INCREASED TURNOVER OF SMALL PROTEOGLYCANS SYNTHESIZED BY HUMAN OSTEOBLASTS DURING CULTIVATION WITH ASCORBATE AND BETA-GLYCEROPHOSPHATE

The small proteoglycan decorin had been localized previously at the d- band in the gap zone of collagen fibrils in nonmineralizing tissues. I n bone matrix this zone is proteoglycan free and is at least in some s pecies the place where mineralization along collagen fibrils starts. T o study the metabolism of the small proteoglycans decorin and biglycan under mineralizing conditions, osteoblasts from human nasal bone were cultured for several weeks in the presence or absence of beta-glycero phosphate and ascorbate. An immediate consequence of the treatment was a reduced expression of decorin, as judged by immune precipitation, w hereas the biosynthesis of biglycan was not affected. Pulse-chase expe riments were performed with osteoblasts embedded in floating type I co llagen gels. In the presence of beta-glycerophosphate and ascorbate, a more rapid turnover of both proteoglycans was noted; the one of bigly can reached statistical significance. Indirect evidence for an enhance d rate of proteoglycan endocytosis was obtained. This effect was not s een in cultured skin fibroblasts. Thus, osteoblasts respond rapidly to mineralizing conditions with alterations of small proteoglycan biosyn thesis and turnover.