E7 protein of human papilloma virus-16 induces degradation of retinoblastoma protein through the ubiquitin-proteasome pathway.

TLDR

Rb protein regulates cell cycle entry, and its loss by mutations or viral oncoprotein interaction drives oncogenic transformation. E7 expression promotes Rb degradation through the ubiquitin‑proteasome system, as demonstrated by pulse‑chase analysis, temperature‑sensitive E1 mutation experiments, and proteasome inhibitor rescue. HPV‑16 E7 immortalizes mammary epithelial cells and markedly reduces Rb levels, with proteasome inhibition restoring Rb and confirming E7‑induced ubiquitin‑dependent degradation, revealing a second oncogenic pathway beyond Rb sequestration.

Abstract

Rb protein is a critical regulator of entry into the cell cycle, and loss of Rb function by deletions, mutations, or interaction with DNA viral oncoproteins leads to oncogenic transformation. We have shown that the human papilloma virus (HPV)-16 E7 gene is sufficient to induce the immortalization of mammary epithelial cells (MECs). Surprisingly, the steady-state level of Rb protein in these immortal cells was drastically decreased. Here, we used pulse-chase analysis to show that the in vivo loss of Rb protein in E7-immortalized MECs is a consequence of enhanced degradation. Expression of HPV16 E7 in a cell line with a temperature-sensitive mutation in the E1 enzyme of the ubiquitin pathway demonstrated that degradation of Rb was ubiquitin dependent. Treatment of E7-immortalized MECs with aldehyde inhibitors of proteasome-associated proteases led to a marked stabilization of Rb protein, particularly the hypophosphorylated form. Taken together, our results provide evidence for HPV-16 E7-induced enhanced degradation of Rb protein via a ubiquitin-proteasome pathway and suggest a second mechanism of oncogenic transformation by E7, in addition to its previously identified ability to sequester Rb from E2F. Our analyses also show that normal Rb levels are regulated by the ubiquitin-proteasome degradation pathway.