Cataracts in experimentally diabetic mouse: morphological and apoptotic changes

LncRNA MALAT1 Regulates miR-144-3p to Facilitate Epithelial-Mesenchymal Transition of Lens Epithelial Cells via the ROS/NRF2/Notch1/Snail Pathway

Diabetic cataract is a common complication of diabetes. The epithelial-mesenchymal transition (EMT) of lens epithelial cells (LECs) is a key event in the development of diabetic cataracts. Metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) has been reported to be highly expressed in different tissues of diabetic patients. This study is aimed at investigating the function and mechanism of MALAT1 in the regulation of EMT in human LECs under high glucose conditions. MALAT1, α-smooth muscle actin (α-SMA), fibronectin (FN), and nuclear factor erythroid-derived 2-like 2 (NRF2) were highly expressed in the LECs of diabetic cataract patients and in the human LECs under high glucose conditions; meanwhile, the decreased expressions of E-cadherin and zonula occludens 1 (ZO-1) were detected. Knockdown of MALAT1 could significantly reduce ROS, prevent EMT, arrest S phase cell cycle, and suppress the expression of total NRF2 and its nucleus translocation in LECs. Furthermore, after NRF2 was knocked down, total NRF2, α-SMA, and FN in cells, and NRF2, Notch intracellular domain (NICD), and Snail were decreased in the nucleus. Using bioinformatics methods, we predicted that MALAT1 and NRF2 shared the same microRNA-144-3p (miR-144-3p) combining site. Luciferase reporter coupled with qRT-PCR assays revealed that miR-144-3p was a target of MALAT1, which was confirmed to downregulate miR-144-3p in the LECs. In addition, after transfection of miR-144-3p mimics or inhibitor, western blot assay demonstrated that miR-144-3p negatively regulated the expression of total NRF2, α-SMA, and FN in cells, and NRF2, NICD, and Snail in the nucleus without affecting Kelch-like ECH-associated protein 1 (KEAP1). Finally, we confirmed that transfection of shMALAT1 inhibited NRF2 expression, and its mediated EMT could be rescued by miR-144-3p inhibitor; transfection of pcDNA3.1-MALAT1 promoted NRF2 expression, and its mediated EMT could be reversed by miR-144-3p inhibitor. In summary, we demonstrate that MALAT1 regulates miR-144-3p to facilitate EMT of LECs via the ROS/NRF2/Notch1/Snail pathway.

Regulation of transforming growth factor β-mediated epithelial-mesenchymal transition of lens epithelial cells by c-Src kinase under high glucose conditions

Recent studies have reported that high glucose (HG) conditions may contribute to the acceleration of renal cell apoptosis and renal fibrosis by inducing epithelial-mesenchymal transition (EMT) of tubular epithelial cells, in which c-Src kinase and transforming growth factor (TGF)-β are key modulators. In the present study, the roles of c-Src kinase and TGF-β in EMT of lens epithelial cells (LECs) under HG conditions were investigated. Results indicated human lens epithelial B3 (HLE-B3) cells under HG conditions exhibited significantly increased protein expression levels of phosphorylated c-Src (p-Src⁴¹⁸) (P<0.05) and secreted a significantly increased amount of TGF-β compared with HLE-B3 cells under normal glucose conditions (P<0.05). Notably the c-Src inhibitor PP1 and the activin receptor-like kinase 5 (ALK5) inhibitor SB431542 suppressed EMT of HLE-B3 cells. Results indicated that PP1 significantly inhibited the activities of c-Src and ALK5 and the secretion of TGF-β, whereas SB431542 only significantly downregulated the protein expression levels and secretion of TGF-β (P<0.05). Following c-Src knockdown, the protein expression levels of p-Src⁴¹⁸, ALK5 and TGF-β were significantly decreased, the secretion of TGF-β was significantly suppressed (both P<0.05) and EMT was decreased in HLE-B3 cells. These results suggest that c-Src and TGF-β may promote EMT of LECs under HG conditions, with c-Src as the upstream regulatory molecule. Thus, the signal axis of c-Src/TGF-β in EMT of LECs may be a potential novel therapeutic target for the prevention of diabetic subcapsular cataract.

