Both Rb and E7 are regulated by the ubiquitin proteasome pathway in HPV-containing cervical tumor cells

High-risk human papillomaviruses (HPVs) are etiologically linked to human c ervical and oral cancers. The E6 and E7 oncoproteins encoded by HPV target host cell tumor suppressor proteins. E6 induces proteolysis of p53 through the ubiquitin-proteasome pathway. Recent studies showed that overexpression of E7 caused proteolytic degradation of the tumor suppressor Rb. However, unlike p53, Rb is not regulated by proteolysis in normal cells. In addition , it was unclear whether in its natural context E7 regulates Rb through the ubiquitin-proteasome pathway. Therefore, we sought to determine whether Rb is regulated by the ubiquitin-proteasome pathway in HPV-containing tumor c ells. We carried out a detailed analysis in Caski cells, that are derived f rom HPV-containing cervical cancer tissues. Studies with various protease i nhibitors revealed that Rb is regulated specifically by the ubiquitin-prote asome pathway in HPV-containing cervical tumor cells. Several inhibitors of the 26S proteasome significantly increased the level of Rb in the Caski ce lls. Rb controls cell growth by forming complexes with the E2F-family trans cription factors. Surprisingly, in spite of a significant accumulation of t he hypophosphorylated form of Rb, no Rb/E2F complex was detectable in the p roteasome inhibitor treated cells. Further analysis revealed that there was an increased accumulation of the E7 oncoprotein. We showed that the protea some inhibitors simultaneously blocked the proteolysis of E7 and Rb, sugges ting that E7 is also regulated by the ubiquitin-dependent proteolysis in ce rvical cancer cells. Taken together, this study suggests that targeted inhi bition of Rb proteolysis will be required for restoring Rb function in HPV- containing cervical cancer cells.