Free fatty acid processing diverges in human pathologic insulin resistance conditions

Abstract

BACKGROUNDPostreceptor insulin resistance (IR) is associated with hyperglycemia and hepatic steatosis. However, receptor-level IR (e.g., insulin receptor pathogenic variants, INSR) causes hyperglycemia without steatosis. We examined 4 pathologic conditions of IR in humans to examine pathways controlling lipid metabolism and gluconeogenesis.METHODSCross-sectional study of severe receptor IR (INSR, n = 7) versus postreceptor IR that was severe (lipodystrophy, n = 14), moderate (type 2 diabetes, n = 9), or mild (obesity, n = 8). Lipolysis (glycerol turnover), hepatic glucose production (HGP), gluconeogenesis (deuterium incorporation from body water into glucose), hepatic triglyceride (magnetic resonance spectroscopy), and hepatic fat oxidation (plasma β-hydroxybutyrate) were measured.RESULTSLipolysis was 2- to 3-fold higher in INSR versus all other groups, and HGP was 2-fold higher in INSR and lipodystrophy versus type 2 diabetes and obesity (P < 0.001), suggesting severe adipose and hepatic IR. INSR subjects had a higher contribution of gluconeogenesis to HGP, approximately 77%, versus 52% to 59% in other groups (P = 0.0001). Despite high lipolysis, INSR subjects had low hepatic triglycerides (0.5% [interquartile range 0.1%-0.5%]), in contrast to lipodystrophy (10.6% [interquartile range 2.8%-17.1%], P < 0.0001). β-hydroxybutyrate was 2- to 7-fold higher in INSR versus all other groups (P < 0.0001), consistent with higher hepatic fat oxidation.CONCLUSIONThese data support a key pathogenic role of adipose tissue IR to increase glycerol and FFA availability to the liver in both receptor and postreceptor IR. However, the fate of FFA diverges in these populations. In receptor-level IR, FFA oxidation drives gluconeogenesis rather than being reesterified to triglyceride. In contrast, in postreceptor IR, FFA contributes to both gluconeogenesis and hepatic steatosis.TRIAL REGISTRATIONClinicalTrials.gov NCT01778556, NCT00001987, and NCT02457897.FUNDINGNational Institute of Diabetes and Digestive and Kidney Diseases, US Department of Agriculture/Agricultural Research Service 58-3092-5-001.

Keywords: Adipose tissue; Endocrinology; Glucose metabolism; Insulin.

Figure 1. CONSORT flow diagram.

Subjects with pathogenic variants of the insulin receptor (INSR), lipodystrophy, type 2 diabetes, and obesity were recruited in the context of multiple clinical trials. Subjects who received stable isotope tracers for measurement of lipolysis, hepatic glucose production, and gluconeogenesis were included in this cross-sectional study. *ClinicalTrials.gov identifiers NCT01778556 (study 1) and NCT00001987 (study 2).

Figure 2. Measures of lipid metabolism.

Lipolysis (A), measured as glycerol rate of appearance (R_a) per kilogram of body weight, was highest in subjects with pathogenic variants of the insulin receptor (INSR, black squares) compared with those with lipodystrophy (white circles), type 2 diabetes (gray triangles), or obesity (gray diamonds). Similarly, plasma glycerol concentration (B) was highest in subjects with INSR pathogenic variants, although it was only significantly different from subjects with obesity. FFA turnover, measured as palmitate R_a per kilogram of body weight (C), plasma palmitate concentrations (D), and total FFA concentrations (E) were comparable in subjects with INSR pathogenic variants and lipodystrophy. Subjects with type 2 diabetes had higher palmitate (D) and total FFA (E) compared with those with obesity, and higher total FFA compared with those with lipodystrophy. Statistical comparisons were performed using mixed models with post hoc pairwise comparisons and Tukey’s correction for multiplicity.

Figure 3. Measures of glucose metabolism.

Hepatic glucose production (A), measured as glucose rate of appearance per kilogram of body weight, was higher in subjects with pathogenic variants of the insulin receptor (INSR, black squares) and lipodystrophy (white circles) compared with those with type 2 diabetes (gray triangles) and obesity (gray diamonds). The percentage of hepatic glucose derived from gluconeogenesis (fractional gluconeogenesis), as well as absolute rates of gluconeogenesis and glycogenolysis, were measured using incorporation of deuterium into newly formed glucose after labeling of body water with ²H₂O. Fractional gluconeogenesis (B) was higher in subjects with INSR pathogenic variants compared with all other groups. Accordingly, the absolute rate of gluconeogenesis (C) was highest in subjects with INSR pathogenic variants. The absolute rate of glycogenolysis (D) was highest in subjects with lipodystrophy. Plasma lactate (E), a substrate for gluconeogenesis, was highest in subjects with lipodystrophy, whereas plasma alanine (F), another gluconeogenic substrate, was highest in subjects with INSR pathogenic variants. Statistical comparisons were performed using mixed models with post hoc pairwise comparisons and Tukey’s correction for multiplicity.

Figure 4. Measure of hepatic and plasma fat, and hepatic fatty acid oxidation.

Hepatic triglyceride content (A), measured using magnetic resonance spectroscopy, and plasma triglycerides (B) were normal in subjects with pathogenic variants of the insulin receptor (INSR, black squares), and elevated, consistent with nonalcoholic fatty liver disease and severe dyslipidemia, in subjects with lipodystrophy (white circles). Plasma β-hydroxybutyrate (C), a measure of hepatic FFA oxidation, was higher in subjects with INSR pathogenic variants compared with all other groups, including lipodystrophy, type 2 diabetes (gray triangles), and obesity (gray diamonds). Statistical comparisons were performed using mixed models with post hoc pairwise comparisons and Tukey’s correction for multiplicity.