Parathyroid hormone-related protein, chromogranin A, and calcitonin gene products in the neuroendocrine skin carcinoma cell lines MKL1 and MKL2

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-Molecule RNA Sequencing Reveals IFNγ-Induced Differential Expression of Immune Escape Genes in Merkel Cell Polyomavirus-Positive MCC Cell Lines

Merkel cell carcinoma (MCC) is a rare and highly aggressive cancer, which is mainly caused by genomic integration of the Merkel cell polyomavirus and subsequent expression of a truncated form of its large T antigen. The resulting primary tumor is known to be immunogenic and under constant pressure to escape immune surveillance. Because interferon gamma (IFNγ), a key player of immune response, is secreted by many immune effector cells and has been shown to exert both anti-tumoral and pro-tumoral effects, we studied the transcriptomic response of MCC cells to IFNγ. In particular, immune modulatory effects that may help the tumor evade immune surveillance were of high interest to our investigation. The effect of IFNγ treatment on the transcriptomic program of three MCC cell lines (WaGa, MKL-1, and MKL-2) was analyzed using single-molecule sequencing via the Oxford Nanopore platform. A significant differential expression of several genes was detected across all three cell lines. Subsequent pathway analysis and manual annotation showed a clear upregulation of genes involved in the immune escape of tumor due to IFNγ treatment. The analysis of selected genes on protein level underlined our sequencing results. These findings contribute to a better understanding of immune escape of MCC and may help in clinical treatment of MCC patients. Furthermore, we demonstrate that single-molecule sequencing can be used to assess characteristics of large eukaryotic transcriptomes and thus contribute to a broader access to sequencing data in the community due to its low cost of entry.

Identification of natural product modulators of Merkel cell carcinoma cell growth and survival

Merkel cell carcinoma (MCC) is a rare, but aggressive skin cancer the incidence of which has increased significantly in recent years. The majority of MCCs have incorporated Merkel cell polyomavirus (VP-MCC) while the remainder are virus-negative (VN-MCC). Although a variety of therapeutic options have shown promise in treating MCC, there remains a need for additional therapeutics as well as probes for better understanding MCC. A high-throughput screening campaign was used to assess the ability of > 25,000 synthetic and natural product compounds as well as > 20,000 natural product extracts to affect growth and survival of VN-MCC and VP-MCC cell lines. Sixteen active compounds were identified that have mechanisms of action reported in the literature along with a number of compounds with unknown mechanisms. Screening results with pure compounds suggest a range of potential targets for MCC including DNA damage, inhibition of DNA or protein synthesis, reactive oxygen species, and proteasome inhibition as well as NFκB inhibition while also suggesting the importance of zinc and/or copper binding. Many of the active compounds, particularly some of the natural products, have multiple reported targets suggesting that this strategy might be a particularly fruitful approach. Processing of several active natural product extracts resulted in the identification of additional MCC-active compounds. Based on these results, further investigations focused on natural products sources, particularly of fungal origin, are expected to yield further potentially useful modulators of MCC.

ALK is frequently phosphorylated in Merkel cell carcinoma and associates with longer survival

Merkel cell carcinoma (MCC) is a rare skin cancer with only limited therapeutic options for advanced disease. We previously showed that oncogene ALK is frequently expressed at the RNA level in MCC and further that ALK positivity by immunohistochemistry is frequent and correlates strongly with Merkel cell polyomavirus (MCPyV) positivity. In this study, we investigated whether ALK receptor is active in MCC tumor samples and MCC cell lines, and whether ALK would be a prospective treatment target in MCC. We utilized tissue microarrays constructed from 136 primary MCC tumor samples as well as nine previously established MCC cell lines to determine the presence of ALK and phosphorylated ALK (p-ALK) via immunohistochemistry. Almost half of the analyzed MCC tumors displayed ALK phosphorylation (47.8%). Analysis of MCC tumor samples revealed that the presence of p-ALK correlated to MCPyV positivity, younger age, nonexistence of metastases at diagnosis and ultimately to better MCC-specific survival. In contrast to MCC tumor samples only two out of nine MCC cell lines showed only low ALK phosphorylation by immunohistochemistry. Our study reveals clear disparity in ALK activity between patient derived tumors and cell line samples and therefore, more advanced disease models such as xenografts are necessary to resolve whether ALK is a useful treatment target in MCC.

