Merkel Cell Polyomavirus Small T Antigen Activates Noncanonical NF-κB Signaling to Promote Tumorigenesis

Abstract

Multiple human polyomaviruses (HPyV) can infect the skin, but only Merkel cell polyomavirus (MCPyV) has been implicated in the development of a cancer, Merkel cell carcinoma (MCC). While expression of HPyV6, HPyV7, and MCPyV small T antigens (sT), all induced a senescence-associated secretory phenotype (SASP), MCPyV sT uniquely activated noncanonical NF-κB (ncNF-κB), instead of canonical NF-κB signaling, to evade p53-mediated cellular senescence. Through its large T stabilization domain, MCPyV sT activated ncNF-κB signaling both by inducing H3K4 trimethylation-mediated increases of NFKB2 and RELB transcription and also by promoting NFKB2 stabilization and activation through FBXW7 inhibition. Noncanonical NF-κB signaling was required for SASP cytokine secretion, which promoted the proliferation of MCPyV sT–expressing cells through autocrine signaling. Virus-positive MCC cell lines and tumors showed ncNF-κB pathway activation and SASP gene expression, and the inhibition of ncNF-κB signaling prevented VP-MCC cell growth in vitro and in xenografts. We identify MCPyV sT–induced ncNF-κB signaling as an essential tumorigenic pathway in MCC. Implications: This work is the first to identify the activation of ncNF-κB signaling by any polyomavirus and its critical role in MCC tumorigenesis.

Characterization of ALTO-encoding circular RNAs expressed by Merkel cell polyomavirus and trichodysplasia spinulosa polyomavirus

“…Notably, a number of pathways previously implicated in MCPyV infection were amongst the most highly upregulated genes. NFKB2, RELB, and NFKBIA were all strongly induced, which suggested an activation of non-canonical, and inhibition of canonical, NF-κB signaling, which has been reported to be a critical function of the MCPyV sT antigen [ 46 ]. HSP70-6/HSP70B, an ortholog of HSP70, which has been shown to bind to MCPyV LT and promote MCPyV replication, was also significantly upregulated [ 47 ].…”

“…Characteristics of these cell lines have been described previously [ 8 , 63 ]. WaGa shows high expression of MCPyV early region genes [ 8 , 46 ]. HeLa, HEK293 and HEK293T cells (ATCC) were cultured in DMEM and additional 10% FBS.…”

“…RNA-Seq was performed as previously described [ 46 ]. 293T cells (6cm plate) were transected with 1 μg of the indicated plasmid.…”

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

“…Similarly, PDTC is generally identified as an NFκB inhibitor, but has also been reported to affect proteasomal activity and, depending on conditions, act as a pro- or anti-oxidant 31 , 32 . MCV polyoma virus small T antigen has variously been reported to inhibit 33 or activate 34 canonical NFκB or to activate non-canonical NFκB 35 . Effects of both PDTC and disulfiram were equipotent against VP-MCC and VN-MCC cells and may be due to their other targets.…”

From Merkel Cell Polyomavirus Infection to Merkel Cell Carcinoma Oncogenesis

“…MCPyV sT drives cellular transformation in rat fibroblasts by promoting hyperphosphorylation of eukaryotic translation-initiation factor 4E-binding protein (4E-BP1) (Shuda et al, 2011 ; Wu et al, 2015 ; Velasquez et al, 2016 ). The LSD domain was also linked to activation of the non-canonical NF-κB pathway, induction of a senescence associated secretory phenotype (SASP), and enhanced MCC cell proliferation (Zhao et al, 2020 ). Besides those functions attributed to the LSD domain, numerous other oncogenic functions have been ascribed to MCPyV sT.…”