RETINOIC ACID AND CYCLIC-AMP SYNERGISTICALLY INDUCE THE EXPRESSION OFLIVER BONE KIDNEY-TYPE ALKALINE-PHOSPHATASE GENE IN L929 FIBROBLASTICCELLS

In L929 mouse fibroblastic cells, liver/bone/kidney type alkaline phos phatase (L/B/K-ALP) enzymic activity is induced by all-trans-retinoic acid at concentrations between 10(-6) and 10(-5) M. At lower concentra tions, retinoic acid is incapable of inducing this enzymic activity pe r se, but increases cyclic AMP (cAMP)-mediated induction. This effect is observed after incubation of the retinoid with dibutyryl cAMP, 8-br omo cAMP or forskolin. The synergism is dependent on the order of addi tion of retinoic acid and the activator of the cAMP pathway. Contempor aneous addition of the two agents, or addition of cAMP prior to retino ic acid (but not addition of retinoic acid before cAMP), is necessary to produce this synergistic interaction. The synergism results in incr eased steady-state levels of L/B/K-ALP mRNA and it is the consequence of increased transcriptional activity of the gene. The expression of t he mouse L/B/K-ALP gene is regulated by the presence of two leader exo ns, 1A and 1B, resulting in the synthesis of two alternatively spliced mRNAs that are different only in part of their 5' untranslated region [Studer, Terao, Gianni and Garattini (1991) Biochem. Biophys. Res. Co mmun. 179, 1352-13601. PCR amplification and nuclear run-on experiment s performed using probes specific for each leader exon demonstrate tha t treatment of these cells with retinoic acid, forskolin or dibutyryl cAMP, and with the combination of the retinoid and one of the cAMP-ele vating agents, leads to the accumulation of nascent and mature L/B/K-A LP mRNA containing exon 1 B. The synergistic induction of the transcri ption of the L/B/K-ALP gene is well correlated with quantitative and q ualitative changes of retinoic-acid-receptor mRNAs mediated by cAMP.