TRANSFORMATION BY HUMAN PAPILLOMAVIRUS-16 E6 AND E7 - ROLE OF THE INSULIN-LIKE GROWTH-FACTOR-1 RECEPTOR

Human papillomavirus-16 E6 and E7 inactivate the tumor suppressors p53 and pRB, respectively, and cooperate during malignant transformation, but the downstream molecular events remain incompletely understood, U sing fibroblast cell lines derived from mice with a homozygous disrupt ion of the insulin-like growth factor-1 receptor (IGF-1R) gene (R-cell s) and their wild-type (WT) littermates, we have stably transfected pl asmids encoding E6 and E7 proteins and examined their transforming pot ential in these cells, Consistent with previous studies using NIH3T3 c ells, pooled cultures of E7-transfected WT cells readily formed coloni es after suspension in soft agar, In contrast, R- cells were not trans formed by E7, E6 had little transforming activity in WT (WT/E6) or R- (R-/E6) cells, However, transfection of R- cells with E6 plus E7 resul ted in extensive colony formation, Because IGF-1R and E6 appear to be functionally equivalent in this transformation assay and both have bee n implicated in antiapoptotic responses, we investigated the apoptotic responses of the cells after exposure to the potent protein kinase C inhibitor, staurosporine. Compared to WT cells, R- cells were relative ly resistant to staurosporine-induced apoptosis, but susceptibility to staurosporine was decreased in both WT/E6 and R-/E6 cells relative to WT and R- cells transfected with mock vector, respectively, In fibrob last cells from p53 gene knockout mice, transfection with E6 also conf erred relative resistance to staurosporine-induced apoptosis, Our data suggest that transformation by E7 requires the participation of the I GF-1R and that E6 may assist E7 in transforming R- cells by functional ly substituting for the IGP-1R, Because IGF-1R activated by its ligand s (IGF-1 and IGF-2) protects cells from apoptosis, the role of the IGF -1R and E6 in transformation by E7 is probably related to the recruitm ent of survival pathways, In addition, because E6 suppressed apoptosis in p53 knockout cells, our data also suggest that E6 may participate in a p53-independent process that protects cells from apoptosis.