SODIUM-BUTYRATE INHIBITS THE PHOSPHORYLATION OF THE RETINOBLASTOMA GENE-PRODUCT IN MOUSE FIBROBLASTS BY A TRANSCRIPTION-DEPENDENT MECHANISM

In the chemically transformed mouse fibroblasts BP-A31, the retinoblas toma protein (pRb) is hypophosphorylated at quiescence and becomes hyp erphosphorylated after approximately 6 h of serum stimulation. Phospho rylation of pRb was blocked if sodium butyrate was added together with serum or within 3 h afterwards. Actinomycin D added 3 h after serum s timulation did not prevent pRb phosphorylation, but it reversed the in hibitory effect of butyrate. These observations indicate that sodium b utyrate acts by turning on the expression of gene(s) coding for protei ns which prevent the accumulation of hyperphosphorylated pRb. Such but yrate-induced inhibitor(s) may interfere with the phosphorylation of p Rb by cyclin-dependent kinases. Treatment of quiescent BP-A31 cells wi th serum in the presence of sodium butyrate has led to an increased ce ll content of the Waf1/CIP1 mRNA (coding for a cyclin-dependent kinase inhibitory protein) compared with serum alone, suggesting a possible role of p21Waf1/CIP1. In contrast, the mitogen activated protein kinas e (enzyme which has been shown to phosphorylate pRb) was constitutivel y active in BP-A31 cells, and its activity was not significantly affec ted by a less than or equal to 3 h incubation with sodium butyrate. (C ) 1996 Wiley-Liss, Inc.