Inhibition of PI3K by copanlisib exerts potent antitumor effects on Merkel cell carcinoma cell lines and mouse xenografts

Casein kinase 1α mediates phosphorylation of the Merkel cell polyomavirus large T antigen for β-TrCP destruction complex interaction and subsequent degradation

Merkel cell polyomavirus (MCPyV) is a double-stranded tumor virus that is the main causative agent of Merkel cell carcinoma (MCC). The MCPyV large T antigen (LT), an essential viral DNA replication protein, maintains viral persistence by interacting with host Skp1-Cullin 1-F-box (SCF) E3 ubiquitin ligase complexes, which subsequently induces LT's proteasomal degradation, restricting MCPyV DNA replication. SCF E3 ubiquitin ligases require their substrates to be phosphorylated to bind them, utilizing phosphorylated serine residues as docking sites. The MCPyV LT unique region (MUR) is highly phosphorylated and plays a role in multiple host protein interactions, including SCF E3 ubiquitin ligases. Therefore, this domain highly governs LT stability. Though much work has been conducted to identify host factors that restrict MCPyV LT protein expression, the kinase(s) that cooperates with the SCF E3 ligase remains unknown. Here, we demonstrate that casein kinase 1 alpha (CK1α) negatively regulates MCPyV LT stability and LT-mediated replication by modulating interactions with the SCF β-TrCP. Specifically, we show that numerous CK1 isoforms (α, δ, ε) localize in close proximity to MCPyV LT through in situ proximity ligation assays (PLA) and CK1α overexpression mainly resulted in decreased MCPyV LT protein expression. Inhibition of CK1α using short hairpin RNA (shRNA) and treatment of a CK1α inhibitor or an mTOR inhibitor, TORKinib, resulted in decreased β-TrCP interaction with LT, increased LT expression, and enhanced MCPyV replication. The expression level of the CSNK1A1 gene transcripts is higher in MCPyV-positive MCC, suggesting a vital role of CK1α in limiting MCPyV replication required for establishing persistent infection.

IMPORTANCE

Merkel cell polyomavirus (MCPyV) large tumor antigen is a polyphosphoprotein and the phosphorylation event is required to modulate various functions of LT, including viral replication. Therefore, cellular kinase pathways are indispensable for governing MCPyV polyomavirus infection and life cycle in coordinating with the immunosuppression environment at disease onset. Understanding the regulation mechanisms of MCPyV replication by viral and cellular factors will guide proper prevention strategies with targeted inhibitors for MCPyV-associated Merkel cell carcinoma (MCC) patients, who currently lack therapies.

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 Approaches for Non-Melanoma Skin Cancer: Standard of Care and Emerging Modalities

Skin cancer encompasses a range of cutaneous malignancies, with non-melanoma skin cancers (NMSCs) being the most common neoplasm worldwide. Skin exposure is the leading risk factor for initiating NMSC. Ultraviolet (UV) light induces various genomic aberrations in both tumor-promoting and tumor-suppressing genes in epidermal cells. In conjunction with interactions with a changed stromal microenvironment and local immune suppression, these aberrations contribute to the occurrence and expansion of cancerous lesions. Surgical excision is still the most common treatment for these lesions; however, locally advanced or metastatic disease significantly increases the chances of morbidity or death. In recent years, numerous pharmacological targets were found through extensive research on the pathogenic mechanisms of NMSCs, leading to the development of novel treatments including Hedgehog pathway inhibitors for advanced and metastatic basal cell carcinoma (BCC) and PD-1/PD-L1 inhibitors for locally advanced cutaneous squamous cell carcinoma (cSCC) and Merkel cell carcinoma (MCC). Despite the efficacy of these new drugs, drug resistance and tolerability issues often arise with long-term treatment. Ongoing studies aim to identify alternative strategies with reduced adverse effects and increased tolerability. This review summarizes the current and emerging therapies used to treat NMSC.

