RETINOIC ACID STIMULATES MATRIX CALCIFICATION AND INITIATES TYPE-I COLLAGEN-SYNTHESIS IN PRIMARY CULTURES OF AVIAN WEIGHT-BEARING GROWTH-PLATE CHONDROCYTES

The effect of retinoic acid (RA) on primary cultures of growth plate c hondrocytes obtained from weight-bearing joints was examined. Chondroc ytes were isolated from the tibial epiphysis of 6- to 8-week-old broil er-strain chickens and cultured in either serum-containing or serum-fr ee media. RA was administered at low levels either transiently or cont inuously after the cells had become established in culture. Effects of RA on cellular protein levels, alkaline phosphatase (AP) activity, sy nthesis of proteoglycan (PG), matrix calcification, cellular morpholog y, synthesis of tissue-specific types of collagen, and level of matrix metalloproteinase (MMP) activity were explored. RA treatment generall y increased AP activity, and stimulated mineral deposition, especially if present continuously. RA also caused a shift in cell morphology fr om spherical/polygonal to spindle-like. This occurred in conjunction w ith a change in the type of collagen synthesized: type X and II collag ens were decreased, while synthesis of type I collagen was increased. There was also a marked increase in the activity of MMP. Contrasting e ffects of continuous RA treatment on cellular protein levels were seen : they were enhanced in serum-containing media, but decreased in serum -free HL-1 media. Levels of RA as low as 10 nM significantly inhibited PG synthesis and caused depletion in the levels of PG in the medium a nd cell-matrix layer. Thus, in these appendicular chondrocytes, RA sup pressed chondrocytic (PC, cartilage-specific collagens) and enhanced o steoblastic phenotype (cell morphology, type I collagen, alkaline phos phatase, and mineralization). (C) 1997 Wiley-Liss, Inc.