Differential cell cycle response of nontumorigenic and tumorigenic human papillomavirus-positive keratinocytes towards transforming growth factor-β₁

Abstract.

Human papillomaviruses (HPVs) are causative agents of a number of malignancies in humans, including cervical cancer. Their tumorigenic potential is linked 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 show here that nontumorigenic and tumorigenic HPV-positive keratinocytes (HPK) exhibit striking differences in the response of cell cycle regulatory genes towards transforming growth factor beta-β₁. Treatment with this agent 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 efficient reduction in viral E6/E7 oncogene expression. This was associated with increased pRb levels, exhibiting sustained hypophosphorylation, and a permanent growth arrest in the G₁ phase of the cell cycle. In contrast, tumorigenic HPKs exhibited only a modest rise in p53 protein levels and a substantially reduced induction of the p15^INK4 and p21^WAF1 genes, which was linked to a lesser degree of viral oncogene repression. In addition, tumorigenic HPKs rapidly resumed cell growth after a transient G₁ arrest, concomitantly with the reappearance of hyperphosphorylated pRb. These results support the notion that the progression of HPV-positive cells to a malignant phenotype is associated with increased resistance to growth inhibition by transforming growth factor-β₁. This is linked in the tumorigenic cells to a lack of persistent G₁ arrest, inefficient induction of several cell cycle control genes involved in growth inhibition, and inefficient repression of the growth-promoting viral E6/E7 oncogenes.