Targeted Therapy in Mesotheliomas: Uphill All the Way

Mesothelioma (MM) is an aggressive and lethal disease with few therapeutic opportunities. Platinum-pemetrexed chemotherapy is the backbone of first-line treatment for MM. The introduction of immunotherapy (IO) has been the only novelty of the last decades, allowing an increase in survival compared to standard chemotherapy (CT). However, IO is not approved for epithelioid histology in many countries. Therefore, therapy for relapsed MM remains an unmet clinical need, and the prognosis of MM remains poor, with an average survival of only 18 months. Increasing evidence reveals MM complexity and heterogeneity, of which histological classification fails to explain. Thus, scientific focus on possibly new molecular markers or cellular targets is increasing, together with the search for target therapies directed towards them. The molecular landscape of MM is characterized by inactivating tumor suppressor alterations, the most common of which is found in CDKN2A, BAP1, MTAP, and NF2. In addition, cellular targets such as mesothelin or metabolic enzymes such as ASS1 could be potentially amenable to specific therapies. This review examines the major targets and relative attempts of therapeutic approaches to provide an overview of the potential prospects for treating this rare neoplasm.

Genetic Tools for Cell Lineage Tracing and Profiling Developmental Trajectories in the Skin

The epidermis is the body's first line of protection against dehydration and pathogens, continually regenerating the outermost protective skin layers throughout life. During both embryonic development and wound healing, epidermal stem and progenitor cells must respond to external stimuli and insults to build, maintain, and repair the cutaneous barrier. Recent advances in CRISPR-based methods for cell lineage tracing have remarkably expanded the potential for experiments that track stem and progenitor cell proliferation and differentiation over the course of tissue and even organismal development. Additional tools for DNA-based recording of cellular signaling cues promise to deepen our understanding of the mechanisms driving normal skin morphogenesis and response to stressors as well as the dysregulation of cell proliferation and differentiation in skin diseases and cancer. In this review, we highlight cutting-edge methods for cell lineage tracing, including in organoids and model organisms, and explore how cutaneous biology researchers might leverage these techniques to elucidate the developmental programs that support the regenerative capacity and plasticity of the skin.

The highly selective and oral phosphoinositide 3-kinase delta (PI3K-δ) inhibitor roginolisib induces apoptosis in mesothelioma cells and increases immune effector cell composition

Targeting aberrantly expressed kinases in malignant pleural mesothelioma (MPM) is a promising therapeutic strategy. We here investigated the effect of the novel and highly selective Phosphoinositide 3-kinase delta (PI3K-δ) inhibitor roginolisib (IOA-244) on MPM cells and on the immune cells in MPM microenvironment. To this aim, we analyzed the expression of PI3K-δ by immunohistochemistry in specimens from primary MPM, cell viability and death in three different MPM cell lines treated with roginolisib alone and in combination with ipatasertib (AKT inhibitor) and sapanisertib (mTOR inhibitor). In a co-culture model of patient-derived MPM cells, autologous peripheral blood mononuclear cells and fibroblasts, the tumor cell viability and changes in immune cell composition were investigated after treatment of roginolisib with nivolumab and cisplatin. PI3K-δ was detected in 66/89 (74%) MPM tumors and was associated with reduced overall survival (12 vs. 25 months, P=0.0452). Roginolisib induced apoptosis in MPM cells and enhanced the anti-tumor efficacy of AKT and mTOR kinase inhibitors by suppressing PI3K-δ/AKT/mTOR and ERK1/2 signaling. Furthermore, the combination of roginolisib with chemotherapy and immunotherapy re-balanced the immune cell composition, increasing effector T-cells and reducing immune suppressive cells. Overall, roginolisib induces apoptosis in MPM cells and increases the antitumor immune cell effector function when combined with nivolumab and cisplatin. These results provide first insights on the potential of roginolisib as a therapeutic agent in patients with MPM and its potential in combination with established immunotherapy regimen.

An Investigation of Structure-Activity Relationships and Cell Death Mechanisms of the Marine Alkaloids Discorhabdins in Merkel Cell Carcinoma Cells

A library of naturally occurring and semi-synthetic discorhabdins was assessed for their effects on Merkel cell carcinoma (MCC) cell viability. The set included five new natural products and semi-synthetic compounds whose structures were elucidated with NMR, HRMS, and ECD techniques. Several discorhabdins averaged sub-micromolar potency against the MCC cell lines tested and most of the active compounds showed selectivity towards virus-positive MCC cell lines. An investigation of structure-activity relationships resulted in an expanded understanding of the crucial structural features of the discorhabdin scaffold. Mechanistic cell death assays suggested that discorhabdins, unlike many other MCC-active small molecules, do not induce apoptosis, as shown by the lack of caspase activation, annexin V staining, and response to caspase inhibition. Similarly, discorhabdin treatment failed to increase MCC intracellular calcium and ROS levels. In contrast, the rapid loss of cellular reducing potential and mitochondrial membrane potential suggested that discorhabdins induce mitochondrial dysfunction leading to non-apoptotic cell death.

