Elevated stearoyl-CoA desaturase-1 expression in skeletal muscle contributes to abnormal fatty acid partitioning in obese humans

Abstract

Obesity and type 2 diabetes are strongly associated with abnormal lipid metabolism and accumulation of intramyocellular triacylglycerol, but the underlying cause of these perturbations are yet unknown. Herein, we show that the lipogenic gene, stearoyl-CoA desaturase 1 (SCD1), is robustly up-regulated in skeletal muscle from extremely obese humans. High expression and activity of SCD1, an enzyme that catalyzes the synthesis of monounsaturated fatty acids, corresponded with low rates of fatty acid oxidation, increased triacylglycerol synthesis and increased monounsaturation of muscle lipids. Elevated SCD1 expression and abnormal lipid partitioning were retained in primary skeletal myocytes derived from obese compared to lean donors, implying that these traits might be driven by epigenetic and/or heritable mechanisms. Overexpression of human SCD1 in myotubes from lean subjects was sufficient to mimic the obese phenotype. These results suggest that elevated expression of SCD1 in skeletal muscle contributes to abnormal lipid metabolism and progression of obesity.

Figure 1. Abnormal lipid metabolism in muscle from obese subjects

A) Histological cross-sections of rectus abdominus muscles obtained from lean and obese humans. B) Fatty acid metabolism in muscle strips from lean (n = 8, BMI, 24.0 ± 0.79) and obese (n = 11, BMI, 58.0 ± 4.02) humans. The partitioning of FA between opposing metabolic pathways was evaluated by dividing the rate (nmol/g wet weight/h) of oleate esterified into TAG by the rate of oleate oxidized. Linear regression analysis revealed a strong correlation (r² = 0.76, p = 0.01) between FA partitioning and body mass index of the donor subject.

Figure 2. Relationship between muscle fatty acid metabolism and SCD1 expression

Muscle samples were obtained from lean and obese humans with a body mass index ranging from 22–60 kg/m². Gene expression data from Table 1 were combined with results from in vitro metabolic assays (detailed in Hulver et al. [2003]) to show strong correlations between SCD1 mRNA levels (A) rates of intramuscular triacylglycerol (IMTG) synthesis and (B) fatty acid oxidation. The relationships between SCD1 activity and (C) body mass index and (D) serum insulin levels were evaluated using rectus abdominus specimens that were harvested from a second cohort of subjects.

Figure 3. Monounsaturated fatty acid content of muscle lipids is increased with obesity

The desaturation index (16:1/16:0 and 18:1/18:0) was measured in various muscle lipid fractions. Total fatty acids (A) and (B) were extracted from rectus abdominus skeletal muscle obtained from lean (n = 7, BMI, 23.6 ± 1.1 kg/m²) and obese (n = 7, BMI, 65.1 ± 7.4 kg/m²). Fatty acyl-CoAs (C) and (D) were extracted from rectus abdominus muscle obtained from lean (n = 8, BMI, 23.8 ± 0.58) and obese (n = 8, BMI, 53.8 ± 3.5). The FA composition of (E) intra-myocellular glycerolipids and (F) mitochondrial-derived acylcarnitines are expressed as a percent of the total. Values represent the mean ± SEM, and differences between groups were analyzed by Student’s t test, *p < 0.05.

Figure 4. Fatty acid metabolism and SCD1 expression in cultured HSKMC from lean and obese subjects

Myoblasts were isolated from human rectus abdominus muscle obtained from lean or obese subjects. Day 8 myotubes were incubated 3 hr at 37°C in DFM with 100 μM [¹⁴C]oleate and 0.25% BSA. A) Fatty acid oxidation was determined by measuring ¹⁴C-label incorporation into CO₂ and acid soluble metabolites. B) Glycerolipid synthesis was determined by measuring ¹⁴C-label incorporation into triacylglycerol (TAG) and phospholipid (PL). C) Relationship between fatty acid partitioning ([¹⁴C]-oleate incorporation into TAG (nmol/mg protein/h) divided by the amount oxidized] and body mass index of the donor subject. Metabolic assays were performed in triplicate and values represent the mean ± SEM from 7–9 subjects. D) Total RNA was isolated and SCD1 mRNA expression was quantified by RTQ-PCR. Relative expression levels were normalized to 18S mRNA and data are presented as means ± SEM of cells from 7 subjects per group. E) Cell extracts were prepared from day 8 myotubes and used for Western blot analysis of SCD1 protein abundance. Differences between groups were analyzed by Student’s t test. *p < 0.05, **p < 0.01.

Figure 5. Characterization of primary human myotubes

Differentiated HSKMC exhibit multi-nucleated (arrows) morphology that is characteristic of mature myotubes. Fluorescent photomicrographs (200×) show that immuno-detection of the sarcomeric protein, myosin (green) was similar between cells from (A) lean and (B) obese subjects. Myotubes were incubated overnight with 500 μM fatty acid (oleate:palmitate 2:1) bound to 0.5% BSA and then fixed and stained for neutral lipids. Phase contrast photomicrographs (100×) revealed less intense staining in myotubes that originated from (C) lean compared to (D) obese donors. (E) Higher magnification (200×) showing lipid droplets in mature myotubes. (F) Myotubes from obese subjects that were incubated overnight with 0.5% BSA minus fatty acid displayed few lipid droplets. (G) Phase contrast photomicrographs (40×) of day 8 myotubes that were incubated with antibodies against human SCD1 and mouse sarcomeric myosin. Proteins were visualized using secondary antibodies conjugated to distinct fluorophores. Fluorescent photomicrographs show co-expression of (H) SCD1 (red) and (I) sarcomeric myosin (green) in multinucleated myotubes. Negative controls that were performed without addition of primary antibody did not present a fluorescent signal (data not shown).

Figure 6. hSCD1 overexpression in primary HSKMC alters FA partitioning

Preconfluent myoblasts were transiently transfected with pcDNA3.0-hSCD1 or the empty parent vector. After 7 days in DFM, cells were harvested to obtain total RNA. Overexpression of SCD1 was confirmed in differentiated myotubes by (A) semiquantitative PCR of mRNA and (B) Western blot analysis of protein. β-actin mRNA and GAPDH protein were used as loading controls. Metabolic assays were performed following a 3 hr incubation with 100 μM [¹⁴C]oleate and 0.25% BSA. (C) Fatty acid oxidation was determined by measuring ¹⁴C-label incorporation into CO₂. (D) Glycerolipid synthesis was determined by measuring ¹⁴C-label incorporation into triacylglycerol (TAG) and phospholipid (PL). All assays were performed in triplicate and values represent the mean ± SEM from four separate experiments. Differences between groups were analyzed by Student’s t test, *p < 0.05, **p < 0.01. (E) ACCβ mRNA levels were quantified by RTQ-PCR and are expressed as relative units in cells transfected with SCD1 versus empty vector controls. Cyclophilin B was measured as an endogenous control. Data are representative of five experiments and differences between groups were analyzed by Student’s t test, *p < 0.05. (F) Total and phosphorylated AMPK and ACCβ were analyzed by immunoblotting with antibodies against the AMPKα 1 and 2 catalytic subunits, ACCβ, AMPK-thr172, and ACCβ-ser79. GAPDH protein expression was not different between groups.