Advances in Brief E7 Protein of Human Papilloma Virus-16 Induces Degradation of Retinoblastoma Protein through the Ubiquitin-Proteasome Pathway'

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 onco proteins leads to oncogenic transformation. We have shown that the human papifloma virus (HPV)-16 E7 gene is sufficient to induce the immortalization of mammary epithelial eels (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 proteinin E7-immortalizedMECSis a consequenceof enhanced degradation. Expression of HPV16 E7 in a cell line with a temperature sensitive mutation in the El enzyme of the ubiqultin pathway demon strated that degradation of Rb was ubiquitin dependent. Treatment of E7-immortallzed MECs with aldehyde inhibitors of proteasome-associ atM 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 ubiq wtin-proteasome pathway and suggest a second mechanism of oncogemc 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.