Single-cell dissection of Merkel cell carcinoma heterogeneity unveils transcriptomic plasticity and therapeutic vulnerabilities

Merkel cell carcinoma (MCC), a rare but aggressive skin cancer, remains a challenge in the era of precision medicine. Immune checkpoint inhibitors (ICIs), the only approved therapy for advanced MCC, are impeded by high primary and acquired resistance. Hence, we dissect transcriptomic heterogeneity at single-cell resolution in a panel of patient tumors, revealing phenotypic plasticity in a subset of treatment-naive MCC. The tumor cells in a "mesenchymal-like" state are endowed with an inflamed phenotype that portends a better ICI response. This observation is also validated in the largest whole transcriptomic dataset available from MCC patient tumors. In contrast, ICI-resistant tumors predominantly express neuroepithelial markers in a well-differentiated state with "immune-cold" landscape. Importantly, a subtle shift to "mesenchymal-like" state reverts copanlisib resistance in primary MCC cells, highlighting potential strategies in patient stratification for therapeutics to harness tumor cell plasticity, augment treatment efficacy, and avert resistance.

Current and preclinical treatment options for Merkel cell carcinoma

INTRODUCTION

Merkel cell carcinoma (MCC) is a rare, highly aggressive form of skin cancer with neuroendocrine features. The origin of this cancer is still unclear, but research in the last 15 years has demonstrated that MCC arises via two distinct etiologic pathways, i.e. virus and UV-induced. Considering the high mortality rate and the limited therapeutic options available, this review aims to highlight the significance of MCC research and the need for advancement in MCC treatment.

AREAS COVERED

With the advent of the immune checkpoint inhibitor therapies, we now have treatment options providing a survival benefit for patients with advanced MCC. However, the issue of primary and acquired resistance to these therapies remains a significant concern. Therefore, ongoing efforts seeking additional therapeutic targets and approaches for MCC therapy are a necessity. Through a comprehensive literature search, we provide an overview on recent preclinical and clinical studies with respect to MCC therapy.

EXPERT OPINION

Currently, the only evidence-based therapy for MCC is immune checkpoint blockade with anti-PD-1/PD-L1 for advanced patients. Neoadjuvant, adjuvant and combined immune checkpoint blockade are promising treatment options.

Current treatments and emerging therapies of human polyomavirus-associated skin diseases: a comprehensive review

Since Merkel cell polyomavirus (MCPyV) was linked as the predominant etiology of Merkel cell carcinoma (MCC) in 2008, three additional human polyomaviruses (HPyV) have been definitively linked to cutaneous diseases-trichodysplasia spinulosa virus (TSPyV) and human polyomavirus 6 and 7 (HPyV6, HPyV7). TSPyV contributes to the development of trichodysplasia spinulosa (TS), and HPyV6/7 is associated closely with the eruption of pruritic and dyskeratotic dermatoses (PDD). Clinically, MCC is treated with surgical excision and radiation with adjuvant chemotherapy, although newer treatment options include immune checkpoint inhibition. These novel immunotherapies hold promise for the treatment of metastatic MCC, but resistance and side effects prevent a significant proportion of patients from realizing their benefits. Based on previous case reports, the standard of care for the less deadly but disfiguring cutaneous disease TS include immunosuppressant (IS) reduction, the use of antivirals such as cidofovir (CDV) or valganciclovir (VGCV), or a combination of these treatments. Similar treatments were attempted for PDD, but oral acitretin was found to be most effective. As MCC, TS, and PDD are rare diseases, further research is required for effective treatments. In this review, we summarize clinical trials, preclinical studies, and case reports that present outcomes and side effects of current and emerging treatments for HPyV-associated cutaneous diseases, offering a comprehensive resource for clinical application and prospective clinical trials.

