EFFECT OF TUMOR-NECROSIS-FACTOR-ALPHA ON THE EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR-I AND INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN-4 INMOUSE OSTEOBLASTS

Tumor necrosis factor-alpha (TNF-alpha) is a cytokine produced by immu ne cells, which has multiple effects on bone cells and is therefore th ought to mediate changes in bone metabolism occurring during inflammat ion. In the present study we have investigated the effect of TNF-alpha on the secretion of insulin-like growth factor I (IGF-I) and IGF bind ing protein 4 (IGFBP-4) by clonal mouse osteoblasts (MC3T3-E1 cells) u sing subconfluent in vitro cultures and serum-free conditions. The IGF -I was determined by radioimmunoassay under conditions eliminating the interference of IGFBPs. Treatment of MC3T3-E1 cultures with TNF-alpha for 24 h resulted in a dose-dependent decrease in IGF-I secretion (ma ximally to 34 +/- 9.7% of control with 60 pmol/l TNF-alpha: mean +/- s o). The TNF-cu treatment also resulted in decreased messenger ribonucl eic acid (mRNA) levels of IGF-I at 4 and 24 h, as detected by Northern analysis. Because basal secretion of IGFBPs is very low in MC3T3-E1 c ells, effects of TNF-alpha on IGFBP secretion were studied in cultures in which IGFBP-4 expression was increased by calcitriol (1,25(OH)(2)D -3) treatment. The presence of TNF-alpha (600 pmol/l) inhibited this c alcitriol-induced stimulation of IGFBP-4 mRNA levels from 4 h onwards, with complete inhibition of the calcitriol effect occurring at 24 h, We also observed a dose-dependent inhibition of calcitriol-stimulated IGFBP-4 secretion into the culture medium (as detected by Western liga nd blot), with the maximal inhibition occurring with 600 pmol/l TFN-al pha to 25 +/- 7% of control levels. These TNF-alpha effects were not p revented by indomethacin treatment, suggesting that they are not depen dent on prostaglandins. The DNA synthesis was reduced to 62 +/- 8% of the control value by 600 pmol/l TNF-alpha. We conclude that secretion of IGFs and IGFBPs by osteoblasts can be modulated by TNF-alpha, which in turn may be responsible for some of the known effects of TNF-alpha on osteoblastic cell proliferation and differentiation.