Stimulation of IGF-binding protein-1 secretion by AMP-activated protein kinase

Insulin-like growth factor-binding protein-1 (IGFBP-1) is stimulated during intensive exercise and in catabolic conditions to very high concentrations, which are not completely explained by known regulators such as insulin and glucocorticoids. The role of AMP-activated protein kinase (AMPK), an important signaling system in lipid and carbohydrate metabolism, in regulating IGFBP-1 was studied in H4-II-E rat hepatoma cells. Arsenic(III) oxide and 5-aminoimidazole-4-carboxamide-riboside (AICAR) were used as activators. AICAR (150 microM) stimulated IGFBP-1 secretion twofold during a 5-h incubation (P = 0.002). Insulin (100 ng/ml) inhibited IGFBP-1 by 80% (P < 0.001), but this was completely abolished in the presence of 150 microM AICAR. The effect of dexamethasone in stimulating IGFBP-1 threefold was additive to the effect of AICAR (P < 0.001) and, in the presence of AICAR, was incompletely inhibited by insulin. In conclusion AMPK is identified as a novel regulatory pathway for IGFBP-1, stimulating secretion and blocking the inhibitory effect of insulin.     

Dexamethasone inhibits feedback regulation of the mitogenic activity of tumor necrosis factor, interleukin-1, and epidermal growth factor in human fibroblasts

Tumor necrosis factor (TNF), interleukin-1 (IL-1), and epidermal growth factor (EGF) were mitogenic for human diploid FS-4 fibroblasts. Dexamethasone amplified the growth-stimulating action of all three agents. Amplification of the growth-stimulating action was maximal when dexamethasone was added along with TNF or EGF; no amplification was seen if the addition of dexamethasone was delayed for more than 3 hr. Prolonged simultaneous treatment with TNF and EGF resulted in less growth stimulation than treatment with EGF alone. Dexamethasone abolished this apparent antagonistic interaction between TNF and EGF. Dexamethasone also inhibited the antiviral action of TNF against encephalomyocarditis (EMC) virus in FS-4 cells. TNF and IL-1 increased the steady state level of interferon (IFN)-beta 2 mRNA but failed to induce detectable levels of IFN-beta 1 mRNA in FS-4 cells. Dexamethasone inhibited the increase of IFN-beta 2 mRNA levels by IL-1 or TNF. Inhibition of IFN-beta synthesis is likely to be responsible for the inhibition of the TNF-induced antiviral state by dexamethasone. Since IFNs suppress cell growth, inhibition of endogenous IFN-beta synthesis may also be responsible for the amplification by dexamethasone of the growth-stimulating action of TNF and IL-1. Amplification of the mitogenic action of EGF by dexamethasone appears to be mediated by different mechanism.     

Dexamethasone inhibits receptor-activated phosphoinositide breakdown in rat basophilic leukemia (RBL-2H3) cells

The activation of rat basophilic leukemia cells for histamine release is accompanied by Ca2+ influx and arachidonic acid release. IgE receptor but not A23187 ionophore stimulation of these cells also resulted in phosphoinositide breakdown. In these experiments, the culture of these cells with dexamethasone inhibited IgE- and ionophore-mediated histamine release. The concentration for 50% of maximal inhibition was 12 nM, and prolonged exposure to the drug was required, with maximal effect observed in 8 to 15 hr. The inhibitory effect of dexamethasone was reversible (t1/2 for recovery was 16 hr). Dexamethasone blocked the IgE-mediated 45Ca2+ influx and the release of [14C]-arachidonic acid (IC50 of 1 nM and 10 nM respectively). Dexamethasone inhibited the IgE receptor-mediated phosphoinositide breakdown (IC50 of 5 nM). It also decreased arachidonic acid release after A23187 stimulation demonstrating an effect on phospholipase A2. Therefore, exposure of the cells to dexamethasone results in the inhibition of both phospholipase A2 and phospholipase C pathways of arachidonic acid generation.     

Short- and longer-term regulation of very-low-density lipoprotein secretion by insulin, dexamethasone and lipogenic substrates in cultured hepatocytes. A biphasic effect of insulin.

