Resistance of transformed mouse keratinocytes to growth inhibition by glucocorticoids

Glucocorticoid hormones are strong inhibitors of normal keratinocyte proliferation, but established mouse skin papillomas and carcinomas become resistant to these hormones. The biological effect of glucocorticoids is mediated through a highly specific glucocorticoid receptor (GR). To study the possible mechanisms of glucocorticoid resistance of transformed mouse keratinocytes, we evaluated GR expression and function in non-tumorigenic (3PC), papilloma-producing (MT 1/4 and P1/17), and squamous 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 keratinocyte cell lines studied appeared to be completely resistant to the growth inhibition by the glucocorticoid fluocinolone acetonide (FA), whereas the untransformed cell line 3PC was very sensitive to FA. Despite the glucocorticoid resistance, all the tumorigenic keratinocyte cell lines expressed high levels of GR mRNA and protein. Southern blot analysis and direct sequencing of the DNA-binding domain of the GR gene revealed no significant changes in GR gene structure in transformed keratinocytes. To test the functional capability of GR, we compared the effect of FA on the expression of glucocorticoid-responsive genes. FA strongly induced metallothionein 1 expression in 3PC cells, slightly induced metallothionein 1 expression in P1/17 and Ca3/7 cells, and did not affect its expression in MT 1/4 and Ca8/29 cells. These data suggest that resistance to the growth inhibition of glucocorticoids is an important feature of tumorigenic keratinocyte cell lines. It is likely that this hormone-resistant phenotype is a result of alteration of GR function but not of GR expression or gene structure.