Keratinocyte growth conditions modulate telomerase expression, senescence, and immortalization by human papillomavirus type 16 E6 and E7 oncogenes

Abstract

Keratinocytes undergo a finite number of divisions in culture before senescing. The high-risk human papillomavirus (HPV) E6 and E7 oncoproteins prevent keratinocyte senescence and extend life span by interacting with p53 and pRb, respectively, and also by transcriptionally activating the human telomerase reverse transcriptase (hTERT) gene, which encodes the catalytic subunit of telomerase. We correlated telomerase activity, which was measured by a highly sensitive and quantitative real-time quantitative-PCR-based telomeric repeat amplification protocol assay, with telomere length and the expression of hTERT, p16(INK4a), and HPV-16 E6 and E7 in keratinocytes grown under two culture conditions. Primary human foreskin keratinocytes (HFKs) cultured in keratinocyte serum-free medium on plastic senesced at approximately 13 population doublings (PDs). Senescence was accompanied by a dramatic increase in p16(INK4A) levels, a marked decrease in telomerase, and only a slight decrease in telomere length. In contrast, HFKs grown in F medium on 3T3 fibroblast feeders maintained elevated telomerase and lower levels of p16(INK4A) for 60 PDs before senescing approximately 81 PDs. E7 was shown to act synergistically with E6 to super induce telomerase expression in a feeder environment-dependent manner. Culture of both HFKs and HFK/16E6E7 cells in the feeder environment significantly increased the number of doublings that these cells could undergo without a significant reduction in telomere length. Finally, transfer of either HFKs or HFK/16E6E7 cells from plastic to the feeder fibroblast culture system significantly induced telomerase activity. This induction in telomerase was fully reversible and largely attributable to the medium. Our results suggest that the influence of keratinocyte culture conditions on the expression of telomerase and p16(INK4A) and on telomere maintenance is responsible, at least partially, for the differences in proliferative capacity, senescence, and HPV-keratinocyte interactions seen in the two culture systems.