomethylating agents have not been fully explored in MCC. We characterized genome-wide DNA methylation in 16 MCC cell lines from both molecular subclasses in comparison with other cancer types and found that the overall profile of MCC is similar to that of small-cell lung carcinoma. Comparison of VP MCC with VN MCC revealed 2,260 differentially methylated positions. The hypomethylating agent decitabine upregulated the expression of antigen-presenting machinery in MCC cell lines and stimulated membrane expression of HLA-A in VP and VN MCC xenograft tumors. Decitabine also induced prominent caspase- and large T antigen‒independent cell death in VP MCC, whereas VN MCC cell lines displayed decreased proliferation without increased cell death. In mouse xenografts, decitabine significantly decreased the size of VP tumors but not that of VN tumors. Our findings indicate that viral status predicts genomic methylation patterns in MCC and that decitabine may be therapeutically effective against MCC through antiproliferative effects, cell death, and increased immune recognition.

Targeted Protein Degradation: An Emerging Therapeutic Strategy in Cancer

Drug discovery in the scope of cancer therapy has been focused on conventional agents that nonselectively induce DNA damage or selectively inhibit the activity of key oncogenic molecules without affecting their protein levels. An emerging therapeutic strategy that garnered attention in recent years is the induction of Targeted Protein Degradation (TPD) of cellular targets by hijacking the intracellular proteolysis machinery. This novel approach offers several advantages over conventional inhibitors and introduces a paradigm shift in several pharmacological aspects of drug therapy. While TPD has been found to be the major mode of action of clinically approved anticancer agents such as fulvestrant and thalidomide, recent years have witnessed systematic endeavors to expand the repertoire of proteins amenable to therapeutic ablation by TPD. Such endeavors have led to three major classes of agents that induce protein degradation, including molecular glues, Proteolysis Targeting Chimeras (PROTACs) and Hydrophobic Tag (HyT)-based degraders. Here, we briefly highlight agents in these classes and key advances made in the field with a focus on clinical translation in cancer therapy.

Targeting histone acetylation in pulmonary hypertension and right ventricular hypertrophy

Epigenetic mechanisms, including DNA methylation and histone post-translational modifications (PTMs), have been known to regulate chromatin structure and lineage-specific gene expression during cardiovascular development and disease. However, alterations in the landscape of histone PTMs and their contribution to the pathogenesis of incurable cardiovascular diseases such as pulmonary hypertension (PH) and associated right heart failure (RHF) remain largely unexplored. This review focusses on the studies in PH and RHF that investigated the gene families that write (histone acetyltransferases), read (bromodomain-containing proteins) or erase (histone deacetylases [HDACs] and sirtuins [SIRT]) acetyl moieties from the ε-amino group of lysine residues of histones and non-histone proteins. Analysis of cells and tissues isolated from the in vivo preclinical models of PH and human pulmonary arterial hypertension not only confirmed significant alterations in the expression levels of multiple HDACs, SIRT1, SIRT3 and BRD4 proteins but also demonstrated their strong association to proliferative, inflammatory and fibrotic phenotypes linked to the pathological vascular remodelling process. Due to the reversible nature of post-translational protein acetylation, the therapeutic efficacy of numerous small-molecule inhibitors (vorinostat, valproic acid, sodium butyrate, mocetinostat, entinostat, tubastatin A, apabetalone, JQ1 and resveratrol) have been evaluated in different preclinical models of cardiovascular disease, which revealed the promising therapeutic benefits of targeting histone acetylation pathways in the attenuation of cardiac hypertrophy, fibrosis, left heart dysfunction, PH and RHF. This review also emphasizes the need for deeper molecular insights into the contribution of epigenetic changes to PH pathogenesis and therapeutic evaluation of isoform-specific modulation in ex vivo and in vivo models of PH and RHF. LINKED ARTICLES: This article is part of a themed issue on Risk factors, comorbidities, and comedications in cardioprotection. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.1/issuetoc.

Targeting BET Proteins With a PROTAC Molecule Elicits Potent Anticancer Activity in HCC Cells

Background and Aim: Bromodomain and extraterminal domain (BET) family proteins are epigenetic regulators involved in human malignances. Targeting BET proteins for degradation using proteolysis-targeting chimera (PROTAC) recently has drawn increasing attention in the field of cancer therapeutics. BET proteins have been found to be overexpressed in HCC cells and tumor tissues. However, the biological activity of BET-PROTACs in hepatocellular carcinoma (HCC) remains unclear. In this study, we investigated anti-HCC activity of BETd-260, a BET-PROTAC molecule using in vitro and in vivo models. Methods: BETd-260-mediated anti-HCC activity was investigated by cell viability, apoptosis assays. Efficacy was examined with a cell lines-derived HCC xenograft model in mice. Anticancer mechanism was investigated by RT-PCR, western blotting and immunohistochemical staining. Results: BETd-260 potently suppressed cell viability and robustly induced apoptosis in HCC cells. BETd-260 reciprocally modulated the expression of several apoptotic genes in HCC cells, i.e., suppressing the expression of anti-apoptotic Mcl-1, Bcl-2, c-Myc, and X-linked inhibitor of apoptosis (XIAP), whereas increasing the expression of pro-apoptotic Bad. BETd-260 treatment led to disruption of mitochondrial membrane integrity, and triggered apoptosis via intrinsic signaling in HCC cells. BETd-260 triggered apoptosis in HCC xenograft tissue and profoundly inhibited the growth of HCC xenograft tumors in mice. Conclusion: Our data suggest that pharmacological targeting of BET for degradation may be a novel therapeutic strategy for the treatment of HCC.