Merkel Cell Polyomavirus T Antigens Induce Merkel Cell-Like Differentiation in GLI1-Expressing Epithelial Cells

Abstract

Merkel cell carcinoma (MCC) is an aggressive skin cancer frequently caused by the Merkel cell polyomavirus (MCPyV). It is still under discussion, in which cells viral integration and MCC development occurs. Recently, we demonstrated that a virus-positive MCC derived from a trichoblastoma, an epithelial neoplasia bearing Merkel cell (MC) differentiation potential. Accordingly, we hypothesized that MC progenitors may represent an origin of MCPyV-positive MCC. To sustain this hypothesis, phenotypic comparison of trichoblastomas and physiologic human MC progenitors was conducted revealing GLI family zinc finger 1 (GLI1), Keratin 17 (KRT 17), and SRY-box transcription factor 9 (SOX9) expressions in both subsets. Furthermore, GLI1 expression in keratinocytes induced transcription of the MC marker SOX2 supporting a role of GLI1 in human MC differentiation. To assess a possible contribution of the MCPyV T antigens (TA) to the development of an MC-like phenotype, human keratinocytes were transduced with TA. While this led only to induction of KRT8, an early MC marker, combined GLI1 and TA expression gave rise to a more advanced MC phenotype with SOX2, KRT8, and KRT20 expression. Finally, we demonstrated MCPyV-large T antigens’ capacity to inhibit the degradation of the MC master regulator Atonal bHLH transcription factor 1 (ATOH1). In conclusion, our report suggests that MCPyV TA contribute to the acquisition of an MC-like phenotype in epithelial cells.

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

“…Therefore, this raft system would be ideal for studying co-factors that collaborate with the MCPyV T antigens to facilitate MCC biogenesis. Gain-of-function studies with transcription factor ATOH1 [ 20 , 26 ] and GLI1 [ 21 ], as well as loss-of-function studies with T antigens [ 28 ], have already provided insight into the potential mechanisms involved in this process. With our model, the biological consequences described in these studies, specifically cell morphology and behavior, can be explored further in a structurally and environmentally relevant model.…”

“…Binding of MCPyV LT to Rb inhibits Rb’s capacity to regulate the activity of the transcription factor E2F, a property that contributes to the transforming capacity of other polyomaviruses [ 18 ]. Multiple investigators have uncovered the transforming/tumorigenic properties of MCPyV ST and/or LT in a variety in vitro and in vivo settings [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ]. Nevertheless, perhaps the strongest evidence for the role of MCPyV ST and LT in the cancer phenotype of MCPyV+ MCC is the observation that silencing their expression induces cell cycle arrest and apoptosis [ 27 ] and causes MCPyV+ MCC cells to undergo phenotypic changes and apparent conversion to a neuron-like cell type [ 28 ], demonstrating the critical role of these viral proteins in MCC.…”

The Merkel Cell Polyomavirus T Antigens Function as Tumor Promoters in Murine Skin

“…Merkel cells arise from epidermal progenitor cells [ 77 , 79 ] and ectopic expression of the Merkel cell specification factor atonal homolog 1 (ATOH1) in epithelial cells induces Merkel cell development [ 80 ]. Interestingly, MCPyV LT expression in keratinocytes has been observed to not only increase ATOH1 expression [ 50 ] but also prevent its degradation [ 52 ]. Furthermore, combined expression of the MCPyV T antigens and cellular genes GLI1 or ATOH1 induces Merkel cell-like phenotypes in vitro [ 52 ] and MCC-like lesions in vivo [ 33 ], respectively.…”

“…Interestingly, MCPyV LT expression in keratinocytes has been observed to not only increase ATOH1 expression [ 50 ] but also prevent its degradation [ 52 ]. Furthermore, combined expression of the MCPyV T antigens and cellular genes GLI1 or ATOH1 induces Merkel cell-like phenotypes in vitro [ 52 ] and MCC-like lesions in vivo [ 33 ], respectively. A recent study reported sequencing evidence that a MCPyV-positive MCC was derived from an epithelial lineage [ 81 ].…”

“…In our transgenic mouse model, epithelial expression of the MCPyV ST and truncated LT antigens induces hyperplasia, proliferation, and spontaneous tumor development [ 32 ]. Furthermore, MCPyV T antigen expression in epithelial cells induces Merkel cell-related gene expression and Merkel cell phenotypes [ 52 ]. There are also several other potentially oncogenic functions of the MCPyV T antigens [ 53 ].…”