Peroxisome proliferator-activated receptor gamma ligands differentially modulate muscle cell differentiation and MyoD gene expression via peroxisome proliferator-activated receptor gamma-dependent and -independent pathways

The effects of distinct classes of peroxisome proliferator-activated recept or gamma (PPAR gamma) ligands on myogenesis and Myo gene expression were ex amined in mouse skeletal muscle C2C12 myoblasts. Treatment of C2C12 cells w ith the PPAR gamma ligand, 15-deoxy-Delta (12,14)-prostaglandin J2 (15d-PGJ 2), repressed morphologically defined myogenesis and reduced endogenous mRN A levels of the myogenic differentiation markers MyoD, myogenin, and alpha -actin. In contrast, two synthetic PPAR gamma ligands, L-805645 and ciglita zone, exhibited no effects. In transient transfection assays, 15d-PGJ2 spec ifically inhibited the expression of a MyoD promoter-luciferase reporter ge ne (MyoDLuc) in a cell type- and promoter-specific manner, indicating that 15d-PGJ2 functions in part by repressing MyoD gene transcription. The inhib ition of MyoD gene expression by 15d-PGJ2 is mediated by the distal region of the MyoD gene promoter. PPAR gamma on its own also inhibited MyoDLuc exp ression and further augmented the 15d-PGJ2 response. In contrast, L-805645 and ciglitazone did not inhibit MyoDLuc expression on their own but did so in the presence of ectopically expressed PPAR gamma. Interestingly, a trans dominant inhibitor of PPAR gamma (hPPAR gamma2 Delta 500) had no effect on the 15d-PGJ2-dependent repression of MyoDLuc expression but overcame L-8056 45/PPAR gamma -dependent repression. Finally, saturating concentrations of L-805645, which did not affect myogenesis, failed to ablate 15d-PGJ2-mediat ed repression of the myogenic program. Thus, distinct PPAR gamma ligands ma y repress MyoD gene expression through PPAR gamma -dependent and -independe nt pathways, and 15d-PGJ2 can inhibit the myogenic program independent of i ts cognate receptor, PPAR gamma.