Effect of phosphodiesterase antagonists on glucocorticoid mediated growth inhibition in murine skin cell lines

The effects of two cyclic nucleotide phosphodiesterase type 4 (PDE4) inhibitors on proliferation of cell lines representing different stages of mouse skin tumorigenesis were studied. Skin papillomas and carcinomas become resistant to the growth inhibition by glucocorticoids. Their control of cellular functions is mediated by a well-known transcription factor, glucocorticoid receptor. The primary aim of the present study was to determine whether the PDE4 inhibitors, that raise intracellular cAMP levels, can increase the sensitivity of mouse skin papillomas and carcinomas to the glucocorticoids. We sought to establish the effect of cAMP signaling on the glucocorticoid receptor function using well-known model representing non-tumorigenic keratinocyte cell line (3PC), papilloma (MT1/2) and squamous cell carcinoma cell line (Ca3/7). These cells were treated with the glucocorticoid fluocinolone acetonide (FA) alone or in concert with PDE4 inhibitors – rolipram or YM976. Results of our study revealed that both PDE4 inhibitors may increase the sensitivity of transformed cell lines to the growth inhibitory effect of FA. In the transformed cell lines, changes in the viability of cells were accompanied by an increase in mRNA level of two negative regulators of the cell cycle – p21 and p27 proteins. Co-treatment with PDE4 inhibitors and FA caused inhibition of an endogenous glucocorticoid-responsive gene (MT-1) expression. Thus, the PDE4 inhibitors exerted a differential effect on non-transformed and transformed keratinocytes and on glucocorticoid receptor signal transduction. These findings warrant further studies to clarify the mechanism by which PDE4 inhibitors modulate glucocorticoid receptor signal transduction in transformed cells.