Alice J Yee’s research while affiliated with University of Southern California and other places

To determine the role of AMP-activated protein kinase (AMPK) activation on the regulation of fatty acid (FA) uptake and oxidation, we perfused rat hindquarters with 6 mM glucose, 10 microU/ml insulin, 550 microM palmitate, and [14C]palmitate during rest (R) or electrical stimulation (ES), inducing low-intensity (0.1 Hz) muscle contraction either with or without 2 mM 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). AICAR treatment significantly increased glucose and FA uptake during R (P < 0.05) but had no effect on either variable during ES (P > 0.05). AICAR treatment significantly increased total FA oxidation (P < 0.05) during both R (0.38 +/- 0.11 vs. 0.89 +/- 0.1 nmol x min(-1) x g(-1)) and ES (0.73 +/- 0.11 vs. 2.01 +/- 0.1 nmol x min(-1) x g(-1)), which was paralleled in both conditions by a significant increase and significant decrease in AMPK and acetyl-CoA carboxylase (ACC) activity, respectively (P < 0.05). Low-intensity muscle contraction increased glucose uptake, FA uptake, and total FA oxidation (P < 0.05) despite no change in AMPK (950.5 +/- 35.9 vs. 1,067.7 +/- 58.8 nmol x min(-1) x g(-1)) or ACC (51.2 +/- 6.7 vs. 55.7 +/- 2.0 nmol x min(-1) x g(-1)) activity from R to ES (P > 0.05). When contraction and AICAR treatment were combined, the AICAR-induced increase in AMPK activity (34%) did not account for the synergistic increase in FA oxidation (175%) observed under similar conditions. These results suggest that while AMPK-dependent mechanisms may regulate FA uptake and FA oxidation at rest, AMPK-independent mechanisms predominate during low-intensity muscle contraction.

Insulin has been shown to alter long-chain fatty acid (LCFA) metabolism and malonyl-CoA production in muscle. However, these alterations may have been induced, in part, by the accompanying insulin-induced changes in glucose uptake. Thus, to determine the effects of insulin on LCFA metabolism independently of changes in glucose uptake, rat hindquarters were perfused with 600 microM palmitate and [1-(14)C]palmitate and with either 20 mM glucose and no insulin (G) or 6 mM glucose and 250 microU/ml of insulin (I). As dictated by our protocol, glucose uptake was not significantly different between the G and I groups (10.3 +/- 0.6 vs. 11.0 +/- 0.5 micromol x g(-1) x h(-1); P > 0.05). Total palmitate uptake and oxidation were not significantly different (P > 0.05) between the G (10.1 +/- 1.0 and 0.8 +/- 0.1 nmol x min(-1) x g(-1)) and I (10.2 +/- 0.6 and 1.1 +/- 0.2 nmol. min(-1) x g(-1)) groups. Preperfusion muscle triglyceride and malonyl-CoA levels were not significantly different between the G and I groups and did not change significantly during the perfusion (P > 0.05). Similarly, muscle triglyceride synthesis was not significantly different between groups (P > 0.05). These results demonstrate that the presence of insulin under conditions of similar glucose uptake does not alter LCFA metabolism and suggest that cellular mechanisms induced by carbohydrate availability, but independent of insulin, may be important in the regulation of muscle LCFA metabolism.

To determine whether changes in long-chain fatty acid (LCFA) oxidative metabolism induced by elevated intracellular carbohydrate availability are due to changes in LCFA uptake or in mitochondrial transport capacity, rat hindquarters were perfused with 500 microM palmitate and [1-14C]palmitate or [1-14C]octanoate as well as with either low (LG) or high (HG) carbohydrate availability. Glucose uptake was higher in the HG vs. LG group (23.6 +/- 1.5 vs 4.7 +/- 0.9 micromol x g(-1) x h(-1), P < 0.05). Palmitate delivery was not significantly different between groups and averaged 97.1 +/- 4.6 nmol x min(-1) x g(-1) (P > 0.05). Fractional and total palmitate uptake values were 60% higher (P < 0.05) in the HG (0.125 +/- 0.012 and 7.4 +/- 1.2 nmol x min(-1) x g(-1)) vs. LG (0.079 +/- 0.009 and 11.8 +/- 1.5 nmol x min(-1) x g(-1)) group. Values of percent and total palmitate oxidized were significantly lower (P < 0.05) in the HG (9.1 +/- 1.1% and 1.31 +/- 0.16 nmol x min(-1) x g(-1)) vs. LG (23.4 +/- 5.2% and 0.76 +/- 0.08 nmol x min(-1) x g(-1)) group. Conversely, values of fractional uptake and percent oxidation of octanoate were not significantly different between groups (P > 0.05). Malonyl-CoA levels were inversely correlated with LCFA oxidation (P < 0.05). These results demonstrate that high carbohydrate availability in muscle is associated with a decrease in LCFA oxidation that is not due to a parallel decrease in LCFA uptake; rather, the decrease in LCFA oxidation could be due to malonyl-CoA inhibition of mitochondrial LCFA transport.

