Mario DiGirolamo

The biology of white adipocyte proliferation.

Expanded adipose tissue mass increases the risk for many clinical conditions including diabetes, hypertension, coronary atherosclerotic heart disease, and some forms of cancer. Therefore, it is imperative that we understand the mechanisms by which fat pads expand. The enlargement of fat cells during the development of obesity has been previously hypothesized to be a triggering factor for the proliferation of new fat cells. There is now a preponderance of evidence that adipose tissue is a source of growth factors such as IGF‐I, IGF binding proteins, TNFα, angiotensin II, and MCSF that are capable of stimulating proliferation. The relative importance of these autocrine/paracrine factors in the normal control of preadipocyte proliferation is unknown. In addition, the proliferative response of preadipocytes to the paracrine milieu is undoubtedly modulated by neural inputs to fat tissue and/or serum factors. Together, these multiple regulatory controls orchestrate overall and region‐specific adipose tissue cellularity responses associated with the development of hyperplastic obesity. Both in vivo and in vitro studies are needed to understand the complex, interacting physiological mechanisms by which growth of this important organ is regulated.

Lactate production in adipose tissue: a regulated function with extra-adipose implications.

Estimates of the quantitative contribution of adipose tissue to whole‐body glucose metabolism, previously reported as 1–3%, have been revised to be on the order of 10–30%, These revised estimates come, in part, from a recognition that adipose tissue uses glucose to produce lactate and pyruvate, in addition to CO2 and triglycerides. Lactate production by adipose tissue is modulated in vitro by changes in glucose, insulin, and epinephrine concentrations. In vivo, lactate production is regulated acutely by the animals nutritional state (fed or fasted) and chronically by the degree of obesity. A strong positive correlation exists between rat fat cell size and relative conversion of glucose to lactate (r = 0.89, P < 0.001). Diabetes is also associated with markedly increased lactate production in adipocytes. Fat cells from obese or diabetic rats (or humans) can metabolize to lactate as much as 50‐70% of the glucose taken up. From these recent studies, a picture is emerging in which the adipose organ may provide lactate for hepatic gluconeogenesis during fasting, and also lactate for hepatic glycogen synthesis after food ingestion. Modulation of adipocyte lactate production and contribution of adipose tissue lactate to the bodys fuel economy in physiological and pathological states are the focus of this review.—DiGirolamo, M.; Newby, F. D.; Lovejoy, J. Lactate production in adipose tissue: a regulated function with extra‐adipose implications. FASEB J. 6: 2405‐2412; 1992.

Insulin resistance in obesity is associated with elevated basal lactate levels and diminished lactate appearance following intravenous glucose and insulin.

Lactate metabolism is altered in obesity. Increasing obesity is associated with increased blood lactate levels after an overnight fast. In contrast, we have recently shown a marked decrease in the capacity for acute lactate generation in obese subjects following an oral glucose load, which we postulated might be linked to altered insulin sensitivity. In the present study, we systematically analyzed the relationship between insulin sensitivity (the Sensitivity Index [SI] derived using the minimal model), body mass index (BMI), and glucose, insulin, and lactate levels in the basal state and following intravenous (IV) glucose and insulin administration in lean and obese subjects. The results showed that SI and BMI were inversely related, as expected. Insulin sensitivity was more tightly associated with glucose, insulin, and lactate levels (both basal and integrated) than obesity per se. A significant inverse relationship was found between SI and basal lactate levels (r = -.56). Moreover, a significant and positive relationship was found between SI and incremental lactate area under the curve (reflecting acute lactate production) (r = .41). In a multiple regression analysis to separate the independent effects of obesity (BMI) and insulin sensitivity, after adjusting for age, sex, and race, SI accounted for 34% of the variance in basal lactate and 24% of the variance in incremental lactate area. Obesity independently accounted for 10% of the variance in basal lactate and 11% of the variance in incremental lactate area, neither of which were statistically significant. We conclude that elevations in basal lactate are associated with the development of insulin resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Meal size and thermic response to food in male subjects as a function of maximum aerobic capacity.

