Rapid response of protein synthesis to insulin in 3T3 cells: effects of protein kinase C depletion and differences from the response to serum repletion

Metabolic control and future opportunities for growth regulation

The last 10 years have seen a significant expansion in the scope of attempts to manipulate the growth of animals (Buttery, Lindsay and Haynes, 1986). The expansion of interest has been driven by a number of factors, both economic and theoretical. At the economic level the need to develop energetically and economically efficient strategies of animal production has been coupled with a renewed awareness of the implications for human health of excessive intakes of saturated fats. Emphasis then has switched from the maximization of weight gain as an end in itself towards a need to promote protein deposition at any given intake and, at the same time, to reduce the fat content of meat and meat products. These twin objectives might be achieved by one of three strategies: the promotion of protein deposition alone, because at any given rate of weight gain this will tend to minimize the rate of fat deposition (the so-called repartitioning effect); the reduction of fat gain (an approach that has received particularly close attention by those concerned primarily with human obesity); or ideally the simultaneous promotion of protein accretion and depression of that of fat.

Nucleocytoplasmic transport

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Translation and the cytoskeleton: a mechanism for targeted protein synthesis

This review describes the critical evidence that in eukaryotic cells polyribosomes, mRNAs and components of the protein synthetic machinery are associated with the cytoskeleton. The role of microtubules, intermediate filaments and microfilaments are discussed; at present most evidence suggests that polyribosomes interact with the actin filaments. The use of non-ionic detergent/deoxycholate treatment in the isolation of cytoskeletal-bound polysomes is described and the conclusion reached that at low salt concentrations this leads to mixed preparations of polysomes derived from both the cytoskeleton and the endoplasmic reticulum. At present the best approach for isolation of cytoskeletal-bound polysomes appears to involve extraction with salt concentrations greater than 130 mM after an initial non-ionic detergent treatment. Such polysomes appear to be enriched in certain mRNAs and thus it is suggested that they are involved in translation of a unique set of proteins. The evidence for mRNA localisation is presented and the role of the cytoskeleton in transport and localisation of RNA discussed. Recent data on the role of the 3' untranslated region in the targeting of mRNAs both to particular regions of the cell and for translation on cytoskeletal-bound polysomes is described. The hypothesis is developed that the association of polysomes with the cytoskeleton is the basis of a mechanism for the targeting of mRNAs and the compartmentalization of protein synthesis.

Evidence for a role for protein kinase C in the stimulation of protein synthesis by insulin in swiss 3T3 fibroblasts

We have examined the role of protein kinase C (PKC) in the stimulation of protein synthesis by insulin using two complementary approaches. In the first, fibroblasts were pretreated with phorbol esters to down-regulate PKC. In these cells, the effects of insulin and of phorbol esters on protein synthesis were completely abolished, although serum still elicited an effect approaching that seen in control cells. Secondly, we used newly developed inhibitors of PKC which, again, blocked the effects of insulin and phorbol esters without greatly reducing the response to serum. Thus PKC apparently plays an important role in the stimulation of translation by insulin.

Stimulation by insulin of protein synthesis in cultured chick embryo fibroblasts

Insulin stimulates protein synthesis in skeletal muscle of young animals and has been reported to exert similar effects on a variety of mammalian cell types in culture. However, with chick embryo fibroblasts we found that the extent of this effect was very sensitive to the culture conditions of the cells prior to the insulin treatment. The most reproducible results were obtained with cells that had been sub-cultured into medium in which fetal calf serum was replaced with 2% horse serum. Insulin only stimulated protein synthesis when added at supra-physiological concentrations. Insulin-like growth factor 1 was effective at much lower concentrations. Since chick embryo fibroblasts can be obtained in good yield, they offer, if treated under appropriate conditions, a suitable system for study of the mechanisms by which insulin and IGF-1 promote the initiation of protein synthesis in eukaryotic cells.

