The relative ability of human papillomavirus type 6 and human papillomavirus type 16 E7 proteins to transactivate E2F-responsive elements is promoter- and cell-dependent.

The human papillomavirus 16 E7 protein (HPV 16 E7) transactivates the adenovirus E2 promoter (AdE2) by altering interactions between retinoblastoma (pRb) family members and the transcription factor E2F. To understand factors limiting the oncogenic potential of HPV 6, the relative ability of HPV 6 E7 as compared to HPV 16 E7 to transactivate the AdE2 promoter was determined. In primary baby rat kidney cells and human foreskin keratinocytes, HPV 16 E7 transactivated the AdE2 promoter to a greater extent than HPV 6 E7, consistent with the observation that HPV 16 E7 binds pRb with greater affinity. HPV 6 E7 gain of function correlated with increasing the affinity of the HPV 6 E7 pRb binding site of conserved region 2 (CR2). In keratinocytes, in contrast to the AdE2 promoter, the abilities of the two E7 proteins to transactivate the B-myb promoter, a promoter regulated by E2F bound to p107/p130, were comparable. Introducing a negative charge into the N-terminus (CR1) and a high affinity pRb binding site into CR2 of HPV 6 E7 resulted in a transactivator with greater activity than HPV 16 E7 for both the AdE2 and B-myb promoters. Both of the promoters were negatively regulated by E2F and transactivation by the E7 proteins required an intact E2F site. In C33-A cells, which contain a mutated pRb, the two E7 proteins had comparable transactivating activity on both the AdE2 and B-myb promoters. The data are consistent with the interpretation that HPV 16 E7 affects interactions of pRb and p107/p130 with the E2F transcription factor, whereas HPV 6 E7 only affects interactions of p107/p130.