Human insulin resistance is associated with increased plasma levels of 12α-hydroxylated bile acids

Abstract

Bile acids (BAs) exert pleiotropic metabolic effects, and physicochemical properties of different BAs affect their function. In rodents, insulin regulates BA composition, in part by regulating the BA 12α-hydroxylase CYP8B1. However, it is unclear whether a similar effect occurs in humans. To address this question, we examined the relationship between clamp-measured insulin sensitivity and plasma BA composition in a cohort of 200 healthy subjects and 35 type 2 diabetic (T2D) patients. In healthy subjects, insulin resistance (IR) was associated with increased 12α-hydroxylated BAs (cholic acid, deoxycholic acid, and their conjugated forms). Furthermore, ratios of 12α-hydroxylated/non-12α-hydroxylated BAs were associated with key features of IR, including higher insulin, proinsulin, glucose, glucagon, and triglyceride (TG) levels and lower HDL cholesterol. In T2D patients, BAs were nearly twofold elevated, and more hydrophobic, compared with healthy subjects, although we did not observe disproportionate increases in 12α-hydroxylated BAs. In multivariate analysis of the whole dataset, controlling for sex, age, BMI, and glucose tolerance status, higher 12α-hydroxy/non-12α-hydroxy BA ratios were associated with lower insulin sensitivity and higher plasma TGs. These findings suggest a role for 12α-hydroxylated BAs in metabolic abnormalities in the natural history of T2D and raise the possibility of developing insulin-sensitizing therapeutics based on manipulations of BA composition.

FIG. 1.

Reminder of BA pathway. The enzyme 12α-hydroxylase (CYP8B1) is responsible for the conversion of CDCA into CA. GCA, glycocholic acid; GCDCA, glycochenodeoxycholic acid.

FIG. 2.

Identification of three factors underlying the correlation pattern of the entire dataset of 200 nondiabetic subjects. The size of the bars (x-axis) is the loading of each individual trait on each of three BA-related characteristics. The 12α-hydroxy/non–12α-hydroxy ratio has the highest positive loading score on the first factor (red); total BA concentration has the lowest score on the second axis (green); and the HI has the highest loading on the third factor (yellow). Results are from a principal component analysis with varimax rotation. ISR, insulin secretion rate; MBP, mean blood pressure; mean I, mean insulin; WHR, waist-to-hip ratio. Fatty liver index is the surrogate index of hepatic steatosis based on an algorithm using BMI, waist circumference, TGs, and γGT validated against liver ultrasound (25).

FIG. 3.

Comparison of BA characteristics between subjects in the top (red bars) and bottom quartile (blue bars) of the distribution of the 12α-hydroxy/non–12α-hydroxy ratio in 200 nondiabetic subjects. Bar plots are median and interquartile range for the variables listed on the left-hand side. ***0.01 ≥ P ≥ 0.001, by Kruskal-Wallis test. GCDCA, glycochenodeoxycholic acid; GDCA, glycodeoxycholic acid.

FIG. 4.

Comparison of metabolic variables between subjects in the top (red bars) and bottom quartile (blue bars) of the distribution of the 12α-hydroxy/non–12α-hydroxy ratio in 200 nondiabetic subjects. Bar plots are median and interquartile range for the variables listed on the left-hand side. *0.05 ≥ P > 0.02; **0.02 ≥ P > 0.01, by Kruskal-Wallis test. FFA, free fatty acid; M/I, clamp-derived insulin sensitivity (in units of µmol ⋅ min⁻¹ ⋅ kg_ffm⁻¹ ⋅ nM⁻¹).

FIG. 5.

Stacked bar plot of plasma levels of the major BAs in 200 nondiabetic subjects (by quartile of insulin sensitivity [Q1–Q4]) and in 35 patients with T2D. 12α-Hydroxylated species are colored in shades of teal; non–12α-hydroxylated species are colored in shades of purple. GCA, glycocholic acid; GCDCA, glycochenodeoxycholic acid; GDCA, glycodeoxycholic acid; HDCA, hyodeoxycholic acid; TCA, taurocholic acid; TCDCA, taurochenodeoxycholic acid; TDCA, taurodeoxycholic acid.

FIG. 6.

Relationship between the 12α-hydroxy/non–12α-hydroxy ratio and insulin sensitivity (A) and plasma TGs (B), and between the HI and plasma TGs (C). All three graphs plot the residuals after adjustment for sex, age, BMI, and glucose tolerance status (diabetic vs. nondiabetic).