A Novel In Vitro Culture Model System to Study Merkel Cell Polyomavirus-Associated MCC Using Three-Dimensional Organotypic Raft Equivalents of Human Skin

Merkel cell polyomavirus (MCPyV) is a human polyomavirus causally linked to the development of Merkel cell carcinoma (MCC), an aggressive malignancy that largely arises within the dermis of the skin. In this study, we recapitulate the histopathology of human MCC tumors in vitro using an organotypic (raft) culture system that is traditionally used to recapitulate the dermal and epidermal equivalents of skin in three dimensions (3D). In the optimal culture condition, MCPyV+ MCC cells were embedded in collagen between the epidermal equivalent comprising human keratinocytes and a dermal equivalent containing fibroblasts, resulting in MCC-like lesions arising within the dermal equivalent. The presence and organization of MCC cells within these dermal lesions were characterized through biomarker analyses. Interestingly, co-culture of MCPyV+ MCC together with keratinocytes specifically within the epidermal equivalent of the raft did not reproduce human MCC morphology, nor were any keratinocytes necessary for MCC-like lesions to develop in the dermal equivalent. This 3D tissue culture system provides a novel in vitro platform for studying the role of MCPyV T antigens in MCC oncogenesis, identifying additional factors involved in this process, and for screening potential MCPyV+ MCC therapeutic strategies.

High-resolution analysis of Merkel Cell Polyomavirus in Merkel Cell Carcinoma reveals distinct integration patterns and suggests NHEJ and MMBIR as underlying mechanisms

Merkel Cell Polyomavirus (MCPyV) is the etiological agent of the majority of Merkel Cell Carcinomas (MCC). MCPyV positive MCCs harbor integrated, defective viral genomes that constitutively express viral oncogenes. Which molecular mechanisms promote viral integration, if distinct integration patterns exist, and if integration occurs preferentially at loci with specific chromatin states is unknown. We here combined short and long-read (nanopore) next-generation sequencing and present the first high-resolution analysis of integration site structure in MCC cell lines as well as primary tumor material. We find two main types of integration site structure: Linear patterns with chromosomal breakpoints that map closely together, and complex integration loci that exhibit local amplification of genomic sequences flanking the viral DNA. Sequence analysis suggests that linear patterns are produced during viral replication by integration of defective/linear genomes into host DNA double strand breaks via non-homologous end joining, NHEJ. In contrast, our data strongly suggest that complex integration patterns are mediated by microhomology-mediated break-induced replication, MMBIR. Furthermore, we show by ChIP-Seq and RNA-Seq analysis that MCPyV preferably integrates in open chromatin and provide evidence that viral oncogene expression is driven by the viral promoter region, rather than transcription from juxtaposed host promoters. Taken together, our data explain the characteristics of MCPyV integration and may also provide a model for integration of other oncogenic DNA viruses such as papillomaviruses.

Combining DNA Damage Induction with BCL-2 Inhibition to Enhance Merkel Cell Carcinoma Cytotoxicity

Merkel cell carcinoma (MCC) is a highly lethal skin cancer. MCC tumors rapidly develop resistance to the chemotherapies tested to date. While PD-1/PD-L1 immune checkpoint blockade has demonstrated success in MCC treatment, a significant portion of MCC patients are nonresponsive. Therefore, the pressing need for effective MCC chemotherapies remains. We screened a library of natural products and discovered that one compound, glaucarubin, potently reduced the viability of Merkel cell polyomavirus (MCPyV)-positive MCCs, while remaining nontoxic to primary human fibroblasts and MCPyV-negative MCC cell lines tested. Protein array and Western blot analyses revealed that glaucarubin induces DNA damage and PARP-1 cleavage that correlates with the loss of viability in MCC cells. However, high basal expression of the antiapoptotic factor BCL-2 allowed a subpopulation of cells to survive glaucarubin treatment. Previous studies have shown that, while targeting BCL-2 family proteins significantly decreases MCC cell viability, BCL-2 antisense therapy alone was insufficient to inhibit tumor growth in patients with advanced MCC. We discovered that treatment with an FDA-approved BCL-2 inhibitor in the context of glaucarubin-induced DNA damage led to near complete killing in multiple MCPyV-positive MCC cell lines that express high levels of BCL-2. The combination of DNA damage-induced apoptosis and BCL-2 inhibition thus represents a novel therapeutic strategy for MCPyV-positive MCCs.