Quercetin inhibited epithelial mesenchymal transition in diabetic rats, high-glucose-cultured lens, and SRA01/04 cells through transforming growth factor-β2/phosphoinositide 3-kinase/Akt pathway

Diabetic cataract (DC), an identified life-threatening secondary complication of diabetes mellitus, has proven to be a dilemma because of its multifactorial caused and progression. An increasing number of studies have shown that in addition to the maillard reaction, enhanced polyol pathway, and oxidative insults, epithelial mesenchymal transition (EMT) is related to the prevalence of DC. Quercetin, a classic flavonoid with multiple pharmacological effects has been reported to possess therapeutic efficacy in the management and treatment of this disease. However, the mechanism underlying its therapeutic efficacy in EMT of lens epithelial cells (SRA01/04) and contribution to resolving DC remains a mystery. Therefore, in this study, we investigated the effects of quercetin on EMT of SRA01/04 and high-glucose (HG)-induced lens opacity accompanied by lens fibrosis induced by type-1 diabetes. Furthermore, we sought to clarify the specific mechanisms underlying these effects. At week 14 after streptozotocin (STZ) intraperitoneal administration, diabetic rats showed lens opacity accompanied with diminished antioxidant function, enhanced polyol pathway activity, and non-enzymatic glycation. Western blotting confirmed EMT in rat SRA01/04 cells with significantly increased α-smooth muscle actin (α-SMA) and decreased E-cadherin expressions. Treatment of the lens with quercetin ameliorated the oxidative stress, inhibited aldose reductase (AR) activation, reduced advanced glycation end product (AGE) production, and finally suppressed EMT in the early stages. Our in vitro results showed that high-glucose activated the transforming growth factor-β2/phosphoinositide 3-kinase/protein kinase B (TGF-β2/PI3K/Akt) signalling and EMT in SRA01/04 cells. Further, induced oxidative stress, activation of aldose reductase, and accumulation of advanced glycation end products were also involved in this process. Quercetin was potent enough to effectively ameliorate the high glucose (HG)-induced EMT of SRA01/04 cells by inhibiting the activation of TGF-β2/PI3K/Akt, enhancing the antioxidant capacity, inhibiting AR activity, and reducing AGE production. From the whole animal to tissues, and finally the cellular level, our results provide considerable evidence of the therapeutic potential of quercetin for DC. This might be due to its inhibition of EMT mediated through inhibition of the TGF-β/PI3K/Akt pathway.

Expression and regulation of microRNA-29a and microRNA-29c in early diabetic rat cataract formation

AIM

To determine the role of microRNA (miRNA)-29a and miRNA-29c in the regulation of apoptosis in early rat diabetic cataract formation.

METHODS

Streptozotocin (STZ)-induced diabetic Sprague-Dawley (SD) rats were used in the study. The expression level of miRNA-29a, miRNA-29c, and BCL2-modifying factor (BMF) in lens epithelial cells (LECs) samples were measured using quantitative real-time polymerase chain reaction. Prediction algorithms of miRanda, TargetScan 6.2, and mirRDB to perform a miRNA gene network analysis were used for the potential miRNA-29a and miRNA-29c targets.

RESULTS

The miRNA-29a and miRNA-29c expression levels were all significantly lower in the control group compared to the 2 and 4wk diabetic samples (P<0.01). The network analysis indicated that one miRNA-29a and miRNA-29c targets was BMF. There was significantly higher expression of BMF mRNA compared to the normal controls (P<0.01).

CONCLUSION

Apoptosis occurs in rat LECs following high blood glucose exposure. It is likely that apoptosis during diabetic cataract formation involves the decreased expression of miRNA-29a and miRNA-29c and the increased expression of BMF.

Sulforaphane-induced transcription of thioredoxin reductase in lens: possible significance against cataract formation

PURPOSE

Sulforaphane is a phytochemically derived organic isothiocyanate 1-isothiocyanato-4-methylsulfinyl-butane present naturally in crucifers, including broccoli and cauliflower. Biochemically, it has been reported to induce the transcription of several antioxidant enzymes. Since such enzymes have been implicated in preventing cataract formation triggered by the intraocular generation of oxy-radical species, the purpose of this investigation was to examine whether it could induce the formation of antioxidant enzymes in the eye lens. Thioredoxin reductase (TrxR) was used as the target of such induction.

METHODS

Mice lenses were cultured for an overnight period of 17 hours in medium 199 fortified with 10% fetal calf serum. Incubation was conducted in the absence and presence of sulforaphane (5 μM). Subsequently, the lenses were homogenized in phosphate-buffered saline (PBS), followed by centrifugation. TrxR activity was determined in the supernatant by measuring the nicotinamide adenine dinucleotide phosphate (reduced) (NADPH)-dependent reduction of 5,5'-dithiobis-2-nitrobenzoic acid (DTNB). Non-specific reduction of DTNB was corrected for by conducting parallel determinations in the presence of aurothiomalate. The reduction of DTNB was followed spectrophotometrically at 410 nm.

