Differentially regulated protein kinase A (PKA) activity in adipose tissue and liver is associated with resistance to diet-induced obesity and glucose intolerance in mice that lack PKA regulatory subunit type IIα

Abstract

The cAMP-dependent protein kinase A (PKA) signaling system is widely expressed and has a central role in regulating cellular metabolism in all organ systems affected by obesity. PKA has four regulatory (RIα, RIIα, RIβ, RIIβ) and four catalytic (Cα, Cβ, Cγ, Prkx) subunit isoforms that have tissue-specific expression profiles. In mice, knockout (KO) of RIIβ, the primary PKA regulatory subunit in adipose tissue or knockout of the catalytic subunit Cβ resulted in a lean phenotype that resists diet-induced obesity and associated metabolic complications. Here we report that the disruption of the ubiquitously expressed PKA RIIα subunit in mice (RIIαKO) confers resistance to diet-induced obesity, glucose intolerance, and hepatic steatosis. After 2-week high-fat diet exposure, RIIαKO mice weighed less than wild-type littermates. Over time this effect was more pronounced in female mice that were also leaner than their wild-type counterparts, regardless of the diet. Decreased intake of a high-fat diet contributed to the attenuated weight gain in RIIαKO mice. Additionally, RIIα deficiency caused differential regulation of PKA in key metabolic organs: cAMP-stimulated PKA activity was decreased in liver and increased in gonadal adipose tissue. We conclude that RIIα represents a potential target for therapeutic interventions in obesity, glucose intolerance, and nonalcoholic fatty liver disease.

Figure 1.

Mean weekly body weights of female (A) and male (B) mice during 14 weeks of ad libitum feeding of either chow (control diet) or high-fat/high-sucrose (HF) diet. Absolute fat mass in female (C) and male (D) WT and RIIαKO mice on control and HF diets as determined by magnetic resonance imaging. Female RIIαKO and WT mice (E) and livers counterstained with Oil Red O and hematoxylin and eosin (F) after 14 weeks of HF diet feeding Oil Red O. Values are mean ± SEM. *, P < .05.

Figure 2.

Glucose tolerance was attenuated and response to insulin tolerance test differed after chronic feeding in female RIIαKO mice HFD feeding. GTT (A) and ITT (B) in female WT and RIIαKO after CD and HFD feeding. Intraperitoneal glucose injection (2 g/kg) and insulin (HumulinR) (0.75 U/kg) was administered, respectively, for GTT and ITT. Blood glucose was measured at baseline and 15, 30, 60, and 120 minutes after glucose injection and at baseline and 15, 30, 45, and 60 minutes after insulin injection. Values are mean ± SEM. *, P < .05.

Figure 3.

DEAE-cellulose column separation and PKA activity assay of inguinal adipose from HFD-fed WT (A) and RIIαKO (B) mice showed substantially increased basal kinase activity in the mutant mice. Line graphs show kinase activity in HPLC-separated fractions with and without added cAMP (5 μM). Type I elutes between 40 and 80 mM (fractions 20–25), type II elutes between 120 and 200 mM (fractions 31–35), and free C subunit elutes just before the type I holoenzyme. Bar graphs show kinase activity (per microgram of protein) in lysate prior to HPLC separation (cumulative kinase activity with and without added cAMP).