The effects of insulin and concanavalin A on the accumulation of a specific mRNA 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.

Specific preservation of biosynthetic responses to insulin in adipose tissue may contribute to hyperleptinemia in insulin-resistant obese mice

Obesity is characterized by whole-body insulin resistance, yet the expression of many insulin-stimulated genes, including leptin, is elevated in obesity. These observations suggest that insulin resistance may depend on tissue type and gene. To address this hypothesis, we examined the regulation of immediate-early gene expression in liver and adipose tissue after injection of insulin and glucose, in lean insulin-sensitive, and in A(y)/a obese insulin-sensitive and obese insulin-resistant mice. Expression of hepatic jun-B mRNA was robustly increased after insulin injection in lean insulin-sensitive a/a mice and insulin-sensitive A(y)/a mice. In contrast, induction of hepatic jun-B and c-fos gene expression by insulin was markedly attenuated in obese insulin-resistant mice. Surprisingly, induction of adipose jun-B and c-fos gene expression by insulin was markedly enhanced in obese insulin-resistant mice. Furthermore, the expressions of jun-B and leptin were also enhanced in insulin-resistant mice after injection of glucose. Leptin mRNA was positively correlated with blood glucose levels and jun-B mRNA in lean but not insulin-resistant mice. Multiple regression analysis indicated that the correlation between leptin mRNA and jun-B mRNA was significant even after removing the effect of blood glucose, but the correlation between leptin mRNA and glucose was no longer significant after removing the effect of jun-B mRNA. These data suggest that some impairments in biosynthetic responses to insulin are manifest primarily in the liver, leading to hyperinsulinemia and stimulating the expression of some adipose insulin-stimulated genes, including leptin. These studies demonstrate the utility of immediate-early gene expression in the analysis of biosynthetic mechanisms of insulin resistance.

Rat glucokinase gene: structure and regulation by insulin

The glucokinase gene is 15.5-kilobases long, appears to be present as a single copy, and contains 10 exons that range in size from 96 to 977 base pairs. The transcription start site was located 127 nucleotides upstream from the translation initiation codon. The 5' flanking DNA contains several regions similar to defined promoter elements. These include a probable "TATA box," an Sp1 binding site, and several elements related to liver-specific gene expression. In addition, we determined that transcription of the glucokinase gene increased at least 20-fold when diabetic rats were treated with insulin for 2 hr.

Molecular cloning and analysis of full-length cDNAs cognate to a rat gene under multihormonal control

Gene 33 is a multihormonally regulated rat gene whose transcription is rapidly and markedly enhanced by glucocorticoids, insulin, or cAMP. A cDNA clone (p216) containing a nearly full-length DNA complementary to the mRNA of this gene was isolated from a cDNA library and sequenced. The cDNA represents the entire mRNA transcript, except for three bases at the 5' terminus. The message is 2970 nucleotides long and can encode a protein of 459 amino acids with a molecular mass of 49,919 Da. The deduced amino acid sequence is rich in proline and serine but bears little homology to any known sequences. The 5' untranslated region, rich in G + C, is 270 nucleotides long and contains two apparently nonfunctional AUG initiator codons. The 3' untranslated sequence, rich in A + U, is 1323 nucleotides long and has a polyadenylation signal 13 bases upstream of the poly(A) tract. Two mRNA transcripts of the gene, which appear to be the consequence of alternative usage of splice sites, have been identified. The less abundant mRNA is truncated by 228 nucleotides in the protein coding region. The reading frame is maintained and this mRNA can code for a protein of 383 amino acids with a calculated molecular mass of 42,204 Da.

