Resistance to UV‐induced apoptosis by β‐HPV5 E6 involves targeting of activated BAK for proteolysis by recruitment of the HERC1 ubiquitin ligase

Abstract

UV exposure is the main etiological agent in the development of non-melanoma skin cancer (NMSC), but mounting evidence suggests a co-factorial role for b-genus HPV types early in tumor initiation or progression. UV damage initiates an apoptotic response, driven at the mitochondrial level by BCL-2 family proteins, that eliminates damaged cells that may accumulate deleterious mutations and acquire tumorigenic properties. BAK is a pro-apoptotic BCL-2 protein that functions ultimately to form pores that permeabilize the mitochondrial outer membrane, thereby committing a cell to death, a process involving changes in BAK phosphorylation and conformation. The E6 protein of b-type HPV5 signals BAK for proteasomal degradation, a function that confers protection from UV-induced apoptosis. We find that HPV5 E6 does not constitutively target BAK for proteolysis, but targets the latter stages of BAK activation, following changes in phosphorylation and conformation. A mutational analysis identified the lysine residue on BAK required for proteolysis, and a functional siRNA screen identified the HECT domain E3 ubiquitin ligase HERC1 as being required for E6-mediated BAK degradation. We show that HERC1 interacts with BAK in E6-expressing cells that have been damaged by UV, and provide evidence that the interaction of HERC1 with BAK requires access to a hydrophobic surface on BAK that binds BH3 domains of BCL-2 proteins. We also show that HERC1 contains a putative BH3 domain that can bind to BAK. These findings reveal a specific and unique mechanism used by the HPV5 E6 protein to target BAK.

Human papillomavirus type 8 oncoproteins E6 and E7 cooperate in downregulation of the cellular checkpoint kinase‐1

“…41,45 Many cellular proteins are known to be targeted for degradation by HPV-E6 utilizing the ubiquitin proteasome pathway. 46,47 Here, we now show that this pathway is not the governing mechanism controlling CHK1 protein levels. Intriguingly, we identified CHK1 expression to be paralleled by expression of the autophagy marker LC3B.…”

Parkin inhibits BAK and BAX apoptotic function by distinct mechanisms during mitophagy

“…The non‐degradative ubiquitination of BAK was supported by the predominantly K11‐linked ubiquitin chains, consistent with the profile of atypical linkages catalysed by Parkin during mitophagy (Cunningham et al , ) and also a recent study reporting that endogenous Parkin does not promote significant proteasomal turnover of mitochondrial substrates (Ordureau et al , ). Ubiquitination of BAK by the E3 ligase HERC1 was shown to promote its proteasomal degradation following irradiation of papillomavirus‐infected cells (Holloway et al , ). Whilst the same residue in BAK was ubiquitinated in these studies, it is likely that the type of chain linkage dictates whether BAK persists, as is the case following Parkin‐mediated ubiquitination, or is degraded, as with HERC1 (Holloway et al , ).…”

Mucosal alpha‐papillomaviruses are not associated with esophageal squamous cell carcinomas: Lack of mechanistic evidence from South Africa, China and Iran and from a world‐wide meta‐analysis

“…Beta‐ and gamma‐papillomaviruses are frequently found both in skin lesions and in healthy skin . Further, the functional data on transforming activity for beta‐papillomaviruses are scarce and limited to HPV5 and HPV38 . Therefore, to substantiate a biological contribution of HPV types (alpha‐, beta‐ or gamma‐) to the transformed phenotype of tumor cells, viral DNA data needs to be corroborated by data on viral activity.…”

“…4 Further, the functional data on transforming activity for beta-papillomaviruses are scarce and limited to HPV5 and HPV38. [75][76][77][78][79][80] Therefore, to substantiate a biological contribution of HPV types (alpha-, beta-or gamma-) to the transformed phenotype of tumor cells, viral DNA data needs to be corroborated by data on viral activity. It should be noted that in contrast to high prevalences for mucosal alpha-papillomaviruses ranging from 23% to 44% previously reported for Africa, China and Iran, 10,11 the most recent studies that also documented significant efforts to prevent potential nucleic acid cross-contamination during tissue sectioning and HPV DNA analysis found <1% of HPV DNA1 ESCC cases.…”