Regulation of Vascular Calcification by Growth Hormone-Releasing Hormone and its Agonists
Rationale: Vascular calcification (VC) is a marker of the severity of atherosclerotic disease. Hormones play important roles in regulating calcification; estrogen and parathyroid hormones exert opposing effects, the former alleviating VC and the latter exacerbating it. So far no treatment strategies have been developed to regulate clinical VC.
Objective: To investigate the effect of growth hormone-releasing hormone (GHRH) and its agonist (GHRH-A) on the blocking of VC in a mouse model.
Methods and Results: Young adult osteoprotegerin deficient (OPG-/-) mice were given daily subcutaneous injections of GHRH-A (MR409) for 4 weeks. Significant reductions in calcification of the aortas of MR409 treated mice were paralleled by markedly lower alkaline phosphatase (ALP) activity and a dramatic reduction in the expression of transcription factors including the osteogenic marker gene Runx2 and its downstream factors, osteonectin and osteocalcin. The mechanism of action of GHRH-A was dissected in smooth muscle cells (SMCs) isolated from human and mouse aortas. Calcification of SMCs induced by osteogenic medium (OM) was inhibited in the presence of GHRH or MR409, as evidenced by reduced ALP activity and Runx2 expression. Inhibition of calcification by MR409 was partially reversed by MIA602, a GHRH antagonist, or a GHRH receptor selective siRNA. Treatment with MR409 induced elevated cytosolic cAMP and its target, protein kinase A (PKA) which in turn blocked NADPH oxidase activity and reduced production of reactive oxygen species (ROS), thus blocking the phosphorylation of NFκB (p65), a key intermediate in the RANKL-Runx2/ALP osteogenesis program. A PKA-selective siRNA or the chemical inhibitor H89 abolished these beneficial effects of MR409.
Conclusions: GHRH-A controls osteogenesis in SMCs by targeting cross talk between PKA and NFκB (p65) and through the suppression of ROS production that induces the Runx2 gene and ALP. Inflammation-mediated osteogenesis is thereby blocked. GHRH-A may represent a new pharmacological strategy to regulate VC.
- growth hormone releasing hormone
- smooth muscle cell
- reactive oxygen species
- Received November 17, 2017.
- Revision received March 20, 2018.
- Accepted April 2, 2018.