Conversion of Sox2-dependent Merkel cell carcinoma to a differentiated neuron-like phenotype by T antigen inhibition

Normal cells can be transformed into cancer cells by viral oncogenes. Reversion of a viral human cancer cell, however, into a differentiated cell by viral oncogene inhibition has not been described. Merkel cell carcinoma (MCC) is a neuroendocrine cancer caused by Merkel cell polyomavirus (MCV) that encodes a T antigen oncogene. When MCV⁺ MCC cells with T antigen knockdown are cocultured with keratinocytes, the MCC phenotype converts to a differentiated neuronal phenotype and loses Merkel cell factor Sox2 and Atoh1 expression. MCV large T activates Sox2 and Atoh1 by its ability to inhibit retinoblastoma. Sox2 inhibition similarly induced this phenotypic conversion of MCC. These findings suggest that MCV induces cancer by dysregulating embryonic Merkel cell differentiation pathways.

Viral cancers show oncogene addiction to viral oncoproteins, which are required for survival and proliferation of the dedifferentiated cancer cell. Human Merkel cell carcinomas (MCCs) that harbor a clonally integrated Merkel cell polyomavirus (MCV) genome have low mutation burden and require viral T antigen expression for tumor growth. Here, we showed that MCV⁺ MCC cells cocultured with keratinocytes undergo neuron-like differentiation with neurite outgrowth, secretory vesicle accumulation, and the generation of sodium-dependent action potentials, hallmarks of a neuronal cell lineage. Cocultured keratinocytes are essential for induction of the neuronal phenotype. Keratinocyte-conditioned medium was insufficient to induce this phenotype. Single-cell RNA sequencing revealed that T antigen knockdown inhibited cell cycle gene expression and reduced expression of key Merkel cell lineage/MCC marker genes, including HES6, SOX2, ATOH1, and KRT20. Of these, T antigen knockdown directly inhibited Sox2 and Atoh1 expression. MCV large T up-regulated Sox2 through its retinoblastoma protein-inhibition domain, which in turn activated Atoh1 expression. The knockdown of Sox2 in MCV⁺ MCCs mimicked T antigen knockdown by inducing MCC cell growth arrest and neuron-like differentiation. These results show Sox2-dependent conversion of an undifferentiated, aggressive cancer cell to a differentiated neuron-like phenotype and suggest that the ontology of MCC arises from a neuronal cell precursor.

PTHrP, calcitonin and calcitriol in a case of severe, protracted and refractory hypercalcemia due to a pancreatic neuroendocrine tumor

A patient with a primary neuroendocrine tumor of the pancreas, presented with severe hypercalcemia. This hypercalcemia of malignancy (HCM) failed to respond to intensive bisphosphonate treatment and needed continuous enhanced diuresis. Only after successful antitumor therapy did the hypercalcemia subside. Hypercalcemia was associated with increased concentrations of plasma PTHrP, calcitonin and 1,25-(OH)(2)D(3). Bone mineral density was markedly increased. We demonstrated the presence of both PTHrP and calcitonin in the tumor at the mRNA and protein level, using RT-PCR, immunohistochemistry and Western blotting. The high levels of plasma PTHrP and the demonstrated predominant renal mechanism in this case of HCM are suspected to be the cause for its refractoriness to bone resorption inhibitors. Our findings furthermore suggest that the tumoral production of calcitonin and PTHrP might have contributed to the increased bone mineral storage of calcium and thus probably attenuated the development of frank hypercalcemia.

