Relative abundance of aldose reductase mRNA in rat lens undergoing development of osmotic cataracts

Aldose reductase activity and glucose-related opacities in incubated lenses from dogs and cats

OBJECTIVE

To determine responses of canine and feline lenses to incubation in a medium with a high glucose concentration.

SAMPLE POPULATION

Lenses from 35 dogs and 26 cats.

PROCEDURE

Glucose concentrations were measured in paired lenses from 25 dogs and 17 cats after incubation for 14 days in high-glucose (30 mmol of glucose/L) or control (6 mmol of glucose/L) medium. Aldose reductase activity was measured spectrophotometrically in the incubated lenses and in freshly frozen lenses from 10 dogs and 9 cats. Two lenses of each group were studied histologically.

RESULTS

Canine and feline lenses in high-glucose medium developed glucose-specific opacities of variable localization and extent. Canine lenses developed equatorial vacuoles, but severity of the lesions was not associated with the age of the dog. Lenses from young cats (< or = 4 years old) developed extensive posterior cortical opacities, whereas those from older cats (> 4 years old) did not. Glucose concentrations were similar in all lenses incubated in high-glucose medium; however aldose reductase activity was significantly lower in lenses from older cats, compared with lenses from young cats and from dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

High aldose reductase activity and glucose-related opacities suggest a central role for this enzyme in the pathogenesis of diabetic cataracts in dogs and cats. Because onset of diabetes mellitus usually occurs in cats > 7 years of age, low activity of aldose reductase in lenses of older cats may explain why diabetic cataracts are rare in this species despite hyperglycemia.

Involvement of aldose reductase in vascular smooth muscle cell growth and lesion formation after arterial injury

Abnormal proliferation of vascular smooth muscle cells (VSMCs) is an important feature of atherosclerosis, restenosis, and hypertension. Although multiple mediators of VSMC growth have been identified, few effective pharmacological tools have been developed to limit such growth. Recent evidence indicating an important role for oxidative stress in cell growth led us to investigate the potential role of aldose reductase (AR) in the proliferation of VSMCs. Because AR catalyzes the reduction of mitogenic aldehydes derived from lipid peroxidation, we hypothesized that it might be a potential regulator of redox changes that accompany VSMC growth. Herein we report several lines of evidence suggesting that AR facilitates/mediates VSMC growth. Stimulation of human aortic SMCs in culture with mitogenic concentrations of serum, thrombin, basic fibroblast growth factor, and the lipid peroxidation product 4-hydroxy-trans-2-nonenal (HNE) led to a 2- to 4-fold increase in the steady-state levels of AR mRNA, a 4- to 7-fold increase in AR protein, and a 2- to 3-fold increase in its catalytic activity. Inhibition of the enzyme by sorbinil or tolrestat diminished mitogen-induced DNA synthesis and cell proliferation. In parallel experiments, the extent of reduction of the glutathione conjugate of HNE to glutathionyl-1,4-dihydroxynonene in HNE-exposed VSMCs was decreased by serum starvation or sorbinil. Immunohistochemical staining of cross sections from balloon-injured rat carotid arteries showed increased expression of AR protein associated with the neointima. The media of injured or uninjured arteries demonstrated no significant staining. Compared with untreated animals, rats fed sorbinil (40 mg. kg(-1). d(-1)) displayed a 51% and a 58% reduction in the ratio of neointima to the media at 10 and 21 days, respectively, after balloon injury. Taken together, these findings suggest that AR is upregulated during growth and that this upregulation facilitates growth by enhancing the metabolism of secondary products of reactive oxygen species.

