Extracellular glucose influences osteoblast differentiation and c-jun expression

Insulin dependent diabetes mellitus, marked by high blood glucose levels an d no insulin secretion, is associated with decreased bone mass and increase d fracture rates. Analysis of bone histology suggests that osteoblast pheno type and function are influenced by diabetes. To determine if elevated extr acellular glucose levels could directly influence osteoblast phenotype we t reated mouse osteoblasts, MC3T3-E1 cells, with 22 mM glucose and analyzed o steoblast gene expression. Collagen I mRNA levels significantly increased w hile osteocalcin mRNA levels decreased 24 h after the addition of glucose. Expression of other genes, actin, osteopontin, and histone H4, was unaffect ed. Effects on collagen I expression were seen as early as 1 h after treatm ent. c-jun, an AP-1 transcription factor involved in the regulation of oste oblast gene expression and growth, was also modulated by glucose. Specifica lly, an increase in c-jun expression was found at 1 h and maintained for 24 h following glucose treatment. Treatment of osteoblasts with an equal conc entration of mannitol completely mimicked glucose treatment effects on coll agen I and c-jun expression, demonstrating that osmotic stress rather than glucose metabolism is responsible for the effects on osteoblast gene expres sion and phenotype. Additional studies using staurosporine and Ro-31-8220 d emonstrate that protein kinase C is required for the glucose up regulation of collagen I and c-jun. Taken together, our results demonstrate that osteo blasts respond to increasing extracellular glucose concentration through an osmotic response pathway that is dependent upon protein kinase C activity and results in upregulation of c-jun and modulation of collagen I and osteo calcin expression. (C) 2000 Wiley-Liss, Inc.