Synthesis of collagen by chondrocytes in suspension culture: modulation by calcium, 3':5'-cyclic AMP, and prostaglandins.

Rabbit articular chondrocytes synthesize type II collagen [3alpha(1)(II)] in vivo and type I collagen [2alpha(1)(I).alpha(2)] in monolayer cultures. In suspension culture the nature of phenotype depends on extracellular Ca(2+). The relationship of Ca(2+) and 3':5'-cyclic AMP (cAMP) in regulation of collagen synthesis has been investigated. In suspension culture, cAMP levels of chondrocytes increase by 2- to 3-fold and then reach basal values regardless of the presence or absence of extracellular Ca(2+). The cells, however, synthesize primarily type II collagen in the absence of CaCl(2) in the medium and type I collagen in medium containing 1.8 mM CaCl(2). If CaCl(2) is added when intracellular cAMP levels are low, the phenotype is type I collagen. These observations minimize the role of cAMP as a second messenger in the chondrocyte culture system. Increasing endogenous cAMP with a phosphodiesterase inhibitor 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 calcium pools and activate the cells to switch their phenotypic expression. Prostaglandins E(2) and F(2)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.