HPV16 E6 and E6AP differentially cooperate to stimulate or augment Wnt signaling

Abstract

The present study investigated the roles of E6 and E6AP in the Wnt pathway. We showed that E6 levels are markedly reduced in cells in which Wnt signaling is activated. Coexpression of wild-type or mutant E6AP (C820A) in Wnt-activated cells stabilized E6 and enhanced Wnt/β-catenin/TCF transcription. Expression of E6AP alone in nonstimulated cells elevated β-catenin level, promoted its nuclear accumulation, and activated β-catenin/TCF transcription. A knockdown of E6AP lowered β-catenin levels. Coexpression with E6 intensified the activities of E6AP. Further experiments proved that E6AP/E6 stabilize β-catenin by protecting it from proteasomal degradation. This function was dependent on the catalytic activity of E6AP, the kinase activity of GSK3β and the susceptibility of β-catenin to GSK3β phosphorylation. Thus, this study identified E6AP as a novel regulator of the Wnt signaling pathway, capable of cooperating with E6 in stimulating or augmenting Wnt/β-catenin signaling, thereby possibly contributing to HPV carcinogenesis.

The autism-linked UBE3A T485A mutant E3 ubiquitin ligase activates the Wnt/β-catenin pathway by inhibiting the proteasome

“…The phosphomimetic UBE3A T485E mutant, which impairs UBE3A ubiquitin ligase activity, was much less effective at stimulating Wnt pathway activation. Consistent with a previous study (40), UBE3A-LD did not activate the pathway ( Fig. 2A and 2B), indicating that UBE3A ubiquitin ligase activity is re-UBE3A activates Wnt signaling quired to stimulate Wnt pathway activation.…”

“…S1E) (55,56), and Wnt signaling regulates proliferation and differentiation of neuronal progenitors (57,58). Our study, and the work of others (40)(41)(42), firmly places UBE3A within the Wnt signaling pathway, and provides fundamental new insights into how excess UBE3A could impair brain development and increase risk for autism.…”

“…As part of a genome-wide small interfering RNA screen, we found that Wnt pathway activation is highly sensitive to knockdown of specific proteasomal subunits (39) (and see new analyses of these data below). Moreover, others found that wild-type (WT) UBE3A can stimulate Wnt pathway activation and protect β-catenin from proteasomal degradation, with these activities dependent on the ubiquitin ligase activity of UBE3A (40)(41)(42).…”

“…T485A enhances Wnt signaling in a cellcontext dependent manner. WT UBE3A, but not ligase-dead (LD) UBE3A, was previously found to stimulate Wnt reporter gene expression in HEK293T cells and to do so independent of Wnt ligand (40,42). Given that a significant number of autism-linked de novo mutations are found in genes associated with the Wnt pathway (14,17), we sought to determine if the autism-linked UBE3A T485A mutation, which disables phosphorylation control and hyperactivates ubiquitin ligase activity (6), had an equal or greater effect on Wnt pathway activation.…”

The Wnt pathway: a key network in cell signalling dysregulated by viruses

“…Rampias further demonstrated that repressed expression of HPV16 E6 and E7 reduced nuclear β‐catenin levels in HPV‐positive cells, whereas expression of these oncogenes enhanced β‐catenin‐TCF/LEF‐mediated transcription, suggesting a critical role of HPV16 E6 and E7 in Wnt signalling activation. Lichtig further demonstrated that HPV16 E6 in cooperation with E3 ubiquitin ligase E6AP increased β‐catenin expression and enhanced β‐catenin‐TCF transcription .…”