1. ¹Istituto di Biologia Cellulare, Consiglio Nazionale delle Ricerche, Monterotondo Scalo (RM), Italy
2. ²Dipartimento di Scienze Motorie e della Salute, Università degli Studi di Cassino, Cassino (FR), Italy

Correspondence: Dr G Falcone and Dr S Alemà, Istituto di Biologia Cellulare, CNR, Via Ramarini 32, 00016 Monterotondo, Italy. E-mails: gfalcone@ibc.cnr.it and alema@ibc.cnr.it

³Current address: Istituto Regina Elena, 00158 Roma, Italy.

Received 13 February 2007; Revised 16 May 2007; Accepted 6 June 2007; Published online 16 July 2007.

Abstract

In this study, we delineate the intracellular signalling pathways modulated by a conditional v-Src tyrosine kinase that lead to unrestrained proliferation and block of differentiation of primary avian myoblasts. By inhibiting Ras–MAPK kinase and phosphatidylinositol 3-kinase with different means, we find that both pathways play crucial roles in controlling v-Src-sustained growth factor and anchorage independence for proliferation. The Ras–MAPK kinase pathway also contributes to block of differentiation independently of cell proliferation since inhibition of this pathway both in proliferating and growth-arrested v-Src-transformed myoblasts induces expression of muscle-specific genes, fusion into multinucleated myotubes and assembly of specialized contractile structures. Importantly, we find that the p38 MAPK pathway is inhibited by v-Src in myoblasts and its forced activation results in growth inhibition and expression of differentiation, indicating p38 MAPK as a critical target of v-Src in growth transformation and myogenic differentiation. Furthermore, we show that downregulation of p38 MAPK activation may occur via Ras–MAPK kinase, thus highlighting a cross-regulation between the two pathways. Finally, we report that the simultaneous inhibition of MAPK kinase and calpain, combined to activation of p38 MAPK, are sufficient to reconstitute largely the differentiation potential of v-Src-transformed myoblasts.