Inhibition of topoisomerase IIα expression by transforming growth factor-β1 is abrogated by the papillomavirus E7 protein

Transforming growth factor-beta (TGF-beta) protects normal cells from etoposide-induced cell death, yet the mechanism has remained speculative. Studies have shown that etoposide modifies the activity of the topoisomerase II alpha (topo II alpha) enzyme, thereby causing DNA damage and inducing cell death. Expression of topo II alpha is necessary for etoposide-induced cell death, and peak expression of topo II alpha normally occurs during the G(2) phase of the cell cycle. We predicted that by arresting growth in the G(1) phase, TGF-beta1 would prevent the induction of topo II alpha expression that normally occurs subsequent to the G(1)-S transition, thereby protecting cells from etoposide-induced cell death. Accordingly, we hypothesized that the inhibition of topo II alpha expression by TGF-beta1 would be dependent on the ability of TGF-beta1 to arrest cell cycle progression in G(1). Using mink lung epithelial cells (Mv1Lu), we found that TGF-beta1 decreases topo II alpha mRNA expression, and the decrease occurs as cells begin to accumulate in the G(1) phase of the cell cycle. Topo II alpha protein expression decreases subsequent to the fall in mRNA expression. In contrast, topo II alpha expression is not affected by TGF-beta1 in cells that fail to undergo G(1) arrest because of inactivation of the retinoblastoma tumor suppressor protein (pRb) by the papillomavirus type 16 E7 protein. Our studies suggest that inhibition of topo II alpha by TGF-beta1 is the principal mechanism that protects mink lung epithelial cells (Mv1Lu) from etoposide-induced toxicity. Furthermore, the inhibition of topo II alpha protein expression by TGF-beta1 is dependent on pRb-mediated cell cycle arrest, suggesting that TGF-beta1 will not reduce the sensitivity of pRb-deficient cancers to etoposide.