RESISTANCE OF TRANSFORMED MOUSE KERATINOCYTES TO GROWTH-INHIBITION BYGLUCOCORTICOIDS

Glucocorticoid hormones are strong inhibitors of normal keratinocyte p roliferation, but established mouse skin papillomas and carcinomas bec ome resistant to these hormones. The biological effect of glucocortico ids is mediated through a highly specific glucocorticoid receptor (GR) . To study the possible mechanisms of glucocorticoid resistance of tra nsformed mouse keratinocytes, we evaluated GR expression and function in non-tumorigenic (3PC), papilloma-producing (MT1/2 and P1/17), and s quamous cell carcinoma-producing (Ca3/7 and Ca8/29) keratinocyte cell lines and analyzed the DNA sequence of GR in glucocorticoid-sensitive and glucocorticoid-resistant keratinocytes. All transformed keratinocy te cell lines studied appeared to be completely resistant to the growt h inhibition by the glucocorticoid fluocinolone acetonide (FA), wherea s the untransformed cell line 3PC was very sensitive to FA. Despite th e glucocorticoid resistance, all the tumorigenic keratinocyte cell lin es expressed high levels of GR mRNA and protein. Southern blot analysi s and direct sequencing of the DNA-binding domain of the GR gene revea led no significant changes in GR gene structure in transformed keratin ocytes. To test the functional capability of GR, we compared the effec t of FA on the expression of glucocorticoid-responsive genes. FA stron gly induced metallothionein 1 expression in 3PC cells, slightly induce d metallothionein 1 expression in P1/17 and Ca3/7 cells, and did not a ffect its expression in MT1/2 and Ca8/29 cells. These data suggest tha t resistance to the growth inhibition of glucocorticoids is an importa nt feature of tumorigenic keratinocyte cell lines. It is likely that t his hormone-resistant phenotype is a result of alteration of GR functi on but not of GR expression or gene structure. (C) 1997 Wiley-Liss, In c.