Adipose tissue metabolism during early pregnancy in the rat: temporal relationships of changes in the metabolic activity, number of insulin receptors, and serum hormone concentrations

Function of phosphoenolpyruvate carboxykinase in mammary gland epithelial cells

Previously, we have shown that Pck1 expression in mammary gland adipocytes and white adipose tissue maintains triglyceride stores through glyceroneogenesis, and these lipids were used for synthesis of milk triglycerides during lactation. Reduced milk triglycerides during lactation resulted in patterning of the newborn for insulin resistance. In this study, the role of Pck1 in mammary gland epithelial cells was analyzed. The developmental expression of Pck1 decreased in isolated mouse mammary gland epithelial cells through development and during lactation. Using HC11, a clonal mammary epithelial cell line, we found that both Janus kinase 2 signal transducers and activators of transcription 5 and the AKT pathways contributed to the repression of Pck1 mRNA by prolactin. These pathways necessitate three accessory factor regions of the Pck1 promoter for repression by prolactin. Using [U-(13)C(6)]glucose, [U-(13)C(3)]pyruvate, and [U-(13)C(3)]glycerol in HC11 cells, we determined that Pck1 functions in the pathway for the conversion of gluconeogenic precursors to glucose and contributes to glycerol-3-phosphate synthesis through glyceroneogenesis. Therefore, Pck1 plays an important role in both the mammary gland adipocytes and epithelial cells during lactation.

Progestins stimulate the differentiation of 3T3-L1 preadipocytes

The effect of progesterone on the differentiation of the 3T3-L1 preadipocytes was investigated and compared with other sex steroids (estradiol and testosterone), with cortisol, with the synthetic progestin R5020 and with the progestin/glucocorticoid antagonist RU38486. At 10(-8) M, progesterone stimulated the activity of glycerol-3-phosphate dehydrogenase and triglyceride deposition. Progesterone, R5020, cortisol, and RU38486 increased triglycerides about 2-fold at 10(-7) M. Only minimal effects were observed with testosterone and estradiol even at 10(-6) M. When the cells were cultured in presence of 10(-5) M metyrapone the effect of progesterone was unchanged, suggesting that the progesterone was not metabolized to a glucocorticoid. Progesterone, R5020 and RU38486 competed efficiently with [3H]dexamethasone for the glucocorticoid receptor in 3T3-L1 cytosol. These results indicate a significant, reproducible dose-dependent effect of progestins on differentiation of the preadipocytes, which appears to be mediated via the glucocorticoid receptor.

Tissue-specific changes in the ability of insulin and noradrenaline to activate pyruvate dehydrogenase in vivo during lactation in the rat

Changes are described in the total pyruvate dehydrogenase (PDH) activity, the proportion of PDH in the active state and its control by insulin and noradrenaline in vivo, in white adipose tissue, liver, skeletal muscle and mammary gland with pregnancy, lactation and on weaning. Lactation resulted in a decrease in total PDH in white adipose tissue and an increase in the mammary gland, whereas the proportion in the active state decreased in muscle and increased in the mammary gland. The ability of insulin to activate PDH of white adipose tissue was lost during lactation, whereas it was retained by the other tissues. The ability of noradrenaline to activate PDH was decreased in white adipose tissue but increased in liver during lactation. These various adaptations should limit the use of glucose and lactate carbon by adipose tissue and skeletal muscle during lactation and thereby facilitate their preferential utilization by the mammary gland.

Regulation of bovine adipose tissue metabolism during lactation. 2. Lipolysis response to milk production and energy intake

Lipolytic adaptations of bovine adipose tissue during late pregnancy, lactation, and dry period were studied in Holsteins. Treatment groups consisted of first lactation daughters of high or low bulls based on Predicted Difference for milk. Heifers were fed either a 60% concentrate, 40% hay diet or a 40% concentrate, 60% hay diet from 0 to 140 d lactation. Feeding a low energy diet for the first 140 d of lactation did not affect adipose tissue lipolytic rates measured in vitro. Epinephrine and norepinephrine responsiveness of bovine adipose tissue increased prior to parturition, increased further in early lactation, then remained elevated during lactation and into the dry period. This responsiveness was unaffected by feeding low energy diets. Basal glycerol release in high genetic merit heifers was 64, 17, 40, 23, 20, and 42% greater than in low genetic merit heifers at -30, -15, 30, 60, 180, and 349 d about parturition. Response to norepinephrine was 15, 20, 18, and 15% greater in high genetic than low genetic merit heifers and response to epinephrine was 12, 20, 14, and 50% greater in high genetic than low genetic merit heifers at 30, 60, 180, and 349 d postpartum. The lack of a dietary energy restriction effect on lipolysis in early lactation suggested that these rates were near the physiological maximum for those animals. Rates of lipolysis were positively related with milk fat production. This study indicates a genetic component in adrenergic regulation of lipolysis in adipose tissue, independent of energy balance, in supporting lactation.

