3,5-diiodothyronine mimics the effect of triiodothyronine on insulin-like growth factor binding protein-4 expression in cultured rat hepatocytes.

We have previously demonstrated that triiodothyronine (T(3)) stimulates hepatic IGFBP-4 expression in rats. Since there is evidence that some of the genes whose expression is regulated by T(3) are also sensitive to 3,5-diiodothyronine (T(2)), we used the adult rat hepatocyte model in primary cultures directly exposed to T(2) to evaluate insulin-like growth factor binding protein-4 (IGFBP-4) expression by Northern and Ligand blot analyses in this study. Our results demonstrate that T(2), like T(3), is able to enhance IGFBP-4 mRNA and protein after 12-24 h of incubation. The potency of the two iodothyronines is comparable as judged by dose-dependence experiments. The T(2)-induced IGFBP-4 increase is independent from ongoing protein synthesis but dependent on active transcription. Since T(3) and T(2) do not affect IGF-I production, it appears that the iodothyronines affect the hepatic IGF system at the IGFBP level. Our data, demonstrating that T(2) mimics the stimulatory effect of T(3) on IGFBP-4 expression by rat hepatocytes, allow us to include IGFBP-4 gene among the genes regulated by the two iodothyronines.

Retinoic acid increases insulin-like growth factor-binding protein-4 expression in cultured rat hepatocytes.

Hepatic insulin-like growth factor binding protein (IGFBP) expression is controlled by diverse factors including thyroid hormone, which enhances IGFBP-4 production in hepatocytes. In the present work, we have investigated whether hepatic IGFBP-4 expression is regulated by retinoic acid (RA), which acts via nuclear receptors belonging to the steroid/thyroid hormone receptor superfamily. Primary cultures of adult rat hepatocytes were incubated with two natural stereoisomers of RA, all-trans RA and 9-cis RA (atRA and 9cRA), and with the synthetic RA receptor (RAR)-selective agonist TTNPB. IGFBP-4 mRNA abundance was measured by Northern blot and protein production was evaluated by Ligand blot on hepatocyte-conditioned culture media. Our results indicate that atRA, 9cRA, and TTNPB increase IGFBP-4 expression by cultured hepatocytes, both at the mRNA and protein level. The RARs play a definite role in this regulation, which is independent from ongoing protein synthesis but dependent on active transcription. AtRA and thyroid hormone act synergistically in increasing hepatic IGFBP-4 expression. Our data establish a role for hormonal factors such as thyronines and retinoids in regulating the hepatic IGF system directly at the IGFBP-4 level.