ROLE OF PHOSPHOLIPASE-C AND PHOSPHOLIPASE-D SIGNALING PATHWAYS IN VASOPRESSIN-DEPENDENT MYOGENIC DIFFERENTIATION

Arg(8)-vasopressin (AVP) is a potent inducer of myogenic differentiati on stimulating the expression of myogenic regulatory factors. To under stand the mechanism of its effect on myogenesis, we investigated the e arly signals induced by AVP in myogenic target cells. In the rat skele tal muscle cell line L6, AVP selectively stimulates phosphatidylinosit ol (Ptdlns) and phosphatidylcholine (PtdCho) breakdown, through the ac tivation of phospholipases C and D (PLC, PLD), as shown by the generat ion of lns(1,4,5)P-3 and phosphatidylethanol (PtdEtOW), respectively. AVP induces the biphasic increase of sn-1,2-diacylglycerol (DAC) consi sting in a rapid peak followed by a sustained phase, and the monophasi c generation of phosphatidic acid (PA). Propranolol (a PA phosphatase inhibitor) and Zn2+ (a PLD inhibitor), abolish the sustained phase of DAC generation. Our data indicate that Ptdlns-PLC activity is mainly r esponsible for the rapid phase of AVP-dependent DAC generation, wherea s the sustained phase is dependent upon PtdCho-PLD activity and PA dep hosphorylation, ruling out any significant role of DAG kinase. Modific ations of PA level correlate with parallel changes of PLC activity, in dicating a possible cross-talk between the two signal transduction pat hways in the intact cell. PLD activation is elicited at AVP concentrat ions two orders of magnitude lower than those required for PLC activat ion. The differentiation of L6 myoblasts into multinucleated fibers is stimulated significantly by AVP at concentrations at which PLD, but n ot PLC, is activated. These data provide the first evidence for an imp ortant role of PLD in the mechanism of AVP-induced muscle differentiat ion. (C) 1997 Wiley-Liss, Inc.