Obesity-induced changes in kidney mitochondria and endoplasmic reticulum in the presence or absence of leptin

Abstract

We investigated obesity-induced changes in kidney lipid accumulation, mitochondrial function, and endoplasmic reticulum (ER) stress in the absence of hypertension, and the potential role of leptin in modulating these changes. We compared two normotensive genetic mouse models of obesity, leptin-deficient ob/ob mice and hyperleptinemic melanocortin-4 receptor-deficient mice (LoxTB MC4R-/-), with their respective lean controls. Compared with controls, ob/ob and LoxTB MC4R-/- mice exhibit significant albuminuria, increased creatinine clearance, and high renal triglyceride content. Renal ATP levels were decreased in both obesity models, and mitochondria isolated from both models showed alterations that would lower mitochondrial ATP production. Mitochondria from hyperleptinemic LoxTB MC4R-/- mice kidneys respired NADH-generating substrates (including palmitate) at lower rates due to an apparent decrease in complex I activity, and these mitochondria showed oxidative damage. Kidney mitochondria of leptin-deficient ob/ob mice showed normal rates of respiration with no evidence of oxidative damage, but electron transfer was partially uncoupled from ATP synthesis. A fourfold induction of C/EBP homologous protein (CHOP) expression indicated induction of ER stress in kidneys of hyperleptinemic LoxTB MC4R-/- mice. In contrast, ER stress was not induced in kidneys of leptin-deficient ob/ob mice. Our findings show that obesity, in the absence of hypertension, is associated with renal dysfunction in mice but not with major renal injury. Alterations to mitochondria that lower cellular ATP levels may be involved in obesity-induced renal injury. The type and severity of mitochondrial and ER dysfunction differs depending upon the presence or absence of leptin.

Keywords: endoplasmic reticulum stress; kidney; leptin; melanocortin-4 receptor (MC4R); mitochondria; nephropathy; obesity.

Fig. 1.

Both transgenic obese mice models (LoxTB MC4R−/− and ob/ob mice) exhibited increased creatinine clearance (A), indicative of renal hyperfiltration, and significant albuminuria (B), a marker of renal dysfunction. Values are means ± SE. *P < 0.05 compared with wild-type (WT) Control. †P < 0.05 compared with WT-LoxTB mice.

Fig. 2.

Representative images of periodic acid-Schiff (PAS)-stained (A) and F4/80-stained (B) kidney sections of ob/ob and LoxTB MC4R−/− mice compared with their respective WT controls under ×400 magnification.

Fig. 3.

Leptin-deficient ob/ob mice and hyperleptinemic LoxTB MC4R−/− mice showed increased lipid accumulation (A) and decreased ATP levels (B) in the kidneys. However, only the LoxTB MC4R−/− mice exhibited increased protein carbonyl levels, a measure of oxidative stress, compared with WT-LoxTB control mice (C). Values are means ± SE. *P < 0.05 compared with WT Control. †P < 0.05 compared with WT-LoxTB mice. ‡P < 0.05 compared with LoxTB MC4R−/− mice.

Fig. 4.

Mitochondrial respiration studies showed impaired mitochondrial state 3 respiration in LoxTB MC4R−/− mice kidneys under complex I substrate glutamate (a glucose substrate), palmitoyl-l-carnitine (an esterified fatty acid), and nonesterified fatty acid palmitoyl CoA+l-carnitine (A). Similar impairments to mitochondrial state 4 respiration were observed in LoxTB MC4R−/− mice kidneys under fatty acid substrates palmitoyl-l-carnitine and palmitoyl CoA+l-carnitine (B). In contrast, LoxTB MC4R−/− mice kidneys demonstrate an increased respiratory control ratio (RCR) under complex II glucose substrate succinate (C) and increase in complex IV (cytochrome c oxidase) activity (D). Although no changes in state 3 respiration or complex IV activity was observed, the ob/ob mice renal mitochondria show increased state 4 respiration under complex II glucose substrate succinate (B) and diminished RCR values under glucose substrates succinate and glutamate (C) but not under fatty acid substrates. State 3 and state 4 respiration values were normalized for citrate synthase (CS) activity in renal mitochondria. Values are means ± SE. *P < 0.05 compared with WT Control. †P < 0.05 compared with WT-LoxTB mice.

Fig. 5.

Western blot and densitometry analysis showed increased expression of proapoptotic endoplasmic reticulum (ER) stress response marker CHOP (A) in LoxTB MC4R−/− mice kidneys with subtle, but not statistically significant, changes in the expression of other ER stress marker XBP-1 (spliced form; B) in LoxTB MC4R−/− and ob/ob mice kidneys with respect to their corresponding lean WT controls. Representative Western blots (n = 3) of ER stress markers [CHOP (A) and XBP-1 (B)] with β-actin used as loading control are shown at the top of each panel, and densitometry values (normalized to β-actin) of ER stress markers are shown at the bottom. Values are means ± SE. †P < 0.05 compared with WT-LoxTB.