GLUCOCORTICOIDS INCREASE OSTEOPONTIN EXPRESSION IN CARDIAC MYOCYTES AND MICROVASCULAR ENDOTHELIAL-CELLS - ROLE IN REGULATION OF INDUCIBLE NITRIC-OXIDE SYNTHASE

In heart muscle, the cytokine-inducible isoform of nitric oxide syntha se (NOS2) is expressed in both cardiac myocytes and microvascular endo thelial cells (CMEC). mRNA levels for both NOS2 and for osteopontin, a multifunctional extracellular matrix phosphoprotein containing an RGD integrin binding domain, are increased in cardiac muscle following in traperitoneal injection of adult rats with lipopolysaccharide. In vitr o, interleukin- 1-beta and interferon-gamma increased osteopontin mRNA levels in CMEC as well as NOS2 expression in both CMEC and cardiac my ocytes. However, osteopontin mRNA levels in heart muscle in vivo, and in cardiac myocytes and CMEC in vitro, also are increased 10-30-fold b y the synthetic glucocorticoid dexamethasone, an agent that suppresses cytokine induction of NOS2 in both cell types. The hexapeptide GRGDSP , which interrupts binding of RGD-containing proteins to cell surface integrins, increased NOS2 mRNA, while a synthetic osteopontin peptide analogue decreased NOS2 mRNA and protein levels in both cytokine-pretr eated cardiac myocytes and CMEC cultures. Also, transfection with a fu ll-length antisense-osteopontin cDNA in cytokine-pretreated CMEC decre ased endogenous osteopontin mRNA and increased NOS2 mRNA levels. These results suggest that osteopontin could regulate the location and exte nt of NOS2 induction in the heart. Increased expression of osteopontin also may be one mechanism by which glucocorticoids suppress NOS2 acti vity in cardiac myocytes and microvascular endothelial cells.