Pathways of glycogen synthesis from glucose during the glycogenic response to insulin in cultured foetal hepatocytes

Lactate and pyruvate stimulate the conversion of glucose to glycogen in hepatocytes by a mechanism that does not involve gluconeogenic flux

Glycogen synthesis in hepatocytes was determined at various concentrations of CO2 and medium HCO3- to modulate cell pH. Glycogen synthesis from glucose was highest in acidic conditions (5% CO2/12.5 mM HCO3-) and lowest in alkaline conditions (2.5% CO2/25 mM HCO3-). Physiological concentrations of lactate/pyruvate (2 mM/0.2 mM) stimulated the conversion of glucose to glycogen in all media examined and mercaptopicolinate, an inhibitor of gluconeogenesis, caused a similar stimulation as lactate/pyruvate. In alkaline media, the stimulation by mercaptopicolinate and by lactate/pyruvate was additive, indicating that the latter is not due to gluconeogenic flux. In acidic media, the stimulation by both lactate/pyruvate and mercaptopicolinate was inhibited by amiloride, an inhibitor of Na+/H+ exchange. Since Na+/H+ exchange is activated when cell pH falls below a certain threshold, it is postulated that lactate and pyruvate stimulate the conversion of glucose to glycogen through cellular acidification.

Glucose contribution to nucleic acid base synthesis in proliferating hepatoma cells: a glycine-biosynthesis-mediated pathway

The coupling of glycolysis to serine and glycine metabolism was studied in fast-growing Zajdela hepatoma cultured cells. During the exponential phase of growth, occurring between 12 and 72 h, cells exhibited a decreased glycogen content together with a high glycolytic activity. Glycogen labelling, evaluated by 1 h-pulse experiments with [U-14C]glucose (5.5 mM), was minimal during the first 48 h and increased 2.5-fold at 72 h and 8-fold at 96 h, at which times it was also stimulated 2-fold by 10 nM insulin. [U-14C]Glucose carbons were incorporated into nucleic acid bases, with maximal incorporation at 72 h, the rate of nucleotide base labelling exceeding that of glycogen during the first 2 days of culture. Incubation of the cells with [U-14C]glucose resulted in the release into the medium of 14C-labelled glycine, the first intermediate formed on the route from serine to DNA. The rate of release per cell decreased as a function of cell growth, concomitantly with an increased rate of glucose carbon incorporation into nucleotide bases. The latter implied the intermediary formation of amino acids since the transaminase inhibitor cycloserine (10 mM), which totally inhibited [14C]glycine release, decreased by 65% nucleotide labelling from [U-14C]glucose. A dose-dependent inhibition by serine of the rate of [U-14C]glucose carbon incorporation into nucleotide bases was observed, which was maximal at 5 mM serine. These metabolic flux measurements indicate that glucose can be used as a precursor of nucleic acid synthesis. These results strongly suggest that this process is to a large extent mediated by a serine/glycine-biosynthesis-mediated pathway, and reinforce the hypothesis that glycolysis contributes to enhancing the provision of precursors required for cell proliferation.

Glycogen synthesis from glucose by direct and indirect pathways in hepatocyte cultures from different nutritional states

The conversion of glucose to glycogen by direct and indirect pathways was determined from the incorporation of [6-3H,U-14C]glucose into glycogen in hepatocyte cultures isolated from fed, fasted or fasted-refed rats. Mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase (PEPCK) was used to determine the extent by which 6-tritium is lost by mechanisms not involving flux through PEPCK. Glucose conversion to glycogen was lower in hepatocytes from fasted and higher in hepatocytes from fasted-refed rats than in hepatocytes from rats fed ad libitum. Insulin increased glycogen synthesis in hepatocytes from all nutritional states, and it decreased the 3H/14C ratio incorporated into glycogen. This increased loss of 6-tritium was only in part mercaptopicolinate-sensitive. Lactate and pyruvate (2 mM + 0.2 mM) increased glycogen deposition, largely by stimulation of glucose conversion to glycogen by the direct pathway. Insulin-induced glucokinase mRNA expression was higher in hepatocytes from fed than from fasted or refed rats whereas PEPCK mRNA expression was lowest in hepatocytes from fasted-refed rats. Hepatocyte cultures derived from different nutritional states express differences in glycogen synthesis from glucose by direct and indirect pathways as well as differences in the extent by which pyruvate cycling accounts for loss of 6-tritium.

The contribution of pyruvate cycling to loss of [6-3H]glucose during conversion of glucose to glycogen in hepatocytes: effects of insulin, glucose and acinar origin of hepatocytes

1. During conversion of [6-3H,U-14C]glucose to glycogen in liver, loss of 6-3H can occur either by cycling via pyruvate (between glycolysis and gluconeogenesis) or by other mechanisms. We used mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase, to determine the extent to which pyruvate cycling contributes to loss of 6-3H during glucose conversion to glycogen in hepatocytes. 2. Mercaptopicolinate increased the 3H/14C ratio in glycogen during incubation of rat, guinea pig, pig and human hepatocytes with [6-3H,U-14C]glucose. The increase in the 3H/14C ratio in glycogen caused by mercaptopicolinate was greater in periportal than in perivenous rat hepatocytes, indicating that cycling of glucose via pyruvate is more prominent in cells with a higher gluconeogenic relative to glycolytic capacity. 3. The effect of mercaptopicolinate on the 3H/14C ratio in glycogen was observed both in the absence and in the presence of insulin, indicating that stimulation of glycogen synthesis by insulin is not associated with inhibition of pyruvate cycling. In rat and guinea pig but not in pig hepatocytes, the effects of mercaptopicolinate on the 3H/14C ratio in glycogen were greater at 10-15 mM glucose than at 30 mM glucose, suggesting diminished cycling via pyruvate at high glucose concentrations. 4. Insulin increased the loss of 6-3H during stimulation of conversion of glucose to glycogen in hepatocytes from all species. This was due in part to an increase in pyruvate cycling and in part to other mechanisms that are not inhibited by mercaptopicolinate. 5. These results suggest that pyruvate cycling is a significant, but not exclusive, component of the loss of 6-3H in the hepatocyte during glucose conversion to glycogen. The extent of pyruvate cycling is dependent on the acinar origin of the hepatocytes and on the glucose concentration and presence of insulin.