1. The precise effects of insulin, dexamethasone and lipogenic precursors on the secretion of very-low-density lipoprotein (VLDL) cholesterol and triacylglycerol were dependent on the age of the culture and the duration of treatment. 2. The rates of secretion of triacylglycerol and cholesterol gradually declined with the age of the culture, although there was no detectable decrease within a given 24 h period. 3. Between 4 h and 24 h after cell preparation, insulin inhibited VLDL secretion. Inhibition was maximal between 6 and 12 h after addition of insulin. Longer-term treatment (24-48 h) with insulin resulted in a stimulation of VLDL secretion. This effect was less apparent when dexamethasone was simultaneously present. The secretion of triacylglycerol and cholesteryl ester was more sensitive to insulin than was that of non-esterified cholesterol. 4. Dexamethasone alone stimulated the secretion of VLDL to an extent which increased with the age of the culture. In young cultures (up to 24 h old) dexamethasone protected against inhibition by insulin, but was ineffective in older cultures. 5. In young cultures the stimulatory effect of lipogenic precursors (lactate and pyruvate) on the secretion of triacylglycerol and cholesterol was more pronounced in the presence of dexamethasone. In cultures older than 24 h, the secretion of these components was less sensitive to short-term stimulation by lactate and pyruvate.     

Insulin action in normal and protein kinase C-deficient rat hepatoma cells. Effects on protein phosphorylation, protein kinase activities, and ornithine decarboxylase activities and messenger ribonucleic acid levels

Insulin and tumor-promoting phorbol esters such as phorbol 12-myristate 13-acetate (PMA) share some biological activities in normal hepatocytes and in some lines of cultured hepatoma cells. To investigate the possibility that some of these common effects might involve a common pathway, we examined the effects of insulin and PMA on several biological processes in normal and protein kinase C-deficient H4IIE rat hepatoma cells. Protein kinase C deficiency was achieved by preincubating the cells in high concentrations of PMA, and was documented by direct enzyme measurement in soluble and particulate cellular fractions, and by analysis of immunoreactive protein kinase C concentrations in whole cellular homogenates. In the protein kinase C-deficient cells, the following actions of insulin remained at near normal levels: stimulated phosphorylation of the ribosomal protein S6; activation of a ribosomal S6 protein kinase; and increases in ornithine decarboxylase activity and mRNA accumulation. PMA stimulated all of these responses in the normal cells, but none of them in the PMA-pretreated cells. We conclude that insulin can exert some of its actions in a normal manner in protein kinase C-deficient H4IIE hepatoma cells (ATCC CRL 1548) and that some of the actions insulin holds in common with PMA may be due to common activation of one or more distal pathways. A candidate for such a distal step is activation of the ribosomal protein S6 protein kinase.     

Glucocorticoid-induced insulin resistance in vitro: inhibition of insulin-stimulated methylaminoisobutyric acid uptake

We have previously developed an in vitro model for the induction of insulin resistance by glucocorticoids using 3T3-L1 fat cells (Grunfeld, Baird, Van Obberghen and Kahn 1981). In this model, glucocorticoid treatment was shown to decrease insulin binding and inhibit the acute stimulation of deoxyglucose uptake by insulin. We now extend the findings in this model to examine insulin stimulated methylaminoisobutyric acid (MAIB) uptake, an event whose expression requires m-RNA and protein synthesis and takes many hours. As previously seen with insulin stimulation of deoxyglucose uptake, one day of exposure to dexamethasone had little effect on insulin stimulation of MAIB uptake. Significant inhibition of insulin-stimulated MAIB uptake was seen after 2 days of exposure, and 3 days were required for the maximum effect of the glucocorticoid. The half-maximal concentration of dexamethasone required for inhibition was 1.6 nM. Exposure to dexamethasone produced a 57% decrease in the maximal response to insulin and a small but consistant shift in the sensitivity to insulin. As seen with the acute effects of insulin, the major locus of glucocorticoid action in inhibiting insulin stimulated MAIB uptake is also after the binding of insulin to its receptor. These data indicate that the inhibitory effects of glucocorticoids on insulin action in fat cells extend to those effects of insulin which require gene expression and are not merely limited to short-term metabolic actions of insulin.     

