Inhibition of Bax activity is crucial for the antiapoptotic function of the human papillomavirus E6 oncoprotein

Abstract

Oncogenic types of human papillomaviruses (HPVs) cause cervical cancer in humans. The antiapoptotic viral E6 gene has been identified as a key factor for maintaining the viability of HPV-positive cancer cells. Although E6 has the potential to modulate many apoptosis regulators, the crucial apoptotic pathway blocked by endogenous E6 in cervical cancer cells remained unknown. Using RNA interference (RNAi), here, we show that targeted inhibition of E6 expression in cervical cancer cells leads to the transcriptional stimulation of the PUMA promoter, in a p53-dependent manner. This is linked to the activation and translocation of Bax to the mitochondrial membrane, cytochrome c release into the cytosol, and activation of caspase-3, in a PUMA-dependent manner. Moreover, inhibition of Bax expression by RNAi efficiently reverts the apoptotic phenotype, which results from inhibition of E6 expression. Thus, interference with the p53/PUMA/ Bax cascade is crucial for the antiapoptotic function of the viral E6 oncogene in HPV-positive cancer cells.

Specific betapapillomaviruses associated with squamous cell carcinoma of the skin inhibit UVB-induced apoptosis of primary human keratinocytes

“…Although we have not tested this, it is most likely that E6 is responsible for this effect, possibly by promoting degradation of Bax, analogous to what has been described for Bak and HPV5 E6 . Downregulation of Bax by HPV16 and HPV18 E6 in PHK and HeLa cells, respectively, has been reported by others (Magal et al, 2005;Vogt et al, 2006), but was not observed in the early-passage HPV16 E6E7-expressing PHKs we used. In contrast, as discussed earlier, we could already observe active Bax in the unirradiated HPV16 E6E7-containing cells, which may explain the ready increase in caspase activity in HPV16-transduced cells immediately after UVB exposure (Fig.…”

“…Previously, most studies have shown that HPV16 E6 deregulates pro-apoptotic proteins, resulting in a reduction in the rate of apoptosis (Alfandari et al, 1999;Filippova et al, 2002Filippova et al, , 2004Magal et al, 2005;Thomas & Banks, 1998Vogt et al, 2006). None of these studies, however, used UV irradiation as a trigger of apoptosis.…”

Human Papilloma Virus 16 E6 RNA Interference Enhances Cisplatin and Death Receptor-Mediated Apoptosis in Human Cervical Carcinoma Cells

“…Prolonged p53 stabilization was achieved by cisplatin in E6-suppressed HeLa cells (Koivusalo et al, 2005;Putral et al, 2005). Moreover, prolonged p53 activation resulted in the activation of the intrinsic pathway in E6-supressed HeLa cells (Vogt et al, 2006), which may occur in E6-suppressed SiHa cells as well. The decreased p21 levels after cisplatin treatment of E6-suppressed SiHa cells may affect apoptosis, because p21 not only mediates p53-induced cell cycle arrest but also can suppress apoptosis after exposure to DNA-damaging agents, such as cisplatin, and death receptor-mediated apoptosis (Koster et al, 2010).…”

Activation of the Enhancer of Zeste Homologue 2 Gene by the Human Papillomavirus E7 Oncoprotein

“…siRNAs either were expressed from vector pSuper or were chemically synthesized (Dharmacon Research). siRNAs were generated against the following target sequences, as previously described (5,18): si16E6/E7, 5 ¶-CCGGACAGAGCCCAUUACA-3 ¶ (HPV16 nucleotides 700-718); si18E6/E7, 5 ¶-CCACAACGUCACACAAUGU-3 ¶ (HPV18 nucleotides 755-773); si16E6, 5 ¶-ACCGUUGUGUGAUUUGUUA-3 ¶ (HPV16 nucleotides 385-403); and si18E6, 5 ¶-CUAACACUGGGUUAUACAA-3 ¶ (HPV18 nucleotides 385-403). EZH2 targeting siRNA siEZH2-1 (13) and vector pSuper-p53 (19) have been characterized in detail before.…”