RESULTS

The activity of TrxR in the lenses incubated with sulforaphane was found to be elevated to 18 times of that observed in lenses incubated without sulforaphane. It was also noticeably higher in the lenses incubated without sulforaphane than in the un-incubated fresh lenses. However, this increase was much lower than that observed for lenses incubated with sulforaphane.

CONCLUSION

Sulforaphane has been found to enhance TrxR activity in the mouse lens in culture. In view of the protective effect of the antioxidant enzymes and certain nutrients against cataract formation, the findings suggest that it would, by virtue of its ability to enhance the activity of such enzymes, prevent the tissue against oxidative stress that leads to cataract formation. Additional studies with the activities of other antioxidant enzymes such as quinone oxidoreductase and the levels of Nrf2 are in progress.

Low glucose under hypoxic conditions induces unfolded protein response and produces reactive oxygen species in lens epithelial cells

Aging is enhanced by hypoxia and oxidative stress. As the lens is located in the hypoglycemic environment under hypoxia, aging lens with diabetes might aggravate these stresses. This study was designed to examine whether low glucose under hypoxic conditions induces the unfolded protein response (UPR), and also if the UPR then generates the reactive oxygen species (ROS) in lens epithelial cells (LECs). The UPR was activated within 1 h by culturing the human LECs (HLECs) and rat LECs in <1.5 mM glucose under hypoxic conditions. These conditions also induced the Nrf2-dependent antioxidant-protective UPR, production of ROS, and apoptosis. The rat LECs located in the anterior center region were the least susceptible to the UPR, whereas the proliferating LECs in the germinative zone were the most susceptible. Because the cortical lens fiber cells are differentiated from the LECs after the onset of diabetes, we suggest that these newly formed cortical fibers have lower levels of Nrf2, and are then oxidized resulting in cortical cataracts. Thus, low glucose and oxygen conditions induce the UPR, generation of ROS, and expressed the Nrf2 and Nrf2-dependent antioxidant enzymes at normal levels. But these cells eventually lose reduced glutathione (GSH) and induce apoptosis. The results indicate a new link between hypoglycemia under hypoxia and impairment of HLEC functions.

Ginkgo biloba extract prevents against apoptosis induced by high glucose in human lens epithelial cells

AIM

To investigate the protective effects of Ginkgo biloba extract (GBE) on high glucose-induced apoptosis of human lens epithelial cells (HLEC) and the possible molecular mechanisms.

METHODS

The cultured HLEC were allotted into 6 groups: normal group, high glucose group, low-, moderate-, and high-dose GBE group, and the bendazac lysine group. Cell viability, cell apoptosis, the activities of cell antioxidases, aldose reductase, caspase-3, the levels of cell antioxidants, and the expressions of Bcl-2 and Bax were assessed by different methods.

RESULTS

After being incubated with high glucose for 24 h, HLEC underwent apoptosis and exhibited significant oxidative stress. In the presence of GBE at different doses, the rate of HLEC apoptosis was lower and the oxidative stress state was significantly ameliorated. The increased ratio of Bax to Bcl-2 was significantly reduced and the activation of caspase-3 was suppressed by GBE in a dose-dependent manner.

CONCLUSION

GBE prevents HLEC from high glucose-induced apoptosis through inhibiting oxidative stress, reducing the ratio of Bax to Bcl-2, and decreasing the activity of caspase-3. Therefore, GBE has a potential protective effect against diabetic cataract formation.

The Etiology of Steroid Cataract

Steroid-induced posterior subcapsular cataracts (PSCs) exhibit three main distinctive characteristics: (i) association only with steroids possessing glucocorticoid activity, (ii) involvement of aberrant migrating lens epithelial cells, and (iii) a central posterior location. The first characteristic suggests a key role for glucocorticoid receptor activation and subsequent changes to the transcription of specific genes. Glucocorticoid receptor activation is associated in many cell types with proliferation, suppressed differentiation, a reduced susceptibility to apoptosis, altered transmembrane transport, and enhancement of reactive oxygen species activity. Glucocorticoids may be capable of inducing changes to the transcription of genes in lens epithelial cells that are related to many of these cellular processes. This review examines the various mechanisms that have been proposed to account for the development of PSC in the context of recent DNA array studies. Additionally, given that the glucocorticoid receptor can also engender wide-ranging indirect activities, glucocorticoids could also indirectly affect the lens through the responses of other cells within the ocular compartment and/or through effects on cells at more remote locations. These indirect mechanisms, which, for example, could be mediated through alterations to the intraocular levels of growth factors that normally orchestrate lens development and maintain lens homeostasis, are also discussed. Although the mechanism of steroid cataract induction remains unknown, glucocorticoid-induced gene transcription events in lens epithelial cells, and also other intraocular or systemic cells, likely interact to generate steroid cataracts. Finally, although evidence for glucocorticoid-protein adduct formation in the lens is inconclusive, the generation of such adducts cannot yet be discounted as a contributing factor and must necessarily be retained in discussions of the etiology of steroid cataract.

