Synthesis of collagen by chondrocytes in suspension culture: Modulation by calcium, 3′:5′-cyclic AMP, and prostaglandins

Abstract

Rabbit articular chondrocytes synthesize type II collagen [3.alpha.1(II)] in vivo and type I collagen [2.alpha.1(I) .cntdot. .alpha.2] in monolayer cultures. In suspension culture the nature of phenotype depends on extracellular Ca2+. The relationship of Ca2+ and cyclic[c]AMP in regulation of collagen synthesis was investigated. In suspension culture, cAMP levels of chondrocytes increase by 2-3-fold then reach basal values regardless of the presence or absence of extracellular Ca2+. The cells synthesize primarily type II collagen in the absence of CaCl2 in the medium and type I collagen in medium containing 1.8 mM CaCl2. If CaCl2 is added when intracellular cAMP levels are low, the phenotype is type I collagen. These observations minimize the role of cAMP as a 2nd messenger in the chondrocyte culture system. Increasing endogenous cAMP with a phosphodiesterase inhibitor [3-isobutyl-1-methylxanthine] or adding exogenous dibutyryl-cAMP leads the cells to synthesize type I collagen, although this effect is significantly less pronounced if the medium contains ethylene glycol bis(.beta.-aminoethyl ether)-N,N''-tetraacetic acid (EGTA). Increased concentrations of cAMP may mobilize the intracellular Ca pools and activate the cells to switch their phenotypic expression. Prostaglandins E2 and F2.alpha., thought to be involved in rheumatoid arthritis and bone resorption, have no significant effect on cAMP content of chondrocytes and alter their collagen phenotype to a small extent.