β2-Adrenergic receptors inhibit the expression of collagen type II in growth plate chondrocytes by stimulating the AP-1 factor Jun-B

Abstract

The sympathetic nervous system can regulate both osteoblast and chondrocyte growth and activity through β2-adrenergic receptors (β2-AR). We have shown previously that β2-AR activate both adenylyl cyclase and mitogen-activated protein kinases ERK1/2 in growth plate chondrocytes prepared from ribs of embryonic E18.5 mice. Here we examined β2-AR inhibition of collagen type II (Col II) expression in growth plate chondrocytes and the molecular pathways involved. Stimulation of β2-AR by isoproterenol inhibited Col II mRNA and protein levels by ∼50% beginning at 2 h, with both remaining suppressed over 24 h. This inhibition was blocked by propranolol and inhibitors of either MEK1 or PKA. Isoproterenol stimulated an AP-1-luciferase reporter and increased the expression of AP-1 factors c-Fos, Fra-1, Fra-2, c-Jun, and Jun-B but had no effect on Jun-D. Stimulation of AP-1 activity was blocked by inhibitors of MEK1 or PKA. siRNA inhibition of AP-1 factors showed that depletion of only Jun-B attenuated isoproterenol-mediated inhibition of Col II. Transfection with jun-B or c-fos showed selective inhibition of Col II mRNA and a Col II luciferase reporter construct by jun-B. Isoproterenol as well as jun-B overexpression in the chondrocytes also inhibited the expression of Sox-6 mRNA and protein, and depletion of Jun-B abrogated β2-AR inhibition of Sox-6. Collectively, these findings demonstrate regulation of chondrocyte differentiation through β2-AR mediated by ERK1/2 and PKA stimulation of the AP-1 factor Jun-B that inhibits the expression of Sox-6 and Col II.