Loss of the Par-1b/MARK2 polarity kinase leads to increased metabolic rate, decreased adiposity, and insulin hypersensitivity in vivo

Abstract

Obesity is a major factor central to the development of insulin resistance and type 2 diabetes. The identification and characterization of genes involved in regulation of adiposity, insulin sensitivity, and glucose uptake are key to the design and development of new drug therapies for this disease. In this study, we show that the polarity kinase Par-1b/MARK2 is required for regulating glucose metabolism in vivo. Mice null for Par-1b were lean, insulin hypersensitive, resistant to high-fat diet-induced weight gain, and hypermetabolic. (18)F-FDG microPET and hyperinsulinemic-euglycemic clamp analyses demonstrated increased glucose uptake into white and brown adipose tissue, but not into skeletal muscle of Par-1b null mice relative to wild-type controls. Taken together, these data indicate that Par-1b is a regulator of glucose metabolism and adiposity in the whole animal and may be a valuable drug target for the treatment of both type 2 diabetes and obesity.

Fig. 1.

Par-1b null mice are growth retarded and have disproportionate reductions in adiposity. (A) Body weights of embryos from 13.5 to 18.5 days postcoitus (d.p.c.) (n = 5–13 per genotype per time point). (B) Body weights of mice from birth to 3 weeks of age (n = 4–19 per genotype per time point). (C) Body weights of Par-1b wild-type (Par-1b^+/+, black bars), heterozygous (Par-1b^+/−, red bars), or null (Par-1b^−/−, gray bars) mice at 1 year of age (n = 7–13 per genotype per sex). (D) Comparison of Par-1b wild-type and null inguinal (I) and epidydymal (E) fat pads. (E) Scatter plot depiction of adiposity in 10- to 12-week-old male Par-1b wild-type and null mice expressed as a ratio of fat mass to body mass as determined by ¹H-MRS. Individual mice for each genotype are shown with averages and P value indicated (n = 18–20 per genotype). (F) Hematoxylin and eosin stain of BAT and WAT from Par-1b wild-type and null mice. (Scale bar, 50 μm.) All values are presented as the averages ± standard error. Student's t test was performed for comparisons between groups. P value designations are as follows: ****, P < 0.05; ***, P < 0.01; **, P < 0.005; *, P < 0.001.

Fig. 2.

Par-1b null mice are resistant to weight gain and are hypermetabolic on a high-fat diet. Par-1b wild-type and null males (n = 5 per genotype) were fed a high-fat diet for 16 weeks starting at 3 weeks of age. (A) Body weight of Par-1b wild-type and null mice during the high-fat diet. Averages are plotted ± standard error. All data points represent statistical differences between wild-type and null mice with P < 0.05. In an independent experiment, male Par-1b null (filled bars) and wild-type mice (open bars) (n = 8–12 per genotype) were fed a high-fat diet for 8 weeks, followed by analysis. Par-1b null mice exhibit increased metabolic rate (B), energy expenditure (C), and food intake (D) on a high-fat diet. (E) Respiratory quotient (VCO₂/VO₂) of Par-1b null mice is not statistically different from wild-type controls. Student t test P values are indicated.

Fig. 3.

Enhanced insulin sensitivity and glucose tolerance in Par-1b null mice. (A) Retro-orbital bleeds were obtained from fed or fasted (12 h) Par-1b^+/+ (open bars) and Par-1b^−/− (filled bars) mice, and serum insulin levels were measured by RIA (n = 18 mice per genotype). (B) Blood glucose levels were determined for fed or fasted mice (n = 18 mice per genotype). Par-1b^+/+ (open bars) and Par-1b^−/− (filled bars). (C) Insulin tolerance tests (ITT) were performed by i.p. injection of 0.30 unit/kg insulin into Par-1b^+/+ (diamonds) and Par-1b^−/− (circles) littermates (n = 14 mice per genotype). Tail bleeds were obtained and glucose levels were monitored at 15-min intervals after insulin injection. Data are plotted as % blood glucose at time 0 before injection. (D) Glucose tolerance tests were performed by intrperitoneal injection of d-glucose at 1 mg/g body weight into Par-1b^+/+ and Par-1b^−/− littermates (n = 10 mice per genotype). Tail bleeds were obtained, and glucose levels were monitored at 20-min intervals after glucose injection. Data are plotted as % blood glucose at time 0 before injection. Standard error is plotted on the y axis for all values in A–D. Student's t test was performed for comparisons between two groups. P values were as follows: ****, P < 0.05; ***, P < 0.01; **, P < 0.005; *, P < 0.001.

Fig. 4.

microPET analysis of ¹⁸F-FDG uptake in Par-1b^+/+ and Par-1b^−/− mice. Representative coronal section image of Par-1b^+/+ and Par-1b^−/− mice 1 h after ¹⁸F-FDG injection. Par-1b^−/− exhibit consistently elevated levels of uptake in intrascapular brown fat (BAT) pads.