Aldose reductase expression as a risk factor for cataract

Abstract

Aldose reductase (AR) is thought to play a role in the pathogenesis of diabetic eye diseases, including cataract and retinopathy. However, not all diabetics develop ocular complications. Paradoxically, some diabetics with poor metabolic control appear to be protected against retinopathy, while others with a history of excellent metabolic control develop severe complications. These observations indicate that one or more risk factors may influence the likelihood that an individual with diabetes will develop cataracts and/or retinopathy. We hypothesize that an elevated level of AR gene expression could confer higher risk for development of diabetic eye disease. To investigate this hypothesis, we examined the onset and severity of diabetes-induced cataract in transgenic mice, designated AR-TG, that were either heterozygous or homozygous for the human AR (AKR1B1) transgene construct. AR-TG mice homozygous for the transgene demonstrated a conditional cataract phenotype, whereby they developed lens vacuoles and cataract-associated structural changes only after induction of experimental diabetes; no such changes were observed in AR-TG heterozygotes or nontransgenic mice with or without experimental diabetes induction. We observed that nondiabetic AR-TG mice did not show lens structural changes even though they had lenticular sorbitol levels almost as high as the diabetic AR-TG lenses that showed early signs of cataract. Over-expression of AR led to increases in the ratio of activated to total levels of extracellular signal-regulated kinase (ERK1/2) and c-Jun N-terminal (JNK1/2), which are known to be involved in cell growth and apoptosis, respectively. After diabetes induction, AR-TG but not WT controls had decreased levels of phosphorylated as well as total ERK1/2 and JNK1/2 compared to their nondiabetic counterparts. These results indicate that high AR expression in the context of hyperglycemia and insulin deficiency may constitute a risk factor that could predispose the lens to disturbances in signaling through the ERK and JNK pathways and thereby alter the balance of cell growth and apoptosis that is critical to lens transparency and homeostasis.

Keywords: AKR1B1; Aldose reductase; Cataract; Diabetes.

Figure 1

AR expression in AR-TG mouse lenses. (A) Western blot of mouse lens homogenates. Heterozygous and homozygous AR-TG mice have increased AR expression compared to wild type mice. (B) Enzymatic activity measurement. Wild type mice have very little AR expression. AR-TG mice heterozygous for the AR transgene have increased AR expression, whereas AR expression is almost double in transgene homozygotes. Data are The mean ± SEM of three experiments (n=3).

Figure 2

Sorbitol concentration of mice lenses. Sorbitol levels in AR-TG and AR-TG/STZ mice were significantly higher than wild type and wild type STZ mice p ≤ 0.01 (**) and p ≤ 0.005 (***). Data are The mean ± SEM of three experiments (n=3).

Figure 3

H&E staining of mice lenses. (A) Wild type (C57/bl6) mouse with normal lens fibers. (B) Diabetic wild type (C57/bl6) mouse with normal lens fibers. (C) AR-TG mouse with normal lens fibers. (D) Diabetic AR-TG mouse with vacuolization of cortical lens fibers. Arrows demonstrate vacuolization. All images were taken at 40x magnification. Images are typical of multiple different animals from each group.

Figure 4

Western blot of lens homogenates. AR-TG and AR-TG/STZ treated mice show increased AR expression compared to wild type (WT) and wild type streptomycin (WT-STZ). WT STZ have similar levels of AR expression compared to WT.

Figure 5

Western blot and densitometry of mice lenses. (A) Western blot of pERK1/2, tERK1/2, and actin. (B) The ratio of pERK/tERK was significantly lower in wild type mice compared to AR-TG mice p ≤ 0.05. The ratio of pERK/tERK was significantly lower in AR-TG/STZ mice compared to AR-TG mice p ≤ 0.05 (*) and p ≤ 0.01 (**). Levels were normalized to actin. Data are the mean ± SEM of two experiments (n=2).

Figure 6

Western blot and densitometry of mice lenses. (A) Western blot of pJNK1/2, tJNK1/2, and actin. (B) There was a significant difference between pJNK/tJNK in wild type (WT) and AR-TG mice with p ≤ 0.01 (**). The ratio of pJNK/tJNK was significantly lower in ARTG/STZ) mice compared to AR-TG mice with p ≤ 0.001 (***)[20]. Levels were normalized to actin. Data are the mean ± SEM of two experiments (n=2).