RETINOIC ACID INDUCES A SHIFT IN THE ENERGETIC STATE OF HYPERTROPHIC CHONDROCYTES

In the epiphyseal growth plate, chondrocyte maturation is accompanied by dramatic alterations in energy metabolism. To explore the relations hip between these two events, we used retinoic acid (RA) to promote ch ondrocyte maturation in culture. The specific question that was addres sed was, does RA treatment of cultured chondrocytes in vitro induce a change in energy status similar to that seen in hypertrophic chondrocy tes in vivo. Maturing chondrocytes isolated from the cephalic region o f day 18 chick embryo sterna were allowed to grow for 7-14 days in mon olayer until confluent and then treated with 10-300 nM RA. Immature ch ondrocytes from the caudal region of sternum were grown in parallel an d served as control cells for the study. We found that in maturing cep halic cell cultures, RA had a rapid and profound effect on oxidative m etabolism. The retinoid caused a reduction in the energy charge ratio (ECR) and the ATP/ADP ratio and a sharp decrease in cell ATP levels. M aximum inhibition was observed when the RA concentration was 10-35 nM. Compared with the adenine nucleotides, creatine phosphate levels were decreased to a lesser extent by RA, although there was substantial in hibition of creatine kinase activity. We expected to find a compensato ry elevation in glycolytic activities; however, the lactate levels in the medium of the treated cells indicated that anaerobic glycolysis wa s depressed. In contrast to the cephalic chondrocytes, when caudal cel l cultures were treated with RA, lactate formation was stimulated and there were minimal effects on oxidative metabolism. To determine the m echanism of inhibition of glycolysis, we measured the activity of pyru vate kinase in RA-treated cephalic cells. We found that the activity o f this key glycolytic enzyme was profoundly and rapidly inhibited by t he retinoid. The unique energy state of the RA-treated chondrocytes wa s termed the minimal energy state. This condition may be expected to i nfluence activities associated with plasma membrane ion pumps and gene transcription. Both these factors would promote chondrocyte hypertrop hy and lead to terminal differentiation.