Fetal serine fluxes across fetal liver, hindlimb, and placenta in late gestation

Eleven studies of fetal serine fluxes were performed in chronically catheterized fetal lambs by continuous infusion of [1-13C]- and [U-14C]serine into a fetal brachial vein. At tracer serine steady state, samples were collected from the fetal abdominal aorta, umbilical vein, fetal hepatic vein, and fetal femoral vein and from the maternal femoral artery and uterine vein. Analyses were performed for plasma serine and glycine concentration, for serine and glycine 13C mole percent enrichment, and for whole blood 14CO2 and O2 concentrations. Uterine and umbilical blood flows were also measured. The placenta had a significant net uptake of fetal serine (2.1 +/- 0.5 mumol.min-1.kg-1, P < 0.01). Fetal plasma serine disposal rate (DR) was 42.5 +/- 3.9 mumol.min-1.kg-1.CO2 production from decarboxylation of fetal plasma serine represented 7.9 +/- 0.5% of DR, or 10.1 +/- 1.2 mumol CO2.min-1.kg-1. Fetal plasma glycine enrichment was 59.7 +/- 4.9% of fetal plasma serine enrichment. There was a significant loss of tracer serine from the fetal circulation into the placenta accounting for approximately 45% of infused tracer. Fifteen percent of this was converted to glycine and released into the umbilical circulation. There was a significant uptake of tracer serine by both fetal liver and fetal hindlimb with a significant CO2 production by both sites with serine oxidation predominantly in the carcass. These results indicate a high fetal serine disposal rate in the lamb, with rapid fetoplacental serine exchange, resulting in a net uptake of fetal serine by the placenta.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations in the antagonistic effects of insulin and glucagon on glycogen metabolism in cultured foetal hepatocytes

The antagonistic effects of insulin and glucagon on glycogen formation and mobilization were studied in cultured 18-day foetal rat hepatocytes with regard to different modes of exposure. Hormone combinations were achieved with a constant dose of 10 nM-insulin (maximal for the glycogenic effect of this hormone) and increasing doses of glucagon [from 0.03 to 10 nM: concn. causing half-maximal response (ED50) = 0.3 nM)]. When insulin and glucagon were added simultaneously, increasing glucagon concentrations progressively depressed the glycogenic effect of insulin and 0.3 nM-glucagon antagonized the insulin effect completely. The maximal glycogenolytic effect of glucagon was observed at concentrations greater than 1 nM. When the two hormones were introduced successively, with an interval of 4 h between additions, the effect of the second hormone was always fully expressed between 4 and 8 h. at which time the effect of the first hormone had ceased; the dominance of glucagon over insulin was also lost, due to cell desensitization to glucagon. Both continuous or intermittent (10 min on/10 min off periods) exposure to insulin and/or glucagon gave similar antagonistic effects, while in cells exposed to insulin plus glucagon alternating with exposure to insulin or glucagon alone, the glycogenic effect of insulin was less or more antagonized respectively by glucagon. Whatever the situation, the results obtained could not be related to antagonism by a glucagon-induced rise in either cyclic AMP levels (ED50 = 3 nM) or cell-surface hormone binding. Thus, depending on the hormonal state and the mode of hormone administration, regulation of glycogenesis in cultured foetal hepatocytes appears to be different from that predicted by the insulin/glucagon molar ratio, which is strikingly altered in the perinatal period.

Role of serine biosynthesis and its utilization in the alternative pathway from glucose to glycogen during the response to insulin in cultured foetal-rat hepatocytes

The role of serine as a possible intermediate of the alternative pathway from glucose to glycogen was investigated under basal and insulin-stimulated conditions in 18-day cultured foetal-rat hepatocytes because these cells cannot use pyruvate-derived metabolites [Bismut & Plas (1989) Biochem. J. 263, 889-895]. Incubation of cells with [U-14C]glucose for 24 h led to a release of labelled serine in the medium concomitantly with a net serine production (100 nmol/24 h per culture). The rate of [14C]serine formation (close to 3 nmol/h per culture) indicated that a large part of newly formed serine originated from glucose. When short-term experiments were performed at day 2, glycogen labelling from [U-14C]serine or [U-14C]glycine, which was increased 3-fold by insulin after 2 h, evidenced their participation as glycogenic precursors. When a double-isotope procedure with [U-14C,3-3H]glucose was used, the direct and the alternative pathways from glucose were found to contribute to glycogenesis by 75 and 25% respectively. Cycloserine (18 mM), a transaminase inhibitor, strongly inhibited glycogen labelling from [U-14C] serine while producing a 70% increase in glucose incorporation by the alternative pathway, in both the presence and the absence of insulin. The inhibitor had no effect on the direct pathway from glucose to glycogen. Supplementation with 1 mM-hydroxypyruvate, a serine-derived metabolite, did not affect direct glucose incorporation, whereas the alternative pathway was stimulated whether insulin was present or not. These results indicate that the sequence glucose----serine----glycogen is operative in cultured foetal hepatocytes. The alternative pathway interferes with hydroxypyruvate utilization, and is likely mediated by the serine aminotransferase pathway, independently of the acute glycogenic action of insulin.