GROWTH-HORMONE AND FIBRONECTIN EXPRESSION IN 3T3 PREADIPOSE CELLS

In the present study we focused on the relationship between GH action and the extracellular matrix in 3T3-F442A preadipose cells. Results from Northern blotting indicated that in serum-free medium, the presence of 2 nM met-human GH down-regulated levels of fibronectin messenger RNA by approximately 40, 60, and 70% as compared with control levels on days 1, 2, and 4, respectively. GH-dependent reduction of levels of collagen alpha-1(I) mRNA expression occurred later and was less pronounced than effects on levels of fibronectin mRNA, suggesting a specificity in the matrix-altering function of GH. Western blot analyses and immunoprecipitation studies revealed that between 2 and 5 days of culture, matrix-associated fibronectin protein was reduced 70 to 90% by GH treatment. Down-regulation of fibronectin protein expression by met-human GH was dose-dependent between 2 and 0.02 nm. The presence of 2 nM insulin or insulin-like growth factor-1 promoted a 30-40% increase in fibronectin levels compared to control cells. The GH-promoted down-regulation of fibronectin expression was eliminated by concomitant addition of insulin. These data demonstrated that GH effects on matrix-associated fibronectin expression were independent of, and in opposition to, effects promoted by insulin and insulin-like growth factor-1. Treatment of culture dishes with fibronectin or collagen inhibited GH-stimulated adipogenesis 50 and 80%, respectively, compared with controls, as judged by levels of glycerol-3-phosphate dehydrogenase activity. Thus, composition of the extracellular matrix was a critical factor in GH-induced adipogenesis of 3T3-F442A fibroblasts. Our results demonstrate that GH action in 3T3 preadipose cells is intimately coupled to the biology of extracellular matrix.