Thyroid hormone upregulates MDM 2 in rat type I fibre: Implications for skeletal muscle mass regulation

Aim Based upon a microarray assay, we have identified that triiodothyronine (T3) upregulates MDM 2 gene expression in the rat skeletal muscle. As MDM 2 protein is an E3 ligase, we hypothesized that this enzyme could play a role in T3 effects on skeletal muscle mass control. Methods To test our hypothesis, male rats (2 months old) were randomly assigned into the following groups: intact controls, treated with 20 physiological doses of T3 for 0.5, 1 and 7 days, or with 5, 20 and 50 physiological doses of T3 for 7 days. For in vitro experiments, myotubes and C2C12 cells were treated with T3 for 3 days. Results After validation of the microarray finding throughout RT ‐ PCR and confirmation that T3 induces increases in MDM 2 protein expression in a dose‐dependent manner, we observed that MDM 2 was upregulated by T3 exclusively in fibre type I. Moreover, detailed histological evaluation showed that MDM 2 overexpression distributes punctiformily along the cross section of the fibre and also inside nuclei. MDM 2 colocalizes with PAX 7 in control muscle and T3 downregulates this myogenic factor. Pharmacological inhibition of MDM 2 in cultured myotubes caused a severe decrease in their diameter (~35%, P < .001 vs Control), enhancing the effect of T3 (from ~12% to ~35%, P < .001) alone upon myotube diameter and mRNA levels of atrogenes. Finally, we observed that FOXO 3 ( MDM 2 target) is kept outside the nucleus under T3 stimulation. Conclusion Our results indicate that MDM 2 might be involved in the pro‐trophic effects of T3 in skeletal muscle.