DIRECT STIMULATORY EFFECT OF INSULIN-LIKE GROWTH-FACTOR-I ON MONOCYTEAND MACROPHAGE TUMOR-NECROSIS-FACTOR-ALPHA PRODUCTION

GH has been demonstrated to play a physiological role in the priming o f macrophages for tumor necrosis factor-alpha (TNF alpha) synthesis. A lthough evidence has been presented that GH exerts this effect by an i ndirect mechanism, the mediators of GH stimulation of TNF alpha synthe sis have not been identified. Because insulin-like growth factor-I (IG F-I) is a major mediator of many GH effects, in the present study we i nvestigated the direct in vitro effect of this growth factor on macrop hage TNF alpha production. Treatment of murine macrophages with physio logical concentrations of IGF-I (0.13-130 nM) enhanced both basal and lipopolysaccharide-stimulated macrophage TNF alpha release and messeng er RNA levels. Induction of basal TNF alpha production was also observ ed after treatment of the cells with supraphysiological concentrations of insulin (130-1300 nM). Exposure of human monocytes to IGF-I led to a similar increase of basal TNF alpha production and messenger RNA ex pression. Preexposure of macrophages with specific antibodies against IGF-I and IGF-I receptor before IGF-I addition resulted in a complete abrogation of the stimulatory effect of IGF-I on TNF alpha production, indicating that specific binding of IGF-I to its receptor is required for macrophage TNF alpha: induction by IGF-I. In contrast to the stim ulatory effect of IGF-I, neither GH (0.1-10 mu g/ml) nor IGF-II (0.13- 130 nM) enhanced macrophage TNF alpha release in vitro. To assess the role of the tyrosine kinase system in mediating IGF-I-induced basal TN F alpha production, macrophages were preincubated with the specific ty rosine kinase inhibitors, genistein and tyrphostin A9, before IGF-I ex posure. Addition of these compounds resulted in a dose-dependent inhib ition of the stimulatory effect of IGF-I on macrophage TNF alpha relea se, indicating that protein tyrosine kinase activation is required for TNF alpha stimulation by IGF-I. Taken together, these results demonst rate that IGF-I is a monocyte/macrophage activating factor that enhanc es TNF alpha production, and that such effect is mediated via the IGF- I receptor and involves tyrosine kinase activation.