GLUCOCORTICOID REGULATION OF AN INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-4 PROTEASE PRODUCED BY A RAT NEURONAL CELL-LINE

Insulin-like growth factor-binding protein-4 (IGFBP-4) is expressed in distinct regions in the rodent brain from the perinatal period into a dulthood and is postulated to modulate the action of the insulin-like growth factors (IGFs) in vivo. This study was initiated to examine the regulation of IGF-binding protein-4 (IGFBP-4) in B104 cells, a rat ne uronal cell line in which IGFBP-4 is the predominant secreted IGFBP. E xposure of B104 monolayer cultures to dexamethasone reduced native IGF BP-4 abundance to less than 10% of that in control medium by 48 h. Imm unoblots showed that the decline in intact 24-kilodalton IGFBP-4 was a ccompanied by an increase in a 16-kilodalton immunoreactive fragment. In addition, IGFBP-4 proteolytic activity in medium was increased afte r exposure of the cells to dexamethasone. The protease was calcium dep endent and appeased to be of the serine protease class, because activi ty could be inhibited by phenylmethylsulfonylfluoride and aprotinin, b ut not antipain, leupeptin, or pepstatin. Although the proteolytically modified IGFBP-4 retained the ability to bind IGFs, the affinities we re approximately 13- and 20-fold lower for IGF-I and IGF-II, respectiv ely. These data indicate that B104 cells produce an IGFBP-4 protease t hat is regulated by glucocorticoids. The actions of this protease redu ce the affinity of IGFBP-4 for the IGFs without abolishing binding. Be cause both the IGFs and glucocorticoids have important roles in brain development, it is possible that some glucocorticoid actions in the br ain could be mediated by proteolysis of IGFBP-4, which, in turn, would alter IGF action.