Abstract
The cytokines are the putative mediators of the catabolic reaction that accompanies infection and trauma. Evidence suggests that their catabolic actions are indirect and potentially mediated through changes in hormonal axis such as the hypothalamo-pituitary-adrenal axis. Insulin-like growth factor I (IGF-I) is a GH-dependent growth factor that regulates the protein metabolism. To determine whether cytokines can directly inhibit the production of IGF-I by the liver, we investigated the regulation of IGF-I gene expression by interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α (10 ng/ml) in a model of rat primary cultured hepatocytes. Hepatocytes were isolated by liver collagenase perfusion and cultured on Matrigel 48 h before experiments. Each experiment was performed in at least three different animals. In the absence of GH, IL-1β and TNF-α did not affect the IGF-I messenger RNA (mRNA) basal levels, whereas IL-6 increased it by a factor of 2.5 after 24 h (P < 0.05). GH (500 ng/ml) alone stimulated the IGF-I gene expression markedly (5- to 10-fold increase) after 24 h (P < 0.001). IL-1β, and TNF-α to a lesser extent, dramatically inhibited the IGF-I mRNA response to GH (IL-1β: −82%, P < 0.001 and TNF-α: −47%, P < 0.01). The half-maximal inhibition of the IGF-I mRNA response to GH was observed for a concentration of IL-1β between 0.1 and 1 ng/ml. Moreover, IL-1β abolished the IL-6-induced IGF-I mRNA response. In contrast, IL-6 did not impair the IGF-I mRNA response to GH. To determine the potential role of the GH receptor (GHR) and the GH-binding protein (GHBP) in this GH resistance, we assessed the GHR and GHBP mRNAs response to these cytokines. GH alone did not affect the GHR/GHBP mRNA levels. IL-1β markedly decreased the GHR and GHBP mRNA levels (respectively, −68% and −60%, P < 0.05). Neither TNF-α nor IL-6 affected the GHR/GHBP gene expression. In conclusion, our results show that IL-1β, and TNF-α to a lesser extent, blunt the IGF-I mRNA response to GH. The resistance to GH induced by IL-1β might be mediated by a decrease of GH receptors, as suggested by the marked reduction of GHR mRNA. These findings suggest that decreased circulating IGF-I, in response to infection and trauma, may be caused by a direct effect of cytokines at the hepatocyte level.
Mechanisms of lymphocyte apoptosis at tick-borne encephalitis