Current In Vitro and In Vivo Models to Study MCPyV-Associated MCC

Merkel cell polyomavirus (MCPyV) is the only human polyomavirus currently known to cause human cancer. MCPyV is believed to be an etiological factor in at least 80% of cases of the rare but aggressive skin malignancy Merkel cell carcinoma (MCC). In these MCPyV+ MCC tumors, clonal integration of the viral genome results in the continued expression of two viral proteins: the viral small T antigen (ST) and a truncated form of the viral large T antigen. The oncogenic potential of MCPyV and the functional properties of the viral T antigens that contribute to neoplasia are becoming increasingly well-characterized with the recent development of model systems that recapitulate the biology of MCPyV+ MCC. In this review, we summarize our understanding of MCPyV and its role in MCC, followed by the current state of both in vitro and in vivo model systems used to study MCPyV and its contribution to carcinogenesis. We also highlight the remaining challenges within the field and the major considerations related to the ongoing development of in vitro and in vivo models of MCPyV+ MCC.

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.

Organotypic Epithelial Raft Cultures as a Three-Dimensional In Vitro Model of Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a rare type of skin cancer for which an in vitro model is still lacking. MCC tumorigenesis is associated either with the integration of Merkel cell polyomavirus into the host genome, or with the accumulation of somatic mutations upon chronic exposure to UV light. Transgenic animals expressing the viral oncoproteins, which are constitutively expressed in virus-related MCC, do not fully recapitulate MCC. Although cell-line-derived xenografts have been established for the two subtypes of MCC, they still present certain limitations. Here, we generated organotypic epithelial raft cultures (OERCs) of MCC by using primary human keratinocytes and both virus-positive and virus-negative MCC cell lines. The primary human keratinocytes and the tumor cells were grown on top of a dermal equivalent. Histological and immunohistochemical examination of the rafts confirmed the growth of MCC cells. Furthermore, gene expression analysis revealed differences in the expression profiles of the distinct tumor cells and the keratinocytes at the transcriptional level. In summary, considering the limited availability of patient samples, OERCs of MCC may constitute a suitable model for evaluating the efficacy and selectivity of new drug candidates against MCC; moreover, they are a potential tool to study the oncogenic mechanisms of this malignancy.

EZH2 inhibition confers PIK3CA-driven lung tumors enhanced sensitivity to PI3K inhibition

Members of the PI3K signaling pathway, especially PIK3CA, the gene encoding the catalytic subunit of the PI3K complex, are highly mutated and amplified in various cancer types, including non-small cell lung cancer. Although PI3K inhibitors have been used in clinics for follicular lymphoma and chronic lymphocytic leukemia, no agents targeting PI3K aberrations in lung cancer have been approved by the FDA so far. In this study, we observed that PIK3CA-E545K, the most common mutation in lung cancer, harbored a modest induction of stem-like properties in lung epithelial cells, and drove development of adenocarcinoma autochthonously when paired with p53 loss in a murine mouse model. We also found that PIK3CA-mutant of amplified lung cancer cells were sensitive to EZH2 inhibition. EZH2 inhibition synergized with PI3K inhibition in human cancer cells in vitro and worked together efficiently in vivo. Mechanistically, EZH2 inhibition cooperated with PI3K inhibition to produce a more potent suppression of phospho-AKT downstream of PI3K. This study suggests a promising combination therapy to combat lung cancers with PIK3CA mutation or amplification. Both copanlisib, the PI3K inhibitor, and tazemetostat, the EZH2 inhibitor, are FDA-approved, which should enhance the clinical translation of this work.

A web-based predictive model for overall survival of patients with cutaneous Merkel cell carcinoma: A population-based study

BACKGROUND

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine carcinoma with a high mortality rate, so it is necessary to create models to predict overall survival of MCC. We developed an easy-to-use web-based calculator to predict the OS of MCC patients based on the nomogram.

METHODS

MCC patients between 2004 and 2015 were collected from the Surveillance, Epidemiology, and End Results (SEER) database and randomly assigned to training and validation cohorts. Patients between 2016-2017 serve as the external validation cohort. Relevant risk factors were identified by univariate and multivariate COX hazards regression methods and combined to produce nomograms. The concordance index (C-index), area under the receiver operating characteristic (AUC) curve, and calibration plots have demonstrated the predictive power of the nomograms. Decision curve analysis (DCA) was used to measure nomograms in clinical practice. Patients were divided into three groups according to the scores of the nomogram.