Oxidative defense enzyme activity and mRNA levels in lenses of diabetic rats

This study examines the mRNA expression and enzyme activity of oxidative defense enzymes during the course of streptozotocin-induced hyperglycemic cataract development. Diabetes was produced in 5-wk-old male Sprague-Dawley rats by administering streptozotocin ip and mRNA expression and enzyme activity were monitored on d 4, 8, 12, 16, 20, 40, 60, and 80; concomitantly, the onset and progress of cataract was followed by digital image analysis. Peak enzyme activity and mRNA expression were attained between d 20 and 40. Although catalase and glutathione peroxidase maintained high levels of mRNA expression through d 60, induction of CuZu-superoxide dismutase was transient, with the activity and mRNA levels returning to baseline values by d 40. There was a pronounced increase in aldose reductase activity, which gradually declined to basal levels by d 60; however, the mRNA levels remained unaltered. Other changes included a progressive loss of lenticular transparency, which declined to 40% of control by d 80. The role of antioxidant defense enzymes and, more interestingly, aldose reductase in combating oxidative stress in diabetic cataractogenesis is discussed.

Regulation of aldose reductase expression in rat astrocytes in culture

Aldose reductase (AR) is known to be responsible for many side effects of diabetes. In the present work, we studied the effects of various extracellular signals on the regulation of the expression of AR in astrocytes in culture, by determining its enzymatic activity or its mRNA level. We found that basic fibroblast growth factor (bFGF), acidic fibroblast growth factor (aFGF), epidermal growth factor (EGF), and hypertonic NaCl were able to increase the expression of AR in astrocytes. A superinduction was found when bFGF was combined with hypertonicity. We also observed that AR activity was independent of glucose concentration in the culture medium. However, when the concentration of glucose in the culture medium was under 1 g/l, bFGF did not increase the activity of AR. Thus, when glucose is depleted, the regulation of AR expression by bFGF does not operate. In addition, AR does not seem to be involved in control of astrocyte proliferation, in contrast to the effects reported on other cell types. These results indicate that AR is expressed in astrocytes and that its expression is upregulated by hypertonicity but also by FGFs and EGF. This suggests that in these cells, AR elicits some regulatory functions.

Isolation of the mouse aldose reductase promoter and identification of a tonicity-responsive element

Aldose reductase (AR; EC 1.1.1.21) is an oxidoreductase that catalyzes the NADPH-dependent conversion of glucose to sorbitol, the first step of the polyol pathway. AR is of great interest due to its implication in the etiology of diabetic complications. In renal medullary cells, AR also plays an osmoregulatory role by accumulating sorbitol to maintain the intracellular osmotic balance during antidiuresis. We have previously cloned the AR cDNA from mouse kidney, and we report here the isolation of the mouse AR gene promoter. Transient transfection of chloramphenicol acetyltransferase reporter constructs containing various 5'-flanking regions of the mouse AR gene in CV1 cells led to the identification of a sequence spanning base pairs -1053 to -1040, required for an enhancer activity in hypertonic compared with isotonic cell culture conditions. This sequence is similar to the tonicity-responsive element first characterized in the betaine-gamma-aminobutyric acid transporter promoter.

Lens epithelial cell mRNA. III. Elevated expression of macrophage migration inhibitory factor mRNA in galactose cataracts

A lens epithelial (LE) cell cDNA clone, designated Clone 156, was isolated from a mature rat LE cDNA library by methods of subtractive hybridization. The cDNA sequence of Clone 156 was 521 nucleotides in length, excluding the poly(T)-tail, and it encoded an open reading frame of 115 amino acids. The translated protein shared extensive sequence similarities with macrophage migration inhibitory factor (MIF) from mouse lens and human T-cell lymphocytes. Northern blot hybridization showed that rat lens MIF mRNA is about 500 nucleotides in length and that its expression in mature rat lens is relatively low in comparison with that in other rat tissues. The expression of MIF mRNA in LE of normal rats and of rats treated by feeding a diet of 50% (w/w) galactose was studied by quantitative RT-PCR. The results showed that the expression of MIF mRNA in a 20-day galactosemic rat LE increased twelvefold as compared to that found in control LE. From the results of this study and from what we know about the locale of epithelial cell differentiation in mature rat lenses, it is being proposed that the increase in abundance of MIF mRNA in the cataractous rat lens is correlated with the enhanced proliferation of the undifferentiated epithelial cells.

