Insulin and phorbol myristic acetate induce ornithine decarboxylase in Reuber H35 rat hepatoma cells by different mechanisms

Phosphoenolpyruvate carboxykinase is induced in growth-arrested hepatoma cells

Phosphoenolpyruvate carboxykinase (PEPCK) mRNA is elevated in H4IIEC3 rat hepatoma cells cultured at high density, suggesting that PEPCK expression and growth arrest may be coordinately regulated. Induction of growth arrest either by contact inhibition (high culture density) or by serum deprivation correlated with significant increases in PEPCK protein and its mRNA. The observation that PEPCK mRNA was induced by contact inhibition in the presence of serum indicates that the effect of high density is independent of insulin or any other serum component. The magnitudes of the changes in PEPCK expression during growth arrest were greatly enhanced in KRC-7 cells, an H4IIEC3 subclone that is much more sensitive to growth arrest than its parental cell line. Restimulation of proliferation in growth-arrested KRC-7 cells, either by addition of serum or insulin to serum-deprived cells or by replating contact-inhibited cells at low density, caused a rapid decrease in PEPCK expression. However, PEPCK mRNA is not always reduced in proliferating cells since treatment of serum-starved cells with epidermal growth factor stimulated entry into the cell cycle but did not affect PEPCK mRNA levels. Finally, dexamethasone induction of PEPCK mRNA was blunted in cells cultured at high density but was unaffected by the presence or absence of serum. Collectively, these data suggest the possibility of cross-talk between the control of PEPCK expression and growth arrest in KRC-7 cells.

Involvement of a pertussis-toxin sensitive G protein in the induction of gene expression by insulin

Binding of insulin to its receptor triggers multiple cellular responses, including changes in metabolism and in gene expression, resulting from the activation of multiple signalling pathways. Pertussis toxin has been shown to block an insulin-stimulated phospholipase C, resulting in an inhibition of the synthesis of phospholipid second messengers by insulin. In the present study, we investigated the significance of this pathway for the induction of growth-related genes by insulin treatment of H35 hepatoma cells. We found that pertussis toxin dramatically inhibits the induction of c-fos mRNA by insulin. Although c-jun and ornithine decarboxylase induction were also inhibited by pertussis toxin, they were much less sensitive than c-fos. These results indicate an important for lipid second messengers in mitogenic signalling by insulin and further demonstrate distinct roles for this pathway in the induction of c-fos and c-jun.

Insulin-mimetic actions of phorbol ester in cultured adult rat hepatocytes. Lack of phorbol-ester-elicited inhibition of the insulin signal

The actions of the phorbol ester phorbol 12-myristate 13-acetate (PMA) on glucose metabolism, amino acid transport and enzyme inductions were studied in primary cultures of adult-rat hepatocytes and compared with the effects of insulin. PMA and insulin stimulated glycolysis 5- and 7-fold respectively. The half-maximal effective dose of PMA was 60 nM. Stimulation of glycolysis was accompanied by an insulin- or PMA-dependent and okadaic acid-sensitive activation of 6-phosphofructo-2-kinase and pyruvate kinase, as well as by an increase in fructose 2,6-bisphosphate. Glucose production from glycogen was decreased to 50% by PMA and to 15% by insulin, whereas glycogen synthesis was stimulated 2- and 7-fold respectively. PMA also increased aminoisobutyrate uptake, induced ornithine decarboxylase and counteracted the glucagon-dependent induction of phosphoenolpyruvate carboxykinase. PMA strongly antagonized the hormonal activation of glycogen synthesis, but all other insulin actions assayed were not decreased by the phorbol ester. Whereas additive effects of PMA and insulin were not detected, PMA and a simultaneous increase in the glucose concentration had additive effects on glycolysis and glycogen metabolism. Cell exposure to insulin resulted in receptor autophosphorylation and a more than 10-fold activation of the receptor tyrosine kinase. PMA did not alter these effects, and also had no effect on the receptor phosphorylation status in the absence of insulin. Long-term (15 h) pretreatment of the cells with PMA abolished all PMA effects, but not the insulin effects. It is concluded that PMA does not generally antagonize the action of insulin in differentiated adult hepatocytes, and that insulin and PMA may use related signal-transduction pathways.

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.

Regulation of mouse ornithine decarboxylase activity by cell growth, serum and tetradecanoyl phorbol acetate is governed primarily by sequences within the coding region of the gene

To determine the genetic elements required for modulation of ornithine decarboxylase (ODC) activity in response to cell growth or treatment with serum or with tetradecanoyl phorbol acetate, ODC-deficient cells were transfected with a series of recombinant DNAs encoding mouse ODC. All of the transfected cells expressing an intact mouse ODC protein displayed regulation of ODC activity, including those expressing a construct deprived of all ODC-specific sequence information except the protein-coding region. ODC mRNA changed much less than enzymatic activity. A mutation of the protein-coding region that converted ODC from an unstable to a stable intracellular protein attenuated the regulatory response. We conclude that post-transcriptional events associated with ODC degradation dominate the response to these stimuli.

Insulin-mediated regulation of tyrosine aminotransferase in rat hepatoma cells: inhibition of transcription and inhibition of enzyme degradation

Insulin induces the enzyme tyrosine aminotransferase (TAT) in Reuber H-35 rat hepatoma cells. A clone of these cells (KRC-7) was used to study the relationship between changes in enzyme activity and hybridizable mRNA, and rates of transcription for TAT in response to insulin. Our results indicate that enzyme activity is inducible by insulin in the presence of an inhibitor of RNA synthesis, suggesting that insulin functions post-transcriptionally to increase enzyme activity. Unexpectedly, insulin causes a decrease in the level of hybridizable TAT mRNA. Glucocorticoids cause an increase in TAT mRNA and insulin inhibits this increase when added either subsequent to or simultaneous with the addition of this agonist. Transcriptional runoffs demonstrate that insulin inhibits transcription of TAT to account for the aforementioned decrease in hybridizable mRNA. To examine the possibility that a post-translational mechanism is responsible for the increase in TAT activity caused by insulin, the rate of degradation of TAT protein was measured using polyclonal antibody. These experiments indicate that the rate of degradation of TAT is decreased about twofold in the presence of insulin, which suggests that part of the observed increase in TAT activity is due to selective post-translational stabilization of TAT. Therefore, insulin regulates TAT in KRC-7 cells by both transcriptional and post-translational mechanisms, the latter being responsible for the increase in activity.