Interaction between glucocorticoids, 8-bromoadenosine 3',5'-monophosphate, and insulin in regulation of synthesis of carbamoyl-phosphate synthetase I in Reuber hepatoma H-35

Regulation of synthesis of carbamoyl-phosphate synthetase I by glucocorticoids, 8-bromoadenosine 3',5'-monophosphate (8-bromo-cAMP), and insulin was investigated in Reuber hepatoma H-35. By measuring the incorporation of [35S]methionine into carbamoyl-phosphate synthetase I and its precursor, we showed that dexamethasone stimulates the enzyme synthesis approximately fivefold. A detectable stimulation was observed at 1 nM of dexamethasone, half-maximal stimulation at 4 nM, and maximal stimulation above 40 nM. Corticosterone was more effective than dexamethasone both for the minimal concentration needed and for the extent of the stimulation. Hydrocortisone was less effective than dexamethasone. 8-Bromo-cAMP also stimulated the enzyme synthesis at a concentration of 3 mM. The effect of 8-bromo-cAMP was suggested to be additive to the effect of dexamethasone. Physiological concentrations of insulin strongly suppressed the stimulatory effect of dexamethasone on the enzyme synthesis but could not completely counteract the effect of dexamethasone. The half-maximal and maximal effects of insulin were observed at 0.5 nM and 5 nM, respectively. Insulin also counteracted the effect of 8-bromo-cAMP on the enzyme synthesis.     

Effects of dexamethasone on the production of insulin-like growth factor-I and insulin-like growth factor binding proteins in primary cultures of rat hepatocytes.

The effects of dexamethasone (Dex) on insulin-like growth factor (IGF)-I and IGF binding protein (IGFBP)-1 production were investigated in primary cultures of rat hepatocytes. Dex enhanced the secretion of IGFBP-1 as measured by ligand blot analysis but did not show any prominent effect on immunoreactive IGF-I secretion. EC50 of Dex on IGFBP-1 secretion was calculated to be 3 x 10(-8) M. The content of IGFBP-1 mRNA in the cells increased greatly in the presence of Dex but the IGF-I mRNA content did not change significantly under the same conditions. Insulin showed the opposite effect of Dex by decreasing the production of IGFBP-1 and the cellular content of IGFBP-1 mRNA. This effect of insulin was observed also with Dex in the medium. These results show that the gene expression of IGF-I and IGFBP-1 is differently regulated by glucocorticoids and insulin in primary cultures of rat hepatocytes. The results most possibly explain the in vivo effects of glucocorticoids and insulin in regulation of IGF-I and IGFBP-1 production by liver.     

Stimulation of adipocyte phospholipid methyltransferase activity by phorbol 12-myristate 13-acetate. Differential regulation of phospholipid methyltransferase and lipolysis.

The present studies demonstrate that treatment of rat adipocytes with the phorbol ester phorbol 12-myristate 13-acetate (PMA) causes a dose-dependent stimulation of phospholipid methyltransferase (PLMT) activity. The stimulatory effect of PMA was not additive with that of isoprenaline or forskolin. The sensitivity of stimulated PLMT activity to inhibition by insulin, however, was decreased in the presence of PMA. The inhibitory effect of a maximal concentration of insulin on PLMT was unchanged in the presence of PMA. In contrast with the effects on PLMT, the lipolytic response of adipocytes to isoprenaline and the anti-lipolytic response to insulin were unaffected by PMA. These data suggest that PLMT is, whereas hormone-sensitive lipase is not, an intracellular target for the action of PMA. The lack of effect of PMA on lipolysis suggests that PLMT and hormone-sensitive lipase can be regulated by separate mechanisms. Furthermore, phorbol esters do not interfere in the regulatory pathway whereby insulin inhibits PMLT or lipolysis.