Expression and regulation of corticotropin-releasing factor receptor type 2β in developing and mature mouse skeletal muscle.

Corticotropin-releasing factor receptor type 2 (CRFR2) is highly expressed in skeletal muscle (SM) tissue where it is suggested to inhibit interactions between insulin signaling pathway components affecting whole-body glucose homeostasis. However, little is known about factors regulating SM CRFR2 expression. Here, we demonstrate the exclusive expression of CRFR2, and not CRFR1, in mature SM tissue using RT-PCR and ribonuclease protection assays and report a differential expression of CRF receptors during C2C12 myogenic differentiation. Whereas C2C12 myoblasts exclusively express CRFR1, the C2C12 myotubes solely express CRFR2. Using cAMP luciferase assays and calcium mobilization measurements, we further demonstrate the functionality of these differentially expressed receptors. Using luciferase reporter assays we show a differential activation of CRFR promoters during myogenic differentiation. Transfections with different fragments of the 5'-flanking region of the mCRFR2β gene fused to a luciferase reporter gene show a promoter-dependent expression of the reporter gene and reveal the importance of the myocyte enhancer factor 2 consensus sequence located at the 3'-proximal region of CRFR2β promoter. Furthermore, we demonstrate that CRFR2 gene transcription in the mature mouse is stimulated by both high-fat diet and chronic variable stress conditions. Performing a whole-genome expression microarray analysis of SM tissues obtained from CRFR2-null mice or wild-type littermates revealed a robust reduction in retinol-binding protein 4 expression levels, an adipokine whose serum levels are elevated in insulin-resistant states. In correlation with the SM CRFR2β levels, the SM retinol-binding protein 4 levels were also elevated in mice subjected to high-fat diet and chronic variable stress conditions. The current findings further position the SM CRFR2 pathways as a relevant physiological system that may affect the known reciprocal relationship between psychological and physiological challenges and the metabolic syndrome.