Regulation of bovine adipose tissue metabolism during lactation. 1. Lipid synthesis in response to increased milk production and decreased energy intake

Effects of genetic selection for milk secretion and dietary energy restriction on adipose tissue lipid synthesis were tested in first lactation Holsteins. Daughters of bulls of 795 kg or 167 kg of Predicted Difference, base 1974, for milk had a 305 d first lactation of 8626 and 7038 kg of milk at 3.02 and 2.71% milk fat, respectively. Animals of each genetic group were allotted to treatments of a diet of 40% hay and 60% concentrate or of 60% hay, 40% concentrate ad libitum from 0 to 140 d of lactation. First lactation heifers fed the low energy diet ate 19% less net energy and produced 7% less milk of 28% more milk fat percent compared with heifers fed the high energy diet. Lipogenesis rates at 15, 30, 60, and 180 d postpartum in heifers fed low energy diets were 107, 66, 21, and 166% of those in heifers fed high energy diets. Rates of lipogenesis in adipose tissue of heifers of high genetic merit were 40, 10, 45, and 66% of those in low genetic merit heifers at 15, 30, 60, and 180 d postpartum. Lipogenesis in bovine adipose tissue homeostatically adapts to decreased energy intake and homeostatically adapts to genetically selected increase in milk fat secretion.

Increased insulin sensitivity and responsiveness of glucose metabolism in adipocytes from female versus male rats

This study was undertaken to examine whether there were sex-associated differences in the action of insulin on glucose metabolism in adipocytes. Insulin binding and the dose-response curves for glucose transport (assessed by measuring the cell-associated radioactivity after 15-s incubation with 50 microM [6-14C]glucose) and [U-14C]glucose (5 mM) metabolism into CO2 and lipids were compared in retroperitoneal adipocytes from age-matched (84 d) male and female rats. In addition, the activity of fatty acid synthetase, one of the key lipogenic enzymes, was determined. Fat cell size was not significantly larger in females than in males (0.238 vs. 0.209 microgram lipid per cell). At insulin concentrations less than or equal to 1.6 nM, adipocytes from females bound significantly more insulin than did adipocytes from males, due to an increased apparent affinity of the receptors for insulin. Accordingly, the sensitivity of glucose transport to insulin was greater in females than in males: insulin concentration eliciting half-maximal stimulation (ED50) = 0.19 nM vs. 0.41 nM. At maximal insulin stimulation the rates of glucose transport (12 times the basal values) were similar in the two sexes. In contrast, the maximal effect of insulin on glucose conversion to CO2 plus lipids was much greater in the adipocytes from females than males (increment over basal: 472 vs. 249 nmol/10(6) cells per 2 h). Fatty acid synthesis contributed approximately 40% of the incremental difference between the two types of adipocytes, while glyceride-glycerol synthesis contributed less than 10%. The insulin dose-response curves for adipocytes from females were shifted to the left for all the metabolic pathways investigated. The mean ED50 for total glucose metabolism in females was 50% of that in males (0.07 nM vs. 0.15 nM). Marked sex-associated differences in the action of insulin on glucose metabolism were also observed in subcutaneous inguinal adipocytes (increment over basal: 137 and 56 nmol/10(6) cells per 2 h, ED50 = 0.13 nM and 0.30 nM in females and males, respectively). The intracellular capacity to metabolize glucose through the fatty acid synthesis pathway, as assessed by FAS activity, was higher in adipocytes from females than in those from males and was greater in retroperitoneal than in inguinal adipocytes. Furthermore, by plotting the individual data, a highly significant correlation (r = 0.92, P less than 0.001) was found between the absolute effect of insulin on glucose metabolism at maximal stimulation and the fatty acid synthetase activity of the cells. These results indicate that the response of glucose metabolism to insulin in adipocytes from female as compared with male rats is characterized by two main features: (a) an increased sensitivity primarily due to an increase in insulin binding, and (b) an increased responsiveness closely associated with a postreceptor increase in the lipogenic capacity of the cell. These findings might be relevant to the differential disposition of male and female rats to develop fatness.

Adaptations of adipose tissue metabolism and number of insulin receptors in pregnant sheep

The endocrine control of adipose tissue metabolism during pregnancy in sheep has been investigated. The number of insulin receptors of sheep adipocytes was increased during pregnancy. There was no apparent change in the concentration of serum insulin during pregnancy in sheep while the rise in serum progesterone concentration was smaller and more gradual than in rats. Net lipid deposition in adipocytes occurred during the first 55 days of pregnancy, probably due primarily to increased lipoprotein lipase activity. Net deposition of lipid had ceased by mid-pregnancy while by 125 days of pregnancy, the rate of fatty acid synthesis in adipose tissue was decreased and the serum fatty acid concentration had risen, suggesting the onset of net lipid mobilization in the tissue. Results are compared with those from other studies with rats; it would appear that different mechanisms regulate lipid deposition during pregnancy in sheep and rats.