Nutritional and hormonal regulation of malic enzyme synthesis in rat mammary gland

Differentiation dependent activation of the myelin genes in purified oligodendrocytes is highly resistant to hypoglycemia

We have previously demonstrated that the developmental upregulation of myelin-specific genes in mixed glial cultures is strongly attenuated by hypoglycemia. The present study was designed to evaluate the effect of hypoglycemia on differentiation-dependent upregulation of myelin genes in purified oligodendrocyte cultures. The expression of major myelin protein genes, i.e., proteolipid protein (PLP), basic protein (BP) and myelin associated glycoprotein (MAG) were monitored by Northern blot analysis. In control cultures maintained at 6 mg/ml of glucose, the expression of all the genes upregulated rapidly, and plateaued at approximately day 4. A similar pattern of differentiation-dependent upregulation was observed for the gene encoding a lipogenic enzyme, i.e., malic enzyme (ME). In contrast to mixed glial cultures, however, this developmental gene upregulation was not significantly affected by severe hypoglycemia (approximately 0.02 mg/ml). The results indicate that the effect of glucose deprivation on oligodendrocyte genes observed in mixed glial cultures is mediated by other cells. The upregulation of the genes in differentiating oligodendrocytes was accompanied by the production of myelin-related membrane that was isolated by density gradient fractionation. In contrast to the effect on gene expression, this anabolic activity was highly dependent on glucose, as seen from a profound suppression by severe hypoglycemia.

Involvement of diminution of glutathione, produced by deficiency of methionine in the diet, in the elevation of malic enzyme level in rat liver

Rats fed on a low protein diet show an increase in the specific activity of malic enzyme and a concomitant decrease of glutathione concentration. We have studied the effect on malic enzyme activity of supplementing of low protein diet with essential amino acids. Only when methionine was excluded from the diet did the specific activity of malic enzyme increase to the same extent as found in rats fed with low protein diet. Immunoprecipitation of malic enzyme indicated that specific activity changes are the result of changes in the amounts of enzyme. Under all dietary conditions studied, the increase in malic enzyme activity is associated with a decrease in the concentration of GSH. To evaluate the possible causative role of GSH in malic enzyme induction, the specific activity of malic enzyme was measured in rats treated with BSO, an inhibitor of GSH biosynthesis. The results show that in BSO-treated rats the decrease of GSH levels is also accompanied by an increase in the activity of malic enzyme.

Integration of lipid metabolism in the mammary gland and adipose tissue by prolactin during lactation

Prolactin deficiency, induced by bromocryptine treatment, brought about reciprocal changes in the ability of adipocytes and acini isolated from lactating rats to synthesize lipids. The capacity to synthesize fatty acids and phospholipids decreased in the mammary gland and increased in adipocytes by bromocryptine treatment. In the mammary gland, the maximum potential activity of the pentose shunt as well as the specific activities of the pathway dehydrogenases were significantly reduced by bromocryptine treatment. Simultaneously, adipose tissue increased its lipogenic capacity but neither the maximum potential of the shunt nor the specific activities of the pentose phosphate shunt dehydrogenases were significantly changed with respect to the control lactating rats. Thus, a differential regulatory mechanism(s) of the pentose phosphate shunt activity appears to operate in these two tissues. Adipocytes from lactating rats showed a poor responsiveness to insulin in terms of lipid synthesis from glucose. In contrast, in adipocytes from bromocryptine treated rats insulin was able to increase lipid synthesis (105%). Sheep prolactin administration 'in vivo' partially reversed the effects of bromocryptine. These data suggest that prolactin mediates adipocytes resistance to insulin during lactation. Phospholipid synthesis, as occurred in fatty acid synthesis, is increased in adipose tissue and decreased in mammary gland by bromocryptine treatment. However, alpha 1-adrenergic stimulation increases phosphatidylinositol turnover to about the same percentages in both mammary gland acini and adipocytes from lactating rats independently of bromocryptine treatment.

Hormonal regulation of malic enzyme expression in primary cultures of foetal brown adipocytes

Insulin increased malic enzyme activity and protein content in foetal brown adipocyte primary cultures through a two fold increase in the rates of enzyme synthesis. Conversely, noradrenaline prevented the induction caused by insulin on enzyme expression through an accelerate rate of malic enzyme degradation. Tri-iodothyronine had no effect on the expression of malic enzyme in cultured foetal brown adipocytes.

