Previous in vivo studies demonstrated significant variations in insulin-like growth factor binding protein-1 (IGFBP-1), IGFBP-2 and IGFBP-4 hepatic mRNAs and/or serum levels depending on the rat thyroid status. In this study we employed cultured hepatocytes from adult rats to demonstrate a possible direct regulation of these genes by tri-iodothyronine (T3). Northern blot analysis revealed that IGFBP-1 and -4 messages were clearly expressed, whereas IGFBP-2 signal was barely detectable. No significant effects on IGFBP-1 mRNA level or on peptide secretion were detected in T3-cultured hepatocytes. In contrast, significant increases in IGFBP-4 mRNA steady-state levels as well as in IGFBP-4 secretion were observed in hepatocytes cultured for 12-24 h in the presence of T3. The T3 effect on IGFBP-4 transcript levels appears to consist of enhanced gene transcription and is independent of ongoing protein synthesis. The T3-increased IGFBP-4 expression in cultured hepatocytes is consistent with our in vivo experiments demonstrating an increase in hepatic IGFBP-4 mRNA and serum IGFBP-4 levels in T3-treated rats. Furthermore, significant decreases in hepatic IGFBP-4 message and serum IGFBP-4 levels were observed in hypothyroid rats compared with euthyroid controls. Our data establish an important direct role for thyroid hormone in regulating IGFBP-4 expression and consequently IGF activity.

The role of carnitine on Sertoli cell metabolism was investigated. Carnitine effects on Sertoli cell lipid metabolism were evaluated by measuring the intracellular levels of non-esterified fatty acids (NEFA) and ketone bodies. The concentration of NEFA in Sertoli cell cultured in the presence of carnitine is significantly reduced as compared to control, while, no significant changes were observed in the concentration of ketone bodies. The functional parameters evaluated to assess the influence of carnitine on Sertoli cell carbohydrate metabolism, i.e., lactate and pyruvate production, lactate dehydrogenase activity and hexose transport, were all significantly increased following carnitine in vitro supplementation. Thus, carnitine appears to drive Sertoli cell intermediary metabolism in an intimately interrelated way, stimulating both fatty acid breakdown and glycolysis. Our results indicate that Sertoli cells are a possible target for a widespread metabolic action of carnitine and strongly support the involvement of carnitine in the regulation of Sertoli cell functions which are related with germ cell "nutrition", convincingly suggesting a direct influence of the compound at testis level.

Previously, we have observed that epidermal growth factor (EGF), a potent mitogen for cultured hepatocytes, stimulates the production of IGF-I and IGF-binding proteins (IGFBPs) by cultured hepatocytes from adult rats. This study was undertaken to investigate the possibility that other growth factors of hepatic origin could specifically be involved in the regulation of IGF-I and IGFBP expression. The effects of transforming growth factor-a (TGF-a), through EGF receptors to induce a mitogenic response, and transforming growth factor-b 1 (TGF-b 1 ), produced by non-parenchymal liver cells and able to inhibit hepatocyte proliferation in vivo and in culture, have been studied in cultured adult rat hepatocytes.Our results demonstrate that TGF-a and TGF-b 1 significantly stimulate IGF-I and IGFBP secretion by cultured hepatocytes but no change in the abundance of IGF-I and IGFBP mRNAs was observed with respect to controls. Cycloheximide is able to inhibit both basal and TGF-stimulated release of IGF-I and a similar effect was elicited by octreotide, the somatostatin analog, known to directly affect hepatic IGF-I gene expression.Our findings show the role of the liver in the secretion of IGF-I and IGFBPs, not only under endocrine and nutritional control but also under autocrine and paracrine control.

Direct effects of PRL on Sertoli cell proliferation were investigated by using Sertoli cell primary cultures isolated from both prepubertal rat and porcine testes. PRL metabolic effects were analyzed in rat Sertoli cell primary cultures. Exposure to physiological doses of PRL resulted in a significant increase (+50-60%) of basal DNA synthesis, as reflected by the pattern of [3H] thymidine incorporation during culture; significant increases in lactate secretion (about 50%), androgen binding protein (ABP) production (about 30%) and basal protein synthesis (25-30%), as reflected in the augmented [14C] valine incorporation, were also evident. Taken together, our present findings, indicating significant effects of PRL on Sertoli cell proliferation and metabolism, demonstrate that Sertoli cells are a potential target for PRL action at testicular level during pre-pubertal development.

Recently, an increase in gammaglutamyltranspeptidase (GGT) activity and mRNA in liver of hypothyroid rats has been reported. The aim of this study was to verify if triiodothyronine (T3) can directly affect GGT expression in primary cultures of rat hepatocytes. Results obtained from adult rat hepatocytes cultured in serum-free medium demonstrate: 1) a rise in GGT mRNA level magnified by dexamethasone during the maintenance of hepatocytes in culture which parallels the stimulation of GGT activity; 2) a negative effect of T3 on GGT activity of cultured hepatocytes which reflects a specific inhibition of GGT gene expression. The T3 effect on GGT expression in cultured hepatocytes is in line with previous observations on hypothyroid rat liver suggesting an important role for thyroid hormone in maintaining the differentiated adult liver phenotype in the rat.