Interaction of insulin and phorbol esters on the regulation of DNA synthesis in rat hepatoma cells

Phorbol esters down-regulate alpha-fetoprotein gene expression without affecting growth in fetal hepatocytes in primary culture

The effects of phorbol esters (phorbol-12,13-dibutyrate, PDB) on alpha-fetoprotein expression and cell growth were assayed by using fetal hepatocytes in primary culture. PDB acts synergistically with epidermal growth factor (EGF) to specifically decrease alpha-fetoprotein (AFP) mRNA levels, without affecting the expression of other genes of the same family, such as albumin and Vitamin D-binding protein (DBP). This effect is PDB-dose dependent, maximal effects being at 10 ng/ml. The implication of protein kinase C (PKC) in this effect seems clear since bisindolylmaleimide (BIS), a specific PKC inhibitor, completely blocks the PDB effect on AFP expression. Nuclear run-on experiments show that the decrease in AFP mRNA levels is mainly due to an inhibition in the transcription rate of the gene. Determination of PKC activities shows that fetal hepatocytes contain mainly Ca(2+)-independent isoenzymes, which patterns of activation was not modified by EGF plus PDB treatment with respect to PDB treatment. We have found that MAPK and JNK activities, c-jun and c-fos mRNA levels and AP-1 binding activity are notably increased when cells are incubated with both EGF and PDB, PDB does not stimulate growth of fetal hepatocytes, measured either as [3H]-thymidine incorporation into DNA or by cell cycle analysis using flow cytometry. All these results suggest that activation of PKC may affect liver gene expression rather than cell growth in fetal hepatocytes.

Insulin induction of pip 92, CL-6, and novel mRNAs in rat hepatoma cells

Insulin directly affects many aspects of cellular metabolism. An additional, poorly studied effect of insulin is the regulation of multiple genes whose products are important in many cellular functions. Using differential screening techniques, we identified insulin-regulated genes induced in insulin-treated rat H4IIE (H4) hepatoma cells. Two of the mRNAs identified were homologous to the previously characterized mouse pip 92 and rat CL-6 immediate-early genes. The pip 92 clone was initially isolated from mitogen-stimulated mouse Balb/c 3T3 fibroblasts, whereas the CL-6 clone was first isolated from regenerating rat liver. In this article, we demonstrated that in rat H4 cells, the transcription rates of both pip 92 and CL-6 are induced by insulin alone. Additionally, we showed that the transcription rates of two other genes, whose sequences are not homologous to any other sequences in gene bank ("novel genes"), were rapidly and transiently induced by insulin. These results demonstrate that insulin regulates the expression of several novel genes with a time-course similar to members of the immediate-early response gene family.

Regulation of gamma-actin gene expression by insulin

Insulin exerts rapid effects on cellular metabolism and can cause morphological changes by inducing rearrangements of cytoskeletal components. The regulation of specific cytoskeletal genes by insulin, however, has not been studied extensively. In the present work insulin was found to rapidly, but transiently, increase transcription of the cytoskeletal gamma-actin gene in rat H4IIE (H4) hepatoma cells. Insulin-induced transcription of the gamma-actin gene was evident within 5 min and was maximal by 15 min at 10-fold above control levels. The stimulation of transcription was transient, with a return towards basal levels by 120 min. Transcription of gamma-actin was increased at insulin concentrations as low as 1 x 10(-11) M and was maximal at 1 x 10(-9) to 1 x 10(-8) M. Transcription of several control genes (skeletal and cardiac alpha-actin and beta-tubulin) were unaltered by insulin administration. Messenger RNA (mRNA) levels for the gamma-actin gene increased, but to a lesser degree than transcription. Since the gamma-actin message is an abundant and stable mRNA, its levels would not be expected to change dramatically from a transient induction of transcription. Like insulin, phorbol esters transiently increased transcription of the gamma-actin gene. In addition, pretreatment of cells with phorbol esters for 24 h reduced the ability of insulin to induce gamma-actin transcription. These data support our hypothesis that insulin and phorbol esters share intracellular signalling pathways in the control of transcription of specific genes.

Short- and long-term effects of insulin on tyrosine aminotransferase gene expression

In the present study the relationship between changes in tyrosine aminotransferase (TAT) enzyme activity, cytoplasmic mRNA levels, and gene transcription in response to both short- and long-term exposure to insulin was investigated. Insulin acutely inhibited transcription of the TAT gene by 50% in serum-deprived rat H4 hepatoma cells. Following this initial 50% decrease in transcription, there was a 2.5-fold induction in TAT activity that could not be accounted for by a concomitant increase in TAT mRNA levels. Insulin had no effect on the half-life of TAT mRNA. Insulin exposure for short periods of time also inhibited the glucocorticoid- and cAMP-induced transcription of the TAT gene. Like insulin, protein synthesis inhibitors acutely inhibited basal and glucocorticoid-induced TAT transcription. TAT activity gradually returned toward basal levels after 8 h of insulin treatment. A second insulin-induced increase in TAT activity (3.5-fold above basal levels) was observed by 24 h of insulin treatment. This second phase of insulin-induced TAT activity was associated with elevated levels of TAT transcription and TAT mRNA levels, and therefore, unlike the earlier stimulation, could be accounted for by changes in gene expression. Thus, the insulin-mediated regulation of the TAT gene in H4 cells is complex. Different transcriptional and post-transcriptional mechanisms are likely to be involved in the biphasic responses to insulin.

Regulation of beta-actin gene transcription by insulin and phorbol esters

Insulin and the phorbol ester, phorbol 12-myristate 13-acetate, induce beta-actin gene transcription in H4 cells. This occurred rapidly, with a maximum 15-fold stimulation following 15 min of insulin (5 x 10(-9) M) or phorbol ester (1 microgram/ml) exposure. The increase in beta-actin transcription was transitory, returning to baseline within 120 min. Pretreatment of cells with phorbol ester for 24 h, reducing functional protein kinase C activity, abolished the ability of phorbol esters to increase beta-actin transcription. When insulin was added to phorbol ester-pretreated cells the insulin-induced increase in beta-actin transcription was reduced by 40-60%. These findings support our hypothesis that a common regulatory signal is utilized by both insulin and phorbol esters for the complete induction of specific genes.

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.