Dual posttranscriptional targets of retinoic acid-induced gene expression

Retinoic acid-induced differentiation of the pre-osteoblastic cell line, UM R 201, is associated with a marked increase in the proficiency of posttrans criptional nuclear processing of alkaline phosphatase mRNA. In this study w e attempted to correlate the posttranscriptional actions of retinoic acid w ith changes in phosphorylation, or abundance of spliceosome components, or both. Treatment with retinoic acid for periods of less than or equal to 4 h resulted in dephosphorylation of nuclear U1 70K protein without affecting its abundance. Peptide mapping showed that U1 70K dephosphorylation was rel ated to the disappearance of one specific phosphopeptide out of four major U1 70K phosphopeptides. A twofold decrease in mRNA expression of an isoform of alternative spl icing factor that inhibits splicing was also observed o ver the same period. Tumor necrosis factor-alpha, which enhances the posttr anscriptional action of retinoic acid, reduced U170K mRNA expression, while an inhibition of retinoic acid action by transforming growth factor-p was associated with a marked increase in U1 70K mRNA levels. Our results draw a ttention to the complex interactions between short- and long-term alteratio ns in the abundance and functional status of U1 70K, as well as SR proteins by growth and/or differentiation factors in the regulation of spliceosome formation and function. J. Cell. Biochem. 72:411-422, 1999. (C) 1999 Wiley- Liss, inc.