Morphological changes in Krebs II ascites tumour cells induced by insulin are associated with differences in protein composition and altered amounts of free, cytoskeletal-bound and membrane-bound polysomes

A three-step sequential detergent/salt extraction procedure was used in order to isolate three distinct subcellular fractions containing free (FP), cytoskeletal-bound (CBP) and membrane-bound polysomes (MBP), respectively, from Krebs II ascites cells (Vedeler et al., Mol Cell Biochem 100: 183-193, 1991). The purpose was to study changes in the distribution of polysomes in these three fractions during long-term incubation with insulin under either stationary conditions or in roller suspension culture. Insulin caused a redistribution of polysomes between FP, CBP and MBP fractions. The hormone appeared to promote an entry of ribosomes into polysomes both in CBP and MBP populations. When cells were grown in stationary culture in the presence of insulin and thus promoted to attach to the substratum and undergo morphological changes, a diversion of ribosomes from CBP into MBP was observed. The level of protein synthesis was apparently very high in this latter fraction since more than 70% of ribosomes were in polysomes. Morphological changes observed following insulin treatment were accompanied by a shift of certain proteins among subcellular fractions (for example actin and p35). The fibronectin content was about 20% higher in attached compared to non-attached cells. The results suggest that morphological changes induced by stimulation with insulin are associated with an increased activity of MBP, presumably reflecting a requirement for an increased synthesis of membrane proteins.

Interaction between mRNA, ribosomes and the cytoskeleton

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c-myc mRNA in cytoskeletal-bound polysomes in fibroblasts

3T3 fibroblasts were treated sequentially with 25 mM-KCl/0.05% Nonidet P40, 130 mM-KCl/0.05% Nonidet P40 and finally with 1% Nonidet P40/1% deoxycholate in order to release free, cytoskeletal-bound and membrane-bound polysomes respectively. The membrane-bound fraction was enriched in the mRNA for the membrane protein beta 2-microglobulin, whereas the cytoskeletal-bound polysomes were enriched in c-myc mRNA. Actin mRNA was present in both free and cytoskeletal-bound polysomes. The results suggest that cytoskeletal-bound polysomes are involved in the translation of specific mRNA species.

Effects of catecholamines on protein synthesis in cardiac myocytes and perfused hearts isolated from adult rats. Stimulation of translation is mediated through the alpha 1-adrenoceptor

Protein-synthesis rates in freshly isolated cardiac myocytes from adult rats were acutely stimulated by 20-30% by 1 microM-adrenaline, by 1 microM-noradrenaline or by 1 microM-phenylephrine, but were not stimulated by 1 microM-isoprenaline. Stimulation by 1 microM-adrenaline was completely prevented by 100 nM-prazosin. Yohimbine was much less effective in preventing stimulation, and 20 microM-DL-propranolol was completely ineffective. The stimulation of protein synthesis by adrenaline was still observed after inhibition of transcription by actinomycin D. None of these manipulations affected myocyte ATP contents. In anterogradely perfused hearts, protein-synthesis rates were stimulated by 1-2 microM-adrenaline in the presence of 10 microM-DL-propranolol (to decrease the beta-adrenergic effects of adrenaline). ATP contents were not altered, but phosphocreatine contents were increased. These observations lead us to conclude that cardiac protein synthesis can be stimulated acutely at the level of translation by alpha 1-adrenergic stimulation. We discuss possible roles for protein kinase C and intracellular alkalinization in the mediation of this effect.

The cyclo-oxygenase inhibitors indomethacin and ibuprofen inhibit the insulin-induced stimulation of ribosomal RNA synthesis in L6 myoblasts

Insulin stimulated total RNA accretion and the incorporation of [3H]uridine into RNA in L6 skeletal-muscle myoblasts. Incorporation of uridine into the rRNA was measured after either separation of 18 S and 28 S rRNA species by agarose-gel electrophoresis or separation of dissociated 40 S and 60 S ribosomal subunits on sucrose density gradients. Both methods showed a stimulation by insulin of uridine incorporation into the RNA of the two subunits. Two non-steroidal anti-inflammatory drugs, indomethacin and ibuprofen, which inhibit the metabolism of arachidonic acid by the cyclo-oxygenase pathway, inhibited the insulin-induced accretion of total cellular RNA and the incorporation of uridine into the RNA of both ribosomal subunits. The effect of insulin was observed both by using a tracer dose of [3H]uridine (5 microM) and in the presence of a high concentration (1 mM) of uridine to minimize possible changes in intracellular precursor pools. Neither insulin nor indomethacin was found to affect the incorporation of uridine into the total intracellular nucleotide pool, or the conversion of uridine into UTP. The ability of inhibitors of arachidonic acid metabolism to prevent insulin-induced increases in RNA metabolism suggests that a prostaglandin or other eicosanoid is involved in the signal mechanism whereby insulin stimulates RNA synthesis.

