The effects of retinoic acid on rat epidermal cells in vitro: changes in patterns of protein phosphorylation in relation to growth and differentiation.

Keratinocytes proliferate, stratify, and differentiate in vitro but if the calcium concentration of the medium is reduced to 0.07 mM Ca2+ (low calcium medium) the cells proliferate but do not stratify or differentiate. Keratinocytes proliferating in low calcium medium synthesized DNA at a higher rate than cultures of stratifying keratinocytes and this correlated with increased phosphorylation of a membrane-associated Mr 23,200 phosphoprotein (pp23) relative to cytosolic phosphoproteins of Mr 26,500 (pp27a and pp27b). In both normal and low calcium medium, all-trans-retinoic acid increased phosphorylation of pp23 relative to that of pp27 and increased DNA synthesis after 12-24 h. These results suggest that the phosphoproteins pp23 and pp27 are cell-cycle regulated and that the changes in phosphorylation were a consequence of a stimulation of cell proliferation by retinoic acid. The half-life of all-trans-retinoic acid in these cultures was about 6 h; increased DNA synthesis and concomitant changes in phosphorylation patterns also resulted from a 6-h pulse of retinoic acid followed by an 18-h washout period.