The Paranuclear Dot Inhibits Apoptosis in Merkel Cell Carcinoma

Authors

• EH Son
• GR Wallen
• L Yang
• RT Tuason
• C Gerrard
• LJ Lee

Abstract

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine skin cancer that is caused by either integration of the Merkel cell polyomavirus or UV damage. Due to its rarity, MCC is under-researched and poorly understood. Staining for Cytokeratin 20 and other intermediate filaments in a paranuclear dot pattern is a diagnostic marker for MCC, however the functional significance of these protein aggregates is unknown. We sought to investigate the structure and function of the paranuclear dot in MCC. Using pan-cytokeratin immunostaining, we found that 94% of MCC tumors contained cells with paranuclear dots. There were more dot-positive cells in virus-positive MCC than in virus-negative MCC. Electron microscopy and immunofluorescent staining showed that the paranuclear dot forms around the centrosome of interphase cells. However, the dot disaggregated during mitosis, suggesting a dynamic structure that reforms with each cell division. Interestingly, dot cytokeratins colocalized with FADD, the cytoplasmic mediator of death receptor signaling. Dot-positive MCC cells were resistant to apoptotic induction by FasL and TNF-ÔÅ°, suggesting that FADD is sequestered in the dot. Overexpression of FADD in dot-positive cells resulted in diffuse cytoplasmic localization of FADD and apoptotic cell death. Microtubule modulating drugs disrupted the dot and sensitized cells to TNF-ÔÅ° induced extrinsic apoptosis. Taken together, the paranuclear dot is an aggregate of intermediate filaments that is maintained by microtubules and functionally sequesters FADD to block death receptor-mediated apoptosis. Therefore, anti-cancer drugs that target microtubules may also enhance anti-tumor immune responses in MCC.

Scientific Focus Area: ACI/IRS

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