Differential cell cycle response of nontumorigenic and tumorigenic human papillomavirus-positive keratinocytes towards transforming growth factor-beta(1)

Human papillomaviruses (HPVs) are causative agents of a number of malignanc ies in humans, including cervical cancer. Their tumorigenic potential is li nked to expression of the viral E6/E7 genes which can interfere with normal cell cycle control by targeting p53, p21(WAF1), p27(KIP1), and pRb. We sho w here that nontumorigenic and tumorigenic HPV-positive keratinocytes (HPK) exhibit striking differences in the response of cell cycle regulatory gene s towards transforming growth factor beta-beta(1). Treatment with this agen t led to an efficient induction of p53 and the growth-inhibitory p15(INK4) and p21(WAF1) genes only in nontumorigenic HPKs and was linked to an effici ent reduction in viral E6/E7 oncogene expression. This was associated with increased pRb levels, exhibiting sustained hypophosphorylation, and a perma nent growth arrest in the G(1) phase of the cell cycle. In contrast, tumori genic HPKs exhibited only a modest rise in p53 protein levels and a substan tially reduced induction of the p15(INK4) and p21(WAF1) genes, which was li nked to a lesser degree of viral oncogene repression. In addition, tumorige nic HPKs rapidly resumed cell growth after a transient G(1) arrest, concomi tantly with the reappearance of hyperphosphorylated pRb. These results supp ort the notion that the progression of HPV-positive cells to a malignant ph enotype is associated with increased resistance to growth inhibition by tra nsforming growth factor-beta(1). This is linked in the tumorigenic cells to a lack of persistent G(1) arrest, inefficient induction of several cell cy cle control genes involved in growth inhibition, and inefficient repression of the growth-promoting viral E6/E7 oncogenes.