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

We have compared the effect of phorbol 12-myristate 13-acetate (PMA) with that of insulin on three targets of insulin action in H4IIEC3 (H4) rat hepatoma cells. These parameters are the phosphorylation state and tyrosine kinase activity of the insulin receptor, the activation state of glycogen synthase, and the accumulation of p33 mRNA. Under conditions where insulin treatment of H4 cells clearly activated receptor serine and tyrosine phosphorylation on the insulin receptor beta-subunit in situ, activated receptor tyrosine kinase activity in vitro, and activated glycogen synthase and p33 mRNA accumulation in situ, PMA alone did not influence the insulin receptor phosphorylation state or tyrosine kinase activity and did not affect glycogen synthase activity, but markedly increased p33 mRNA accumulation. When PMA was added in the presence of insulin, particularly if PMA was preincubated, the receptor phosphorylation state and the tyrosine kinase activity again were not affected, but insulin-activated glycogen synthase was significantly diminished or abolished. In contrast, increased p33 mRNA accumulation by PMA was additive with that of insulin. Thus, under conditions where no effect was observed on the insulin receptor phosphorylation state or the tyrosine kinase activity, PMA acted in an insulin-antagonistic manner on glycogen synthase and in an insulin-like manner on p33 mRNA accumulation, indicating that these actions of PMA are unrelated to early events in the pathway of the insulin action. Effects on glycogen synthase are most readily explained by an effect of protein kinase C-activated phosphorylation of glycogen synthase.

Insulin-mimetic actions of wheat germ agglutinin and concanavalin A on specific mRNA levels

Insulin has pleiotropic effects on sensitive cells, including the regulation of specific mRNA accumulation initiated by the binding of insulin to its plasma membrane receptor. Lectins, such as wheat germ agglutinin (WGA) and concanavalin A (Con A), are known to be insulin mimetic. It is thought that WGA and Con A interact with the insulin receptor or associated membrane glycoproteins which, when activated, lead to insulin-mimetic responses. We attempted to determine whether WGA and Con A could induce the accumulation of a specific messenger RNA (p33-mRNA). Insulin treatment of H4IIE (H4) hepatoma cells increased the concentration of p33-mRNA within 30 min after addition, with a maximum effect of 10- to 15-fold. WGA and Con A also exhibited time- and dose-dependent stimulatory effects on p33-mRNA accumulation with maximal effects of 30- to 40-fold. The effect of insulin was maximal by 1 h and plateaued thereafter, whereas lectins had maximal effects at 2 h after addition to cell cultures. Insulin, WGA, and Con A did not significantly alter the stability (half-life) of p33-mRNA. The addition of RNA synthesis inhibitors blocked the ability of insulin, WGA, and Con A to induce the amount of p33-mRNA. These data suggest that lectins, as well as insulin, induce the synthesis of p33-mRNA in acutely treated H4 hepatoma cells.

Insulin and growth factor effects on c-fos expression in normal and protein kinase C-deficient 3T3-L1 fibroblasts and adipocytes

We investigated the expression of the protooncogene c-fos in 3T3-L1 fibroblasts and adipocytes in response to a variety of growth-promoting agents in normal cells and in cells preincubated with phorbol esters to deplete them of protein kinase C. There was a rapid accumulation of c-fos mRNA in fibroblasts and adipocytes treated with phorbol 12-myristate 13-acetate, platelet-derived growth factor, fibroblast growth factor, fetal calf serum, bombesin, and insulin, especially in the adipocytes. Phorbol 12-myristate 13-acetate pretreatment abolished the increase in c-fos mRNA due to additional phorbol 12-myristate 13-acetate treatment and decreased but did not eliminate the ability of platelet-derived growth factor, fibroblast growth factor, fetal calf serum, bombesin, and insulin to stimulate c-fos mRNA. These data suggested that c-fos mRNA could be induced in serum-deprived 3T3-L1 fibroblasts and adipocytes by at least two separate pathways, one involving protein kinase C and the other independent of protein kinase C. In the very insulin-sensitive 3T3-L1 adipocytes, insulin rapidly and transiently increased c-fos expression (c-fos mRNA appeared by 15 min and disappeared after 60 min) via interaction with its own cellular receptor, rather than by interacting with receptors for one of the insulin-like growth factors. Cycloheximide treatment in combination with insulin or phorbol 12-myristate 13-acetate resulted in superinduction of c-fos mRNA. We conclude that insulin can rapidly stimulate c-fos mRNA accumulation in 3T3-L1 adipocytes and that part of the growth factor-stimulated increase in this mRNA that occurs in protein kinase C-deficient cells may be due to activation of a pathway similar or identical to that activated by insulin.