RETINOIC ACID INDUCES ADULT MUSCLE-CELL DIFFERENTIATION MEDIATED BY THE RETINOIC ACID RECEPTOR-ALPHA

Retinoic acid (RA) is known to induce differentiation in many cell systems. This induction is mediated by nuclear RA receptors (RARs), which act as transactivating factors belonging to the superfamily of steroid/thyroid nuclear receptors. In this study, effects of RA were studied in a mouse myogenic C2 cell line and in primary chicken satellite cells, the myogenic precursor cells in adult muscle. Addition of RA decreased the rate of DNA synthesis in a dose-dependent manner in both cultures. Differentiation was more rapid in cells treated with RA, and these cells exhibited large myotubes after 48 hr of incubation as compared to untreated cells, which were still proliferating. Addition of 10(-8) M RA to C2 cells elevated mRNA levels of myogenin, a skeletal muscle-specific transcription factor. In parallel, activity levels of the muscle-specific creatine kinase were enhanced in the RA-treated cells nearly twofold as compared to the untreated cells. RA treatment of both mouse C2 and chicken satellite cells caused rapid induction of the RAR-alpha mRNA levels. Maximal mRNA levels were observed after 2 to 5 hr followed by a sharp reduction to nearly zero levels at 9 hr. The RAR-alpha mRNA levels augmented in a dose-dependent manner between concentrations of 10(-10) M and 10(-8) M RA, whereas higher concentrations caused mRNA levels to decrease. These results indicate that RA induces differentiation in both adult skeletal muscle primary satellite cells and a myogenic cell line. The rapid and specific induction of RAR-alpha mRNA in these cells upon exposure to RA may suggest that this receptor is the primary target and a mediator of RA.