Kaposi's sarcoma-associated herpesvirus viral protein kinase phosphorylates extracellular signal-regulated kinase and activates MAPK/ERK signaling pathway

Open reading frame 36 (ORF36) of Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a serine/threonine-type viral protein kinase (vPK). Previous studies have examined the functions of KSHV vPK; however, its role in the activation of extracellular signal-regulated kinase (ERK1/2) has not yet been described to date. Using HEK 293 cell lines, we performed a human phospho-kinase array analysis to screen for MAPK signaling pathways kinases that are activated by KSHV vPK. In addition, we investigated the regulator protein phosphorylation of up/downstream ERK1/2 pathway; nuclear translocation of phosphorylated ERK1/2; and regulation of transcription factor, inflammatory cytokine, and pro-/anti-apoptotic factor by KSHV vPK transfection. Here, we demonstrated that KSHV vPK activates ERK1/2 signaling pathway and plays an important role in the activation of MAPK/ERK signaling pathway.

KSHV vPK inhibits Wnt signaling via preventing interactions between β-catenin and TCF4

Viral factors interact with host cellular proteins, leading to dysregulation of signaling pathways. The Wnt pathway is known to participate in embryonic development and oncogenesis under dysregulation conditions. A downstream factor of the Wnt signaling pathway, β-catenin, activates T-cell factor (TCF)-dependent transcription, which contributes to cell proliferation and tumorigenesis. In this study, we demonstrated that viral protein kinase (vPK) encoded by Kaposi's sarcoma-associated herpesvirus inhibits the Wnt signaling pathway without affecting nuclear localization and expression of β-catenin. Coimmunoprecipitation and chromatin immunoprecipitation assays revealed that vPK interacts with β-catenin, reducing the binding affinity on TCF binding regions as well as interactions of β-catenin with TCF4. Overexpression of vPK led to reduced mRNA expression of cyclin D1, a well-known transcriptional product of Wnt signaling, suggesting that vPK effectively regulates the host signaling pathway through direct interactions with cellular proteins.