Lipid partitioning between tissues is important to insulin action, energy balance, and the regulation of body weight and composition. The normal physiology of lipid and lipoprotein fuel partitioning is controlled by the transport and uptake of adipose tissue–derived FFA and lipoprotein-derived TG fatty acids. Tissue-specific changes in the regulation of LPL in obese subjects may play an important role in nutrient partitioning when energy intake exceeds energy expenditure (22
). Thus, the ability to selectively modify LPL in skeletal muscle and/or adipose tissue may influence body weight and composition. The SMLPL−/−
mice were created to provide a model to study how reduced lipid partitioning to skeletal muscle influences insulin-sensitive tissues and the potential role in regulating body weight, body composition, and whole-body insulin sensitivity.
SMLPL−/− mice show a transition of phenotypes between young and old animals. At a young (9–11 weeks) age, body weight and percent fat were unchanged compared with WT controls. With aging, both body weight and percent fat were increased in SMLPL−/− and systemic insulin resistance developed. The transition of the phenotype was accelerated by feeding the young SMLPL−/− mice with HF diets. Such information suggests an increased propensity that lipid storage develops and that accompanies lower lipid deposition in skeletal muscle.
The deletion of LPL in skeletal muscle results in two distinct patterns of clinical and biochemical abnormalities in young mice. The first is an increase in insulin sensitivity in skeletal muscle. The second is the decrease of insulin sensitivity in liver and other tissues. The present data suggest that skeletal muscle, and LPL in particular, acts as an important buffer against excess lipid storage but cannot protect against the ultimate development of obesity or insulin resistance. Despite no change in body composition in young SMLPL−/− mice or whole-body glucose disposal during the hyperinsulinemic clamp, closer inspection of individual tissues revealed that insulin-stimulated 2-DG uptake was significantly greater in skeletal muscle, whereas other insulin-sensitive tissues (WAT, heart) showed remarkable reductions in insulin-stimulated glucose uptake.
The increase in insulin-stimulated glucose uptake in skeletal muscle of young SMLPL−/− mice was associated with reduced TG accumulation in skeletal muscle and increased basal and insulin-stimulated Akt phosphorylation. Interestingly, there were no differences in the accumulation of other metabolic intermediates such as LC acyl-CoAs, ceramides, and DAG in skeletal muscle. Unexpectedly, the significant effect on increased Akt phosphorylation in SMLPL−/− mice was not associated with greater IRS-1 or PI 3-kinase activity. This suggests that LPL may play a role in regulating other lipid-derived factors that affect Akt phosphorylation in skeletal muscle.
Because the total PTEN and SHIP2 levels were not different in SMLPL−/−
versus control mice, it is unlikely that these two negative regulators of Akt activity contribute to the increased Akt phosphorylation we observed in SMLPL−/−
mice. Increased Akt phosphorylation at Ser473 has been associated with the mammalian target of rapamycin(mTOR)-rictor complex (27
). Because there was no change in the Ser–IRS-1 phosphorylation (an mTOR target), it suggests that there may be other regulators of Akt in SMLPL−/−
mice. For example, phosphorylation of Akt at Thr308 and Ser473 can be inhibited by direct binding of two different proteins: carboxyl-terminal modulator protein or TRB3 (29
). Also, a pH domain leucine-rich repeat protein phosphatase (PHLPP) has also been shown to specifically dephosphorylate Ser-473 and inactivate Akt (31
). Interestingly, insulin-stimulated Akt Thr308 phosphorylation was unaffected in SMLPL−/−
mice, suggesting that there might be different combination of pathways affecting Akt activation. In subsequent studies, it will be important to identify which of these possible mediators might serve as the link between reduced LPL-mediated lipid uptake and deposition and enhanced Akt activation in our SMLPL−/−
Increased insulin-stimulated glucose uptake and lower TG in skeletal muscle of young SMLPL−/− mice also suggest that the loss of LPL activity in skeletal muscle in SMLPL−/− mice creates a preference for glucose rather than TG-rich lipoproteins for fuel. Because plasma lipoprotein TG and liver TG were not statistically different for SMLPL−/− and control mice, more TG-rich lipoprotein TG may be processed in liver, WAT, or heart. Noting that LPL activity in WAT tissue was only marginally increased in SMLPL−/− mice and the fat mass remained the same as control mice, WAT tissue could be compensating for the loss of LPL in skeletal muscle by enhancing lipoprotein-dependent lipid uptake and turnover. However, at 9–11 weeks, there was no increase in TG storage in the heart, an important organ in the metabolism of TG-rich lipoproteins. It is very likely at this stage of phenotypic development that the heart increased fatty acid oxidation in response to the reduced lipid partitioning to skeletal muscle.
Despite increased skeletal muscle insulin sensitivity, SMLPL−/− mice also displayed hepatic insulin resistance, as indicated by the reduced capacity to suppress hepatic glucose production upon insulin stimulation. However, TG content in the liver of SMLPL−/− mice was not increased. Gene expression in the liver of SMLPL−/− mice was mostly unchanged, whereas PGC-1α and IL-1β expression was modestly increased, and PPAR-γ and SCD-1 mRNA levels reduced in the liver of SMLPL−/− mice. The increase in IL-1β expression suggests a possible role of this cytokine in mediating insulin resistance in liver. However, when we measured the plasma IL-1β and IL-6 levels in SMLPL−/− mice, there was no indication that the circulating cytokine levels were elevated in the plasma of the mice.
The downregulation of PPAR-γ and SCD-1 in liver of the SMLPL−/−
mice is consistent with the liver not storing excessive TG in the SMLPL−/−
mice. SCD-1 catalyzes the desaturation of saturated fatty acyl-CoAs and is regulated by liver X receptor (LXR) (32
). LXR gene expression, however, was not different in SMLPL−/−
mice. SCD-1 has also been identified along with several other genes involved in lipogenesis as a direct target of PPAR-γ in liver. Furthermore, SCD-1 plays a pivotal role in the regulation of hepatic and plasma lipoprotein TG concentrations, and SCD-1–deficient mice are usually protected against hypertriglyceridemia (33
). Interestingly, SMLPL−/−
mice had a borderline increase of plasma TG, despite the lower SCD-1 mRNA in liver. Moreover, PPAR-γ is usually associated with greater liver TG storage, and hepatic overexpression of this transcription factor leads to steatosis (34
). Given that both PPAR-γ and SCD-1 mRNAs are lower and PGC-1α is higher in the livers of SMLPL−/−
mice, the liver might respond to the excess delivery of FFAs by maintaining a higher rate of gluconeogenesis during insulin infusion in the clamp study.
To summarize and conclude, the loss of SMLPL reduced skeletal muscle TG and increased insulin sensitivity. Despite the lack of change in whole-body insulin sensitivity at a young age, insulin resistance in WAT, liver, and heart was already evident. In the skeletal muscle of young SMLPL−/− mice, insulin signaling at the level of Akt was increased, suggesting that LPL-derived lipids influence both basal and insulin-stimulated Akt activation. Because the reduction of LPL-dependent lipid processing in skeletal muscle must result in the partitioning of TG-rich lipoproteins to other tissues, over time this promotes obesity development, a condition further enhanced by high-fat feeding.