RESULTS

A total of 3480 patients were randomly assigned to the training group and validation group in this study. Meaningful prognostic factors were applied to the establishment of nomograms. The C-index for OS was 0.725 (95% CI: 0.706-0.741) in the training cohort and 0.710 (95% CI: 0.683-0.737) in the validation cohort. In the external validation cohort, C-index was 0.763 (95% CI: 0.734-0.792). The C-index of training cohort, validation cohort and external validation cohort for CSS were 0.743 (95% CI:0.725-0.761), 0.739(95%CI:0.712-0.766) and 0.774 (95%CI:0.735-0.813), respectively. The AUC and calibration plots of 1-, 3-, and 5-year OS rates showed that the nomogram had good predictive power. DCA demonstrated that the nomogram constructed in this study could provide a clinical net benefit. Our calculator demonstrated excellent predictive capabilities for better risk grouping of MCC patients.

CONCLUSION

We created novel nomograms of prognostic factors for MCC, which more accurately and comprehensively predicted 1-, 3-, and 5-year OS/CSS in MCC patients. We established a calculator which can easily and quickly calculate the risk grouping of MCC patients by inputting clinically relevant characteristics. This can help clinicians identify high-risk patients as early as possible, carry out personalized treatment, follow-up, and monitoring, and improve the survival rate of MCC patients.

Updates on the diagnosis, current and future therapeutic options in Merkel-cell carcinoma

Merkel-cell carcinoma (MCC) is a rare and extremely aggressive nonmelanocytic cutaneous neuroendocrine carcinoma. Historically, it has been associated with limited therapy options and poor prognosis. While its incidence has been rising over the last two decades, recent discoveries and a better understanding of its pathogenesis, viral association and immunologic features have allowed for the emergence of new therapies. Surgical excision with or without radiotherapy remains the first-line therapy for primary lesions without evidence of metastatic disease. The majority of MCC cases are regrettably diagnosed at advanced stages and oftentimes require systemic therapy. There have been several significant advances in the treatment of MCC in the last decade. Among these have been the development of immune checkpoint inhibitors targeting the programmed death protein-1 (PD-1)/programmed death ligand-1 (PDL-1). Despite recent success of immunotherapy, nearly 50% of patients diagnosed with MCC still succumb to the disease. Fortunately, there has been a number of new targeted therapies that hold great promise. Among them are phosphatidylinositide-3kinase (Pl3K) inhibitors, adoptive T-cell immunotherapy, activated NK-92 cells infusions and therapeutic vaccines. Additional emerging therapeutic targets include cellular ubiquitin-specific processing protease 7 (Usp7) that restricts viral replication and IFN genes (STING), activation of which promotes an antitumor inflammatory response.

Selective Inhibition of Aurora Kinase A by AK-01/LY3295668 Attenuates MCC Tumor Growth by Inducing MCC Cell Cycle Arrest and Apoptosis

Merkel cell carcinoma (MCC) is an often-lethal skin cancer with increasing incidence and limited treatment options. Although immune checkpoint inhibitors (ICI) have become the standard of care in advanced MCC, 50% of all MCC patients are ineligible for ICIs, and amongst those treated, many patients develop resistance. There is no therapeutic alternative for these patients, highlighting the urgent clinical need for alternative therapeutic strategies. Using patient-derived genetic insights and data generated in our lab, we identified aurora kinase as a promising therapeutic target for MCC. In this study, we examined the efficacy of the recently developed and highly selective AURKA inhibitor, AK-01 (LY3295668), in six patient-derived MCC cell lines and two MCC cell-line-derived xenograft mouse models. We found that AK-01 potently suppresses MCC survival through apoptosis and cell cycle arrest, particularly in MCPyV-negative MCC cells without RB expression. Despite the challenge posed by its short in vivo durability upon discontinuation, the swift and substantial tumor suppression with low toxicity makes AK-01 a strong potential candidate for MCC management, particularly in combination with existing regimens.

Merkel Cell Polyomavirus and Merkel Cell Carcinoma

Viruses are the cause of approximately 15% of all human cancers. Both RNA and DNA human tumor viruses have been identified, with Merkel cell polyomavirus being the most recent one to be linked to cancer. This virus is associated with about 80% of Merkel cell carcinomas, a rare, but aggressive cutaneous malignancy. Despite its name, the cells of origin of this tumor may not be Merkel cells. This review provides an update on the structure and life cycle, cell tropism and epidemiology of the virus and its oncogenic properties. Putative strategies to prevent viral infection or treat virus-positive Merkel cell carcinoma patients are discussed.