The relationship between size of a mixed, liquid meal and the thermic effect of food (TEF) was studied in two groups of nonobese male subjects differing in maximum aerobic capacity (VO2 max). A design using repeated measures was chosen in which each subject received each meal (water, 500 kcal, 1000 kcal, 1500 kcal) on a different morning. TEF was measured by indirect calorimetry for three hours following each meal and was found to increase systematically, in a nonlinear fashion, as meal size was increased. Subjects with a high VO2 max responded to the two higher calorie meals with a greater TEF than did subjects with a low VO2 max. They also showed a greater increase in TEF for any given increase in meal size. This study establishes a precise relationship between meal size and the thermic effect of food. It also identifies an important variable, VO2 max, in determination of the individual thermic response to food. These findings suggest that individuals with a high VO2 max (such as aerobically trained athletes) show a greater caloric expenditure after eating, particularly after a large meal, than do individuals with a low VO2 max. A high thermic response to food could be beneficial in body weight homeostasis.

Insulin Binding and Responsiveness in Fat Cells from Patients with Reduced Glucose Tolerance and Type II Diabetes

Adipose tissue was obtained from 66 individuals including 21 patients with type II diabetes of different severity (16 SU-treated and 5 diet-treated only) as well as 9 obese subjects with reduced glucose tolerance. Adi-pocyte insulin binding, antilipolytic effect of insulin, and glucose incorporation into triglycerides were measured in the diabetic and the obese subjects and the data compared with that of normal controls of similar age and relative weight. Insulin binding per cell was normal in the diabetic patients and was significantly increased at low insulin concentrations in the obese patients with reduced glucose tolerance, suggesting increased affinity. Furthermore, insulin binding correlated negatively with age but, when age was corrected for, did not correlate significantly with fasting insulin or glucose levels, relative body weight, or fat cell size. Insulin sensitivity, measured as the antilipolytic effect of insulin, was similar in all patient groups. Patients with the most severe type II diabetes (SU-treated group) demonstrated, in contrast to the less severely diabetic patients, a marked reduction in both basal and insulin-stimulated glucose incorporation into triglycerides showing the presence of a pronounced postreceptor defect. The insulin effect on glucose incorporation correlated negatively with the fasting glucose levels, suggesting that the postreceptor defect seen in the adipocyte reflectsperturbations in other organs, like muscle or liver, of greater importance for glUCOSe homeostasis.

Effects of a high-fat diet on energy intake and expenditure in rats

The effects of a high-fat diet supplying a constant energy/protein ratio, with and without overeating, on energy intake and expenditure was studied in mature male rats. A control group (LF) received ad libitum access to a low-fat diet. Body weight gain, efficiency of food utilization, and dietary-induced thermogenesis were increased relative to controls in a group with ad libitum access to the high-fat diet (HF-A), but not in a group which was pair fed the diet (HF-P) in amounts (kcal) equal to that of LF animals. However, the individual variability within the HF-A group was high for each measure. An arbitrary separation of that group into 2 subgroups (based on high vs low weight gain) produced one subgroup with increased efficiency, greater weight gain and no change in dietary-induced thermogenesis (HF-AH), and another with no difference in efficiency or in weight gain from the LF group but which had higher dietary-induced thermogenesis (HF-AL). Food intake was slightly, but not significantly, greater for the HF-AH subgroup than for the HF-AL subgroup. We conclude that rats can increase thermogenesis in response to overeating but that the increase is highly variable. The thermogenic response appears to be related to the overeating rather than to the fat content of the diet.

Effects of Streptozocin-Induced Diabetes on Glucose Metabolism and Lactate Release by Isolated Fat Cells From Young Lean and Older, Moderately Obese Rats

Streptozocin-induced diabetes (STZ-D) was produced in male Wistar rats at two stages of development: young, lean rats, weighing 150–220 g (6–8 wk), and older, moderately obese rats, weighing 450–500 g (6–8 mo). A comparable degree of hyperglycemia (420–500 mg/dl) without ketosis was generated by injection of 50 mg/kg i.v. STZ for young, lean rats and 30 mg/kg i.v. for older, fatter rats. The animals were killed 8–11 days after injection. Insulin binding by the isolated adipocytes of both groups was not significantly altered on a per-cell basis by the presence of diabetes. Total adipocyte glucose metabolism, both basal and insulin stimulated, was reduced (63 and 88%, respectively) by the induction of diabetes in young, lean rats. In contrast, the induction ofdiabetes in the older, moderately obese rats had no suppressive effect on total glucose metabolismby their fat cells. Diabetes increased the relative conversion of glucose to lactate by fat cells from both groups of rats, but in absolute terms, the fat cells from the obese diabetic rats produced significantly more lactate from glucose than cells from the lean diabetic rats, both in the absence and presence of insulin. Diabetes did not alter the glucose concentration at which peak insulin response occurred in either group. We conclude that STZ-D in rats, at different stages of development and degrees of adiposity, results in quantitatively different alterations of adipocyte metabolism, which appear to be postreceptor in nature and result in an increase in glucose conversion to lactate.

