The Regulation of Glyceride Synthesis from Pyruvate in Isolated Fat Cells

Abstract

The incorporation of 14C-labelled pyruvate into glyceride fatty acids and glyceride glycerol has been measured in fat cells prepared from epididymal adipose tissues of normal rats, rats starved for 24 h, rats starved for 24 h and refed a carbohydrate diet for 24 h or rats starved for 24 h and refed a fat diet for 24 h. Rates of fatty acid synthesis were decreased in starved and starved, fat refed cells and increased in starved, carbohydrate refed cells. I n all four dietary states, addition of palmitate to the incubation medium resulted in increased rates of glyceride glycerol formation and decreased rates of fatty acid synthesis from 14C-labelled pyruvate. In normal cells these effects of palmitate were maximal below 1 mM palmitate. Dietary-induced alterations in incorporation of 14C-labelled pyruvate into glyceride glycerol, maximised by addition of palmitate, correlated well with corresponding alterations in the activity of extramitochondrial phosphoenolpyruvate carboxykinase in the cells, but did not correlate with alterations in the activity of pyruvate carboxylase or mitochondrial phosphoenolpyruvate carboxykinase. Use of marker enzymes allowed an assessment to be made of the distribution of these enzymes between extramitochondrial and mitochondrial spaces.The incubated rat epididymal fat pad, using pyruvate as the sole substrate, has been shown to be able to synthesise both the glycerol and fatty acid portions of glycerides [l-51. Isotopic evidence has been presented that the formation of glyceride glycerol from pyruvate proceeds through the carboxylation of pyruvate to oxalacetate, the formation of phosphoenolpyruvate from oxalacetate, and subsequently the production of glycerol phosphate through a partial reversal of glycolysis [6,7]. It has been suggested that this pathway may be important for the maintenance of glyceride turnover in metabolic situations associated with a decreased availability of glucose [6,8,9]. The presence of both pyruvate carboxylase and phosphoenolpyruvate carboxykinase has been demonstrated in both soluble and mitochonEnzymes. Citrate (isocitrate) hydro-lyase or aconitate hydratase (EC 4.2

Acute regulation of 5′-AMP-activated protein kinase by long-chain fatty acid, glucose and insulin in rat primary adipocytes

“…Most fructose transport is through GLUT5 that is entirely in the plasma membrane with no modulation of its membrane abundance by insulin [34]. Also the utilization of pyruvate that enters the cells via a monocarboxylate transporter is unaffected by palmitate [1]. (iii) The attenuation by Compound C of the effects of palmitate on glucose utilization, 2-DOG transport and on the palmitate ‘anabolic effect’ as well as its attenuation of the activation of AMPK by palmitate implies that AMPK is a mediator of these effects.…”

“…The other essential TAG precursor is glycerol 3-phosphate. Adipocytes have minimal expression of glycerol kinase and normally glyceroneogenesis from 3-carbon precursors is low [1]. Therefore cellular glycerol 3-phosphate is largely derived from plasma glucose whose entry into adipocytes is mainly facilitated by the insulin-activated glucose transporter GLUT4.…”

De Novo Lipogenesis and Stearoyl-CoA Desaturase Are Coordinately Regulated in the Human Adipocyte and Protect against Palmitate-induced Cell Injury

“…Early studies in rats showed palmitate to stimulate glucose incorporation into TG fatty acids (10,11). Several studies of high fat feeding in rats and mice, whether or not specifically high in saturated fat, show induction of DNL at a transcriptional or flux level (12)(13)(14)(15).…”

Lipogenesis in rabbit isolated fat-cells

“…The role of the extraordinarily high mitochondrial phosphoenolpyruvate carboxylase activity in rabbit fat-cells remains unclear at present. The abundance of this activity does not appear to confer any extra ability to carry out 'glyceroneogenesis' (Reshef et al, 1970) since the incorporation of pyruvate or lactate carbon into glyceride glycerol shown in Table 1 was no higher than that found in rat cells (Saggerson & Tomassi, 1971) and was very low compared with the incorporation of glucose into glyceride glycerol.…”

“…The procedures for the measurement of the incorporation of '4C-labelled substrates have been described (Saggerson & Tomassi, 1971;Saggerson, 1972a,b). Fat-cell DNA was determined as described by Saggerson (1972a).…”