Polyol and water accumulation in muscle of galactose-fed rats

Skeletal muscle contains high levels of aldose reductase that catalyzes the reduction of galactose to the polyol galactitol. Galactitol and water were measured in muscle of rats fed a high galactose diet with or without addition of the aldose reductase inhibitor sorbinil. Galactitol, measured in isolated samples of muscle by HPLC, reached steady-state levels (5.9 +/- 1.0 mg/g tissue) within 3 days. Muscle water, determined in vivo by magnetic resonance imaging, increased (51 +/- 5%, P < 0.02) to steady-state levels within 7 days. Both the increased galactitol and water remained constant for the 4-month duration of this study. Aldose reductase activity also remained constant. Sorbinil prevented both the increase in galactitol and the increase in water. These results suggest that the increase in water is due to the osmotic effects of galactitol accumulation and demonstrate that galactitol and water accumulation neither up-regulate nor down-regulate aldose reductase expression in skeletal muscle.

Abnormal expression of aldose reductase mRNA in fiber cells of cataractous rat lens. Analysis by in situ hybridization

Aldose reductase (AR), an enzyme of the polyol pathway, has been implicated in the pathogenesis of diabetic and galactosemic cataracts. AR mRNA is a specific transcript of the lens epithelial cells. However, in addition to its presence at high levels in the epithelial cells at the equator, it is also found at significant concentrations in fiber cells at the lens bow. In this study we extended our previous work, and examined the distribution of AR mRNA by in situ hybridization in lens of rats maintained on a 50% galactose diet for up to 20 days, then reversed to a normal diet for an additional 20 days. It was found that within 8 days on galactose the posterior AR mRNA dissipated, with no clear increase in this transcript in the lens epithelial cells. By 16 to 17 days on galactose, the surviving fiber cells in the equatorial region appeared to express AR mRNA; these cells in the normal lens contain insignificant amounts of AR mRNA, as we have previously demonstrated. AR mRNA in the fiber cells of the cataractous lens appears to be of the same base length as that found in the control, indicating that the AR mRNA in these cells may not represent a degraded transcript. Upon reversal of the cataracts, the posterior AR mRNA re-accumulates, and the fiber cells at the equator appear to have lost the AR mRNA that accumulated during the period of exposure to galactose. The data demonstrate that during formation of galactose-cataract, surviving cortical fiber cells express significant levels of AR mRNA, while following withdrawal from galactose these fiber cells lose that activity.

Altered aldose reductase gene regulation in cultured human retinal pigment epithelial cells

Aldose reductase (AR2), a putative "hypertonicity stress protein" whose gene is induced by hyperosmolarity, protects renal medullary cells against the interstitial hyperosmolarity of antidiuresis by catalyzing the synthesis of millimolar concentrations of intracellular sorbitol from glucose. Although AR2 gene induction has been noted in a variety of renal and nonrenal cells subjected to hypertonic stress in vitro, the functional significance of AR2 gene expression in cells not normally exposed to a hyperosmolar milieu is not fully understood. The physiological impact of basal AR2 expression in such cells may be limited to hyperglycemic states in which AR2 promotes pathological polyol accumulation, a mechanism invoked in the pathogenesis of diabetic complications. Since AR2 overexpression in the retinal pigment epithelium has been associated with diabetic retinopathy, the regulation of AR2 gene expression and associated changes in sorbitol and myo-inositol were studied in human retinal pigment epithelial cells in culture. The relative abundance of aldehyde reductase (AR1) and AR2 mRNA was quantitated by filter hybridization of RNA from several human retinal pigment epithelial cell lines exposed to hyperglycemic and hyperosmolar conditions in vitro. AR2 but not AR1 mRNA was significantly increased some 11- to 18-fold by hyperosmolarity in several retinal pigment epithelial cell lines. A single cell line with a 15-fold higher basal level of AR2 mRNA than other cell lines tested demonstrated no significant increase in AR2 mRNA in response to hypertonic stress. This cell line demonstrated accelerated and exaggerated production of sorbitol and depletion of myo-inositol upon exposure to 20 mM glucose. Therefore, abnormal AR2 expression may enhance the sensitivity of cells to the biochemical consequences of hyperglycemia potentiating the development of diabetic complications.