Altered muscle fatty acid (FA) metabolism may contribute to the presence of muscle insulin resistance in the genetically obese Zucker rat. To determine whether FA uptake and disposal are altered in insulin-resistant muscle, we measured palmitate uptake, oxidation, and incorporation into di- and triglycerides in isolated rat hindquarters, as well as muscle plasma membrane fatty acid-binding protein (FABP(PM)) content of lean (n = 16, fa/+) and obese (n = 15, fa/fa) Zucker rats (12 weeks of age). Hindquarters were perfused with 7 mmol/l glucose, 1,000 micromol/l albumin-bound palmitate, and albumin-bound [1-(14)C]palmitate at rest (no insulin). Glucose uptake was 42% lower in the obese than in the lean rats and indicated the presence of muscle insulin resistance. Fractional and total rates of palmitate uptake were 42 and 74% higher in the obese than in the lean rats and were associated with higher muscle FABP(PM) content (r(2) = 0.69, P < 0.05). The percentage of palmitate oxidized was not significantly different between groups. FA disposal to storage was altered according to fiber type. When compared with lean rats, the rate of triglyceride synthesis in red muscle was 158% higher in obese rats, and the rate of palmitate incorporation into diglycerides in white muscle was 93% higher in obese rats. Pre- and postperfusion muscle triglyceride levels were higher in both red and white muscles of the obese rats. These results show that increased FA uptake and altered FA disposal to storage may contribute to the development of muscle insulin resistance in obese Zucker rats.

Studies show that uptake of long-chain fatty acids (LCFA) across the plasma membranes (PM) may occur partly via a carrier-mediated process and that the plasma membrane fatty acid-binding protein (FABP(PM)) may be a component of this system. To test the hypothesis that FABP(PM) is involved in transsarcolemmal transport of LCFA in muscle, we measured palmitate uptake in giant sarcolemmal vesicles and palmitate binding to PM proteins in rat muscles, (1) in the presence of increasing amounts of unbound palmitate and (2) in the absence or presence of antibody to FABP(PM). Both palmitate uptake and binding were found to be saturable functions of the unbound palmitate concentration with calculated Vmax values of 10.5 +/- 1.2 pmol/mg protein/15 sec and 45.6 +/- 2.9 nmol/mg protein/15 min and Km values of 12.8 +/- 3.8 and 18.4 +/- 1.8 nmol/L, respectively. The Vmax values for both palmitate uptake and binding were significantly decreased by 75-79% in the presence of a polyclonal antibody to the rat hepatic FABP(PM). Antibody inhibition was found to be dose-dependent and specific to LCFA. Glucose uptake was not affected by the presence of the antibody to FABP(PM). Palmitate uptake and binding were also inhibited in the presence of trypsin and phloretin. These results support the hypothesis that transsarcolemmal LCFA transport occurs in part by a carrier-mediated process and that FABP(PM) is a component of this process in muscle.

Previous studies have demonstrated enhanced insulin sensitivity in calorie-restricted [CR, fed 60% ad libitum (AL) one time daily] compared with AL-fed rats. To evaluate the effects of reduced food intake, independent of temporal differences in consumption, we studied AL (unlimited food access)-fed and CR (fed one time daily) rats along with groups temporally matched for feeding [fed 3 meals (M) daily]: MAL and MCR, eating 100 and 60% of AL intake, respectively. Insulin-stimulated glucose transport by isolated muscle was increased in MCR and CR vs. AL and MAL; there was no significant difference for MCR vs. CR or MAL vs. AL. Intramuscular triglyceride concentration, which is inversely related to insulin sensitivity in some conditions, did not differ among groups. Muscle concentration of UDP-N-acetylhexosamines [end products of the hexosamine biosynthetic pathway (HBP)] was lower in MCR vs. MAL despite unaltered glutamine-fructose-6-phosphate aminotransferase activity (rate-limiting enzyme for HBP). These results indicate that the CR-induced increase in insulin-stimulated glucose transport in muscle is attributable to an altered amount, not timing, of food intake and is independent of lower triglyceride concentration. They further suggest that enhanced insulin action might involve changes in HBP.