Myocardial infarction damages heart tissue both during the initial ischemia and the subsequent reperfusion of tissues with oxygen. Corticosteroids can protect cardiac tissue from damage following a heart attack, but the mechanisms by which corticosteroids are cardioprotective have not been clear and negative side effects such as reduced wound healing may result from their use. Corticosteroids exert a variety of actions through binding to the glucocorticoid receptor (GR), a member of the steroid hormone receptor gene family. GR acts as a ligand-dependent transcription factor, but some of the cardioprotective effects mediated by GR-bound corticosteroids are non-transcriptional in nature. Glucocorticoids are commonly used as anti-inflammatory drugs in a variety of conditions, and some of their effects in the heart result from inhibition of the inflammatory response of heart tissue to ischemia and reperfusion. NF-kB is a transcription factor involved in signaling by inflammatory factors such as TNF, and is repressed by glucocorticoids. Annexin-1 is a calcium-dependent phospholipid binding protein whose expression is induced by corticosteroids and inhibits the infiltration of neutrophils into tissue, blocking reperfusion-induced inflammatory heart damage. A non-transcriptional cardioprotective effect of glucocorticoids is activation of NO production by endothelial nitric oxide synthase (eNOS). Glucocorticoids activate eNOS through activation of PI3 kinase and AKT and increased NO produced by eNOS can diffuse into surrounding tissues to prevent clotting and cause vasodilation. The beta-2 adrenergic receptor can also activate PI3 kinase and may synergize with glucocorticoids in this pathway. The atrial natriuretic factor (ANF) is a peptide secreted by the atrial wall in response to increased atrial pressure such as occurs during cardiac failure and to be decreased by myocardial infarction. Glucocorticoids increase the secretion of ANF by acting at the transcriptional level to increase expression of the pro-ANF peptide, perhaps inducing increased water excretion in the kidneys to reduce blood volume and reduce atrial pressure. The exploration of glucorticoid responses may allow the identification of compounds that retain the cardioprotective activities but do not inhibit wound healing. Alternative mechanisms of eNOS activation may also provide a route to identify cardioprotective drugs.