Overexpression of human epidermal growth factor receptor and c-erbB-2 by neuroendocrine cells in normal prostatic tissue

OBJECTIVE

It has been suggested that prostatic neuroendocrine (NE) cells play an important role in the growth and differentiation of the prostate by secreting various neuropeptides and serotonin. However, the mechanism by which NE cells themselves are regulated is virtually unknown. In the present study we evaluated the expression of the human epidermal growth factor receptor (EGFR) family (HER) in prostatic NE cells.

METHODS

Formalin-fixed, paraffin-embedded tissue sections from twenty radical prostatectomy specimens were immunostained with validated rabbit polyclonal antibodies raised against human EGFR and c-erbB-2, using the streptavidin-peroxidase enzyme conjugate method.

RESULTS

A strong immunoreactivity was observed with both antibodies in the cytosol of a few epithelial cells. These cells frequently had a dendritic appearance and were located in the acini and ducts. The EGFR-positive cells were predominant in most cases. Double immunostaining revealed the colocalization of both antigens with chromogranin A, a polypeptide that is expressed by most NE cells. Moreover, EGFR and c-erbB-2 appeared to be colocalized as well as independently expressed by different subpopulations of NE cells.

CONCLUSIONS

The results suggest that prostatic NE cells might be regulated by the HER protein family, probably, in a ligand-specific fashion. This is the first report identifying a potential pathway regulating prostatic NE cells.

Parathyroid hormone-related protein is expressed by prostatic neuroendocrine cells

OBJECTIVE

Parathyroid hormone-related protein (PTHrP) is a regulatory peptide that has been associated with normal fetal growth and differentiation as well as the regulation of fetal calcium. In a variety of cancers, PTHrP has been implicated in the humoral hypercalcemia of malignancy. Recently, we demonstrated that all prostatic adenocarcinomas express PTHrP. In the present study, the localization of PTHrP and its mRNA in nonmalignant prostate tissue was assessed.

METHODS

Formalin-fixed, paraffin-embedded prostatic tissues from 23 patients were evaluated. Immunocytochemistry (ICC) was performed by the streptavidin-peroxidase enzyme conjugate method using a monoclonal antibody, 8B12, generated against fragment 1-34 of the amino-terminal end of PTHrP. Nonradioactive in situ hybridization was carried out using a digoxigenin labeled single-stranded cDNA probe complementary to the sequence coding for PTHrP(15-120).

RESULTS

PTHrP immunoreactivity was observed in the cytosol of a few epithelial cells. Double labeling and serial section ICC with 8B12 and a monoclonal antibody to chromogranin A (a generic neuroendocrine [NE] marker) revealed that PTHrP was present in a subpopulation of prostatic NE cells. In situ hybridization of mirror image sections demonstrated positive signals in prostatic NE cells in complete accordance with the ICC findings.

CONCLUSIONS

The localization and production of PTHrP in prostatic NE cells suggest that this polypeptide may act in an endocrine-paracrine fashion involved in the prostatic growth and differentiation as well as the regulation of calcium in semen and seminal fluid.

Parathyroid hormone-like protein is a secretory product of atrial myocytes

Parathyroid hormone-like protein (PLP) was originally identified from tumors associated with hypercalcemia. Recently, it has been found to be expressed in a stretch-responsive manner in several types of smooth muscle. We studied adult rat heart muscle for the presence of the PLP. Using immunohistology and the PCR, we demonstrated the presence of PLP and its mRNA in all heart chambers. Immunoelectron microscopy demonstrated PLP in secretory vesicles of atrial mycocytes. Using immunoassay, we demonstrated that atria contained a higher concentration of PLP than ventricles. Furthermore, primary cultures of both chambers released PLP into conditioned medium, with atria secreting more than ventricles. Considered with studies of the role of PLP in other tissues, our observations suggest that the production and secretion of PLP by cardiac myocytes represents a calcium-related regulatory function for this stretch-responsive polypeptide in the cardiovascular system. PLP in the heart may be the calcium counterpart for the atrial natriuretic-sodium regulatory axis of the cardiovascular system.