Both cell substratum regulation and hormonal regulation of milk protein gene expression are exerted primarily at the posttranscriptional level

The mechanism by which individual peptide and steroid hormones and cell-substratum interactions regulate milk protein gene expression has been studied in the COMMA-D mammary epithelial cell line. In the presence of insulin, hydrocortisone, and prolactin, growth of COMMA-D cells on floating collagen gels in comparison with that on a plastic substratum resulted in a 2.5- to 3-fold increase in the relative rate of beta-casein gene transcription but a 37-fold increase in beta-casein mRNA accumulation. In contrast, whey acidic protein gene transcription was constitutive in COMMA-D cells grown on either substratum, but its mRNA was unstable and little intact mature mRNA was detected. Culturing COMMA-D cells on collagen also promoted increased expression of other genes expressed in differentiated mammary epithelial cells, including those encoding alpha- and gamma-casein, transferrin, malic enzyme, and phosphoenolpyruvate carboxykinase but decreased the expression of actin and histone genes. Using COMMA-D cells, we defined further the role of individual hormones in influencing beta-casein gene transcription. With insulin alone, a basal level of beta-casein gene transcription was detected in COMMA-D cells grown on floating collagen gels. Addition of prolactin but not hydrocortisone resulted in a 2.5- to 3.0-fold increase in beta-casein gene transcription, but both hormones were required to elicit the maximal 73-fold induction in mRNA accumulation. This posttranscriptional effect of hormones on casein mRNA accumulation preceded any detectable changes in the relative rate of transcription. Thus, regulation by both hormones and cell substratum of casein gene expression is exerted primarily at the post transcriptional level.

Malic enzyme activity in the developing rat brain in relation to thyroid status

Malic enzyme activity in the soluble fraction of neonate brains from mothers fed with propylthiouracil (0.015% w/v) in drinking water from day 12 of the gestation period was significantly lowered (P less than 0.01) as compared to the offspring of normal mothers. Supplementation of triiodothyronine to the neonates from experimental mothers restored the malic enzyme activity to normal levels. However, administration of triiodothyronine to adult control rats did not influence malic enzyme activity in the brains of these animals. Our data suggest that during the initial critical period of brain maturation, malic enzyme is under the control of thyroid hormones. The response of malic enzyme towards thyroid hormones is lost once the brain has matured.

Both cell substratum regulation and hormonal regulation of milk protein gene expression are exerted primarily at the posttranscriptional level

The mechanism by which individual peptide and steroid hormones and cell-substratum interactions regulate milk protein gene expression has been studied in the COMMA-D mammary epithelial cell line. In the presence of insulin, hydrocortisone, and prolactin, growth of COMMA-D cells on floating collagen gels in comparison with that on a plastic substratum resulted in a 2.5- to 3-fold increase in the relative rate of beta-casein gene transcription but a 37-fold increase in beta-casein mRNA accumulation. In contrast, whey acidic protein gene transcription was constitutive in COMMA-D cells grown on either substratum, but its mRNA was unstable and little intact mature mRNA was detected. Culturing COMMA-D cells on collagen also promoted increased expression of other genes expressed in differentiated mammary epithelial cells, including those encoding alpha- and gamma-casein, transferrin, malic enzyme, and phosphoenolpyruvate carboxykinase but decreased the expression of actin and histone genes. Using COMMA-D cells, we defined further the role of individual hormones in influencing beta-casein gene transcription. With insulin alone, a basal level of beta-casein gene transcription was detected in COMMA-D cells grown on floating collagen gels. Addition of prolactin but not hydrocortisone resulted in a 2.5- to 3.0-fold increase in beta-casein gene transcription, but both hormones were required to elicit the maximal 73-fold induction in mRNA accumulation. This posttranscriptional effect of hormones on casein mRNA accumulation preceded any detectable changes in the relative rate of transcription. Thus, regulation by both hormones and cell substratum of casein gene expression is exerted primarily at the post transcriptional level.