Influence of ambient glucose and insulin concentrations on adipocyte insulin binding

To elucidate factors of importance for insulin binding, fat cells from humans and rats were incubated under various experimental conditions for different periods of time. Human adipocytes incubated for 24 hours in the absence of insulin showed no significant difference in insulin binding compared with cells from freshly excised tissue. After 48 hours, however, an increased rate of binding (average 54%; P less than 0.05) was obtained. The addition of insulin (2000 microU/ml) to the culture medium resulted in a decrease in insulin binding (average 33%; P less than 0.05) compared with cells maintained in the absence of insulin. There was no apparent difference in receptor affinity, indicating that the altered binding was due to a change in receptor number. In the absence of insulin, elevating the glucose concentration of the medium from 0.8 mM to 22.4 mM did not significantly influence insulin binding. Rat adipocytes showed similar but more rapid changes. Thus, incubation for 24 hours without insulin caused an increase in insulin binding (average 37%; P less than 0.05). This up-regulation was seen even in a high glucose concentration (28 mM) but was completely prevented by the presence of insulin in the medium. Furthermore, when rat adipocytes were incubated with insulin in the presence of a high glucose concentration (28 mM) there was a significant further decrease in insulin binding compared with that of parallel incubations performed in 5.6 mM glucose. Thus, even in the absence of TRIS buffer, insulin-dependent regulation of the number of binding sites is shown for both human and rat adipocyte tissue in vitro. Although this perturbation could be directly due to hormone-receptor interaction at the membrane level, the finding of rat adipocytes that the ambient glucose concentration can modulate this effect suggests the importance of post-receptor events.

Insulin Sensitivity Accounts for Glucose and Lactate Kinetics After Intravenous Glucose Injection

Mathematical modeling was used to explore the interaction between glucose, insulin, and lactate during the frequently sampled intravenous glucose tolerance test (FSIGTT). Insulin-modified FSIGTs were performed in 25 lean volunteers, and an additional 5 volunteers underwent FSIGTs with glucose injection alone to illustrate the effect of insulin on both glucose and lactate kinetics. The model chosen as the best representation of the system extended the minimal model of glucose kinetics (MM) by including a two-compartment model of lactate kinetics. The model accounted for both glucose and lactate kinetics, provided traditional MM parameters of insulin sensitivity and glucose effectiveness, and descriptive parameters of lactate kinetics. Modeling suggested that lactate production was limited by the rate of glucose disappearance, with no indication of direct effects of insulin on lactate. Inclusion of lactate kinetics had no adverse effect on MM parameters (SG: 0.023 ± 0.009 vs. 0.023 ± 0.010 min−1,SI: 1.01 ± 0.70 vs. 1.03 ± 0.71 × 104 · min−1 · pmol−1 · 1; P < 0.50, lactate model vs. MM), and indicated that ∼1.2% min−1 of total glucose disappearance during the FSIGT is converted to lactate. An additional benefit of including lactate kinetics was the significant improvement in precision in MM parameter estimates as reflected by the fractional standard deviations (FSDs). This effect was most prominent for SG, in which a threefold improvement in parameter precision was observed (FSD: 13.5 ± 3.1 vs. 42.5 ± 48.5; means ± SD). These analyses indicate that kinetic information for both glucose and lactate can be derived from FSIGT data and provide insights into the interaction between glucose, insulin, and lactate under non-steady-state conditions.

Lipoprotein lipase secretion from isolated rat fat cells of different size

Lipoprotein lipase(LPL) release from isolated small fat cells from young rats and large fat cells from older, fatter rats was compared during in vitro incubation at 30 degrees C. Although large fat cells had nearly three times higher cellular LPL activity, they secreted similar amounts of LPL activity into the incubation medium under both basal conditions (Krebs Ringer bicarbonate buffer containing 4% albumin and 6mM glucose) and after stimulation of LPL release by 5% human serum, or serum plus 1 U/ml heparin. These data suggest that previous observations of an altered tissue distribution of LPL in adipose tissue containing large fat cells can be at least in part explained by an alteration at the level of LPL secretion.