Protein synthesis in rat cardiac myocytes is stimulated at the level of translation by phorbol esters

12-O-Tetradecanoylphorbol 13-acetate acutely stimulated the rate of protein synthesis maximally by about 43% in terminally differentiated myocytes freshly isolated from adult rat hearts. Stimulation was rapidly expressed (within 20 min). The relative effects of four phorbol esters on protein synthesis was consistent with a specific effect on protein kinase C. Inhibition of transcription with actinomycin D had no effect on the absolute stimulation of the protein synthesis rate by 12-O-tetradecanoylphorbol 13-acetate. We conclude that protein kinase C may be involved in the regulation of the translational process.

Effect of RNA synthesis inhibitors on insulin-induced protein synthesis by 3T3 cells

1. The increased protein synthesis of quiescent 3T3 cells in response to insulin was separated into three distinct phases based on their response to various inhibitors of RNA synthesis. 2. The first increase in protein synthesis was insensitive to the inhibitors used, and probably resulted from activation of existing protein synthesizing mechanism. 3. The second phase was sensitive to a varying extent to alpha-amanitin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, implying the need for new mRNA synthesis as well as the production of new ribosomes indicated by its further sensitivity to low concentration (10 ng/ml) of Actinomycin D. 4. The final phase was insensitive to inhibitors of new ribosome formation, but still depended on new mRNA. alpha-difluoromethylornithine, an inhibitor of de novo polyamine synthesis, partly inhibited the insulin induced stimulation of protein synthesis.

Increased protein synthesis response to insulin in fibroblasts treated with the diacylglycerol kinase inhibitor R59022

Insulin stimulated protein synthesis in quiescent 3T3 fibroblasts. This effect of the hormone was greater in the presence of the diacylglycerol kinase inhibitor R59022 (10(-5) M) over a range of insulin concentrations from 1 microU to 1 mU/ml; R59022 increased the sensitivity of cells to insulin. The amount of radioactive diacylglycerol recovered from cells prelabelled with [3H]glycerol was increased transiently in response to insulin; the response was larger and prolonged in cells given the kinase inhibitor. The results (i) support the hypothesis that diacylglycerol production is part of the signal pathway by which insulin stimulates protein synthesis and (ii) suggest that inhibition of diacylglycerol breakdown leads to increased sensitivity to the hormone.

Evidence that insulin increases the proportion of polysomes that are bound to the cytoskeleton in 3T3 fibroblasts

The association of polysome redistribution with changes in protein synthesis was investigated in insulin-stimulated fibroblasts. Free polysomes were released by Nonidet-P40 and 25 mM KCl, cytoskeletal-bound polysomes were retained at 25 mM KCl but released at 130 mM, while membrane-bound polysomes were released by deoxycholate. Insulin increased the proportion of polysomes which were retained at 25 mM KCl but had no effect when extraction was carried out at 130 mM KCl, suggesting that more polysomes were associated with the cytoskeleton. Insulin also reduced the amount of actin released from the detergent-insoluble cytoskeleton indicating that the hormone affects microfilament organization.

Effects of insulin, pertussis toxin and cholera toxin on protein synthesis and diacylglycerol production in 3T3 fibroblasts: evidence for a G-protein mediated activation of phospholipase C in the insulin signal mechanism

The rapid increase in protein synthesis that occurs on addition of insulin (1 mU/ml) to stepped-down 3T3 cells was blocked by pre-incubation of the cells with pertussis toxin. Cholera toxin on the other hand stimulated protein synthesis and this effect was insensitive to actinomycin D and inhibited by pre-treatment of the cells with phorbol dibutyrate to deplete cell protein kinase C. Insulin was found to cause a rapid and transient increase in diacylglycerol (DAG) synthesis. The insulin-induced increase in diacylglycerol was blocked by pertussis toxin. Exogenous DAG (10 microM) stimulated protein synthesis within 1 hour. The results suggest that insulin stimulates ribosomal activity through a signal mechanism that involves a G-protein mediated activation of phospholipase C to increase DAG levels.