Contrasting interactions between phorbol ester and insulin on the regulation of glycogen synthase activity and p33 mRNA accumulation in rat hepatoma cells

Regulation of Gene33 expression by insulin requires MEK–ERK activation

Gene33 and its human homologue, mitogen inducible gene-6/receptor-associated late transducer (mig-6, RALT), is a 53-kDa soluble protein that was identified as a hepatic gene regulated by glucocorticoids and insulin. Its mRNA is expressed in numerous tissues in addition to the liver. Mitogen inducibility of Gene33 mRNA has been described in several experimental systems. Recent reports have suggested a role for Gene33 in inhibition of proliferation induced by factors that bind to members of the ErbB family of receptors. In the present work, we examine the regulation of Gene33 protein by insulin in hepatoma cells of rat (H4IIE) and human (HepG2/Hep3B) origin. Inhibition of MEK1 significantly inhibited extracellularly regulated kinase (ERK)1/2 activation and insulin-regulated Gene33 transcription and protein levels in H4IIE cells. Inhibition of phosphatidylinositol 3-kinase (PI3-K) activity alone did not significantly alter transcription of Gene33. In Hep3B and HepG2 cells, insulin did not significantly induce either ERK1/2 activation or Gene33 expression. This work suggests that the MEK-ERK, but not the phosphatidylinositol 3-kinase (PI3-K), pathway plays a direct role in insulin regulation of Gene33 transcription and protein expression.

Mechanisms of aluminum fluoride- and insulin-stimulated p33 mRNA accumulation in rat hepatoma cells: involvement of a G protein and kinase action and demonstration of effects on mRNA turnover

The insulin signalling pathway to control nuclear p33 gene expression was examined. An AlF4-stimulated pertussis toxin-insensitive G protein was shown to be involved. The action of AlF4- was completely blocked by deferoxamine. Insulin action was markedly stimulated in the presence of AlF4-. cAMP and diacylglycerol concentrations were examined as possible regulators but no increases were detected. The effects of AlF4- and of insulin were completely inhibited by the general kinase inhibitor H-7. A second calcium calmodulin protein kinase inhibitor, W-7, had no detectable effect. Insulin and AlF4- were shown to stabilize p33 mRNA.

Stimulation of proliferation of HL60 cells by low concentrations of 12-O-tetradecanoylphorbol-13-acetate and its relationship to the mitogenic effects of insulin

The effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on the growth and differentiation of cultured human acute promyelocytic leukemia (HL60) cells have been studied using cells growing in a fully defined medium consisting of RPMI 1640 supplemented with selenium dioxide, insulin, and either transferrin or ferric citrate. High concentrations of TPA (greater than 1 nM) cause the expected inhibition of proliferation and induction of macrophage-like differentiation. In contrast, in cells deprived of insulin, which continue to grow at a slow rate, lower concentrations of TPA stimulate proliferation without inducing differentiation. A TPA concentration between 0.03 and 0.3 nM will approximately double the long-term rate of thymidine incorporation into DNA and the rate of increase in cell density. Low-TPA becomes progressively less able to stimulate further proliferation as the insulin concentration is increased and is virtually without effect on cells stimulated by an optimal insulin concentration (5 micrograms ml-1). Insulin itself stimulates proliferation to a greater extent than low-TPA, increasing the long-term rate of thymidine incorporation and the rate of increase in cell density by three- to fourfold. The ability of higher concentrations of TPA to induce differentiation is independent of the presence of insulin. Low-TPA also stimulates the short-term incorporation of thymidine (during a 1-h pulse after 1 or 2 days incubation) by three- to fourfold, as compared to a sevenfold stimulation by insulin. The proliferation response to low TPA concentrations provides a useful model for dissecting the signalling pathways that control cell proliferation following stimulation by insulin and activators of protein kinase C.

Long-term effects of insulin on the enzyme activity and messenger RNA of glycogen synthase in rat hepatoma H4 cells: an effect of insulin on glycogen synthase mRNA stability

Insulin induced glycogen synthase activity and decreased glycogen synthase mRNA concentrations in rat hepatoma H4 cells. Total enzyme activity measured with glucose 6-phosphate gradually increased during a 24-h insulin incubation. The time course of glycogen synthase activation measured by the activity ratio (low G-6-P/high G-6-P) in response to insulin was biphasic with the first peak at 15 min and the second peak at 4 to 6 h. When cells were incubated with insulin and cycloheximide, the first peak persisted while the second peak was abolished. These data suggest that the first activation peak derives from the classic effect of insulin via dephosphorylation and the second peak from an insulin-induced protein synthesis of a glycogen synthase activator. Ribonuclease protection assays with a cloned rat liver glycogen synthase cDNA were used to quantitate glycogen synthase mRNA. Insulin unexpectedly decreased glycogen synthase mRNA in a time- and a dose-dependent manner. After incubation with the RNA synthesis inhibitor, 5, 6-dichloro-1-beta-D-ribofuranosyl benzimidazole (DRB) without and with insulin, the half time of glycogen synthase mRNA decreased from 6.0 +/- 0.80 to 3.9 +/- 0.75 h, respectively. Nuclear run-off experiments with isolated nuclei showed no change of transcription of glycogen synthase mRNA. These data suggest that insulin in this system affects glycogen synthase mRNA stability rather than transcription.

Phosphofructokinase 2 and glycolysis in HT29 human colon adenocarcinoma cell line. Regulation by insulin and phorbol esters

Kinetic properties of phosphofructokinase 2 (PFK2) and regulation of glycolysis by phorbol 12-myristate 13-acetate (PMA) and insulin were investigated in highly glycolytic HT29 colon cancer cells. PFK2 was found to be inhibited by citrate and, to a lesser extent, by phosphoenolpyruvate and ADP, but to be insensitive to inhibition by sn-glycerol phosphate. From these kinetic data, PFK2 from HT29 cells appears different from the liver form, but resembles somewhat the heart isoenzyme. Fructose 2,6-bisphosphate (Fru-2,6-P2) levels, glucose consumption and lactate production are increased in a dose-dependent manner in HT29 cells treated with PMA or insulin. The increase in Fru-2,6-P2 can be related to an increase in the Vmax. of PFK2, persisting after the enzyme has been precipitated with poly(ethylene glycol), without change in the Km for fructose 6-phosphate. The most striking effects of PMA and insulin on Fru-2,6-P2 production are observed after long-term treatment (24 h) and are abolished by actinomycin, cycloheximide and puromycin, suggesting that protein synthesis is involved. Furthermore, the effects of insulin and PMA on glucose consumption, lactate production, Fru-2,6-P2 levels and PFK2 activity are additive, and the effect of insulin on Fru-2,6-P2 production is not altered by pre-treatment of the cells with the phorbol ester. This suggests that these effects are exerted by separate mechanisms.