Local Aldosterone Synthesis in Skeletal Muscle

OBJECTIVE To characterize mineralocorticoid receptor (MR) regulation in normal vs. dystrophic skeletal muscle and elucidate the mechanism behind the efficacy of MR antagonists in dystrophic muscle. BACKGROUND Our lab has recently shown MR is present in skeletal muscle fibers and represents a novel therapeutic target for treating both the skeletal and cardiac muscle pathologies of Duchenne muscular dystrophy (DMD). However, the underlying mechanisms and functional role of MR activation in skeletal muscle is still unclear. Excessive MR activation is known to contribute to the pathogenesis of heart failure, presumably due to increased levels of its natural ligand aldosterone. However, clinical trials have shown that addition of an MR antagonist to standard‐of‐care regimens (ACE inhibitors like lisinopril) improves morbidity and mortality in patients, even when circulating aldosterone levels are within the physiological range. There is now a growing body of evidence that shows aldosterone, which was previously thought to only be synthesized in the adrenal cortex can also be synthesized and metabolized locally in extra‐adrenal tissues, including the heart, particularly in pathological states. DESIGN/METHODS Aldosterone synthase (CYP11B2) and other key enzymes in the aldosterone synthesis pathway were tested at the protein level in mouse skeletal muscles and myogenic cultures from normal and dystrophic models, and in normal human myogenic cultures using western blot analysis and immunofluorescence. Sequencing analysis of reverse transcribed PCR (RT‐PCR) of RNA from mouse skeletal muscle and human myotubes was used to confirm expression. RESULTS CYP11B2 is present in mouse skeletal muscle homogenates and up‐regulated in muscles from dystrophic mice ( Figure 1). CYP11B2 does not appear to be coming from the skeletal muscle itself; immunofluorescence of dystrophic mouse muscles suggests this enzyme may be present in a subset of immune cells, which are present at much higher levels in dystrophic muscles ( Figure 1). We have also found type 2 11‐beta hydroxysteroid dehydrogenase (11β‐HSD2), a key regulator of MR specificity, is present in normal human myotubes and is up‐regulated in dystrophic muscle ( Figure 2). CONCLUSIONS CYP11B2 and 11β‐HSD2 levels are significantly increased in dystrophic muscle, suggesting MR activation is in excess of the physiological need. 11β‐HSD2 is expressed within the muscle fibers but CYP11B2 appears to be coming from a population of immune cells up‐regulated in dystrophic tissue due to increased inflammation and muscle degeneration. Support or Funding Information National Institutes of Health [R01 NS082868]