Levels of expression of hexokinase, aldose reductase and sorbitol dehydrogenase genes in lens of mouse and rat

The level of expression of the genes for hexokinase, aldose reductase and sorbitol dehydrogenase was investigated in lenses of mice and rats. These genes represent two separate but interrelated pathways for the metabolism of glucose in the cell. It is hypothesized that the extent of expression of the hexokinase gene may play an important role in the regulation of the levels of glucose in the lens. It is known that if there occurs a build up of intracellular glucose, such as in diabetes mellitus, activation of the aldose reductase/sorbitol dehydrogenase pathway may lead to various diabetic complications, including a lessening of lens clarity. We have therefore determined the levels of expression of the genes for these three enzymes in the lens of both mice and rats. Mice are known to be more resistant than rats to the development of lens opacification during hyperglycemia. By Northern blot hybridization analysis, and by quantitation of the resulting hexokinase, aldose reductase and sorbitol dehydrogenase mRNA hybrids, we found that in the mouse lens the expression of the hexokinase gene exceeded that of the aldose reductase gene by a factor of three, while in the rat it only approached about 1/4 that of the aldose reductase gene. The extent of expression of the SDH gene, however, was equal between the mouse and rat lenses. These results were calculated relative to the level of expression of the alpha A-crystallin gene in those two types of lenses, in order to account for the generally higher genetic expression found in the rat relative to the mouse lens due to its higher content of DNA, henceforth larger mass. The presence of high levels of hexokinase mRNAs relative to aldose reductase mRNAs in the lens would be expected to favor metabolism of glucose via the glycolytic pathway rather than the sorbitol pathway, leading to retardation of development of sugar cataracts in the mouse lens; while the opposite is true for the rat lens.

Effects of osmotic stress and hyperglycemia on aldose reductase gene expression in human renal proximal tubule cells

Sorbitol levels in proximal tubule cells cultured for 96 h in the presence of 16.5 mM and 27.5 mM glucose were significantly elevated when compared to cells cultured in 5.5 mM glucose. No changes in the levels of aldose reductase activity, mRNA and immunoreactivity were observed in cells cultured for up to 168 h in media containing either 5.5 mM or 27.5 mM glucose. In contrast, cells cultured in the presence of hypertonic media (600 mosmol/kg) containing either 5.5 mM or 27.5 mM glucose contained markedly elevated aldose reductase activity, mRNA and immunoreactivity. These results demonstrate that exposure of human renal proximal tubule cells to elevated glucose for up to 168 h does not result in enhancement of aldose reductase gene expression at transcriptional, translational or post-translational levels. However, exposure to a hyperosmotic milieu causes dramatic induction of aldose reductase gene expression.

Increase in aldose reductase mRNA in dog lens epithelial cells under hypertonic conditions

Aldose reductase (AR) mRNA levels increase when dog lens epithelial cells are exposed to hypertonic conditions. Hybridization of mRNA to an AR cDNA, using Northern and slot blots, showed that AR mRNA is elevated at least fourfold when primary dog lens epithelial cells are grown in media (300 mosmol kg-1) supplemented with 150 mM NaCl (600 mosmol kg-1 final). A time course showed an increase in AR mRNA of approximately twofold by 24 hr with a maximum increase of between four- and eightfold by 48 hr. AR mRNA remained elevated for the duration of the experiment, 8 days. The addition of Tolrestat, an inhibitor of aldose reductase, had no effect on the increased level of AR mRNA in these hypertonically stressed cells. Cells grown in media supplemented with 250 mM sorbitol also showed a substantial increase in AR mRNA. These data indicate, as in other cell types, the lens, a target tissue of diabetes, responds to hypersomotic stress with an induction of AR expression and suggests that AR may play a role in intracellular osmotic regulation.