Susceptibility of the ocular lens to nitric oxide: implications in cataractogenesis

Oxides of nitrogen, such as nitric oxide (NO), are now biologically referred to as reactive nitrogen species. The generation of NO gives rise to several other reactive species, such as NO+, NO-, NO2, N2O3, and ONOO- and so forth, which are all capable of inflicting tissue damage. Indeed, NO generation is known to be associated with retinal degeneration and glaucoma. Its level has also been found to increase in the aqueous and vitreous humors in diabetes. We hypothesize that such an increase would have a detrimental effect on the biochemistry and metabolism of tissues, including the lens, bathed by the aqueous containing elevated levels of NO. The primary aim of our investigations was, therefore, to examine the susceptibility of the lens to damage by NO in vitro in the presence of nitroaspirin, a novel NO donating agent. The extent of physiologic damage to the lens was initially assessed by determining the integrity of its active transport mechanism. The overall status of tissue metabolism was determined by measuring the adenosine triphosphate (ATP) levels. The levels of glutathione (GSH) and glutathione disulfide, reflecting the status of its antioxidant reserve, were also determined. That NO is indeed deleterious to the lens was apparent by the inhibition of the active transport of Rb(+). This was associated with a substantial decrease in the contents of ATP and GSH, the decrease in the latter directly suggesting that the NO effects are caused by oxidative stress. That the effects are caused by NO generated from nitroaspirin was proven by a substantial increase in NO level in the medium during incubation of the lenses with nitroaspirin, as compared to the controls. The results, therefore, were highly suggestive of a contribution of the oxides of nitrogen in cataract formation associated with diabetes and other aging diseases.

Upregulation of glyoxalase I fails to normalize methylglyoxal levels: a possible mechanism for biochemical changes in diabetic mouse lenses

Glyoxalase I is the first enzyme in a two-enzyme glyoxalase system that metabolizes physiological methylglyoxal (MGO). MGO reacts with proteins to form irreversible adducts that may lead to crosslinking and aggregation of lens proteins in diabetes. This study examined the effect of hyperglycemia on glyoxalase I activity and its mRNA content in mouse lens epithelial cells (mLE cells) and in diabetic mouse lenses and investigated the relationship between GSH and MGO in organ cultured lenses. mLE cells cultured with 25 mM D-glucose (high glucose) showed an upregulation of glyoxalase I activity and a higher content of glyoxalase I mRNA when compared with either cells cultured with 5 mM glucose (control) or with 20 mM L-glucose + 5 mM D-glucose. MGO concentration was significantly elevated in cells cultured with high D-glucose, but not in L-glucose. GSH levels were lower in cells incubated with high glucose compared to control cells. Glyoxalase I activity and mRNA levels were elevated in diabetic lenses compared to non-diabetic control mouse lenses. MGO levels in diabetic lenses were higher than in control lenses. Incubation of lenses with buthionine sulfoximine (BSO) resulted in a dramatic decline in GSH but the MGO levels were similar to lenses incubated without BSO. Our data suggest that in mouse lenses MGO accumulation may occur independent of GSH concentration and in diabetes there is an upregulation of glyoxalase I, but this upregulation is inadequate to normalize MGO levels, which could lead to MGO retention and chemical modification of proteins.

Prevention of cataract by pyruvate in experimentally diabetic mice

Previous studies have demonstrated that administration of pyruvate prevents cataract formation in diabetic rats. It is known that the induction of cataractous process in this case is initiated by aldose reductase (AR) catalyzed synthesis and accumulation of excessive sorbitol in the lens fibres and epithelium and their consequent osmotic hydration. Synthesis of this and other polyols is competitively inhibited by pyruvate. The objective of the present investigations was hence to determine whether pyruvate would have a similar protective effect in species where cataract formation is relatively independent of sorbitol synthesis such as in humans where the lens AR activity is extremely low, especially with glucose as a substrate. The Km of AR for glucose is known to be very high. The possible protective effect of pyruvate in the low AR models was conceived on the basis of our previous findings suggesting that it can also exert substantial antiglycating as well as antioxidant effects. The present studies have hence been conducted with mice, a species known to be low in lens AR, similar to that in humans. As stipulated, pyruvate administration has indeed been found to offer a significant protection against development of diabetic cataract in this model also. The effect correlated with the inhibition of protein glycation as well as of oxidative stress. The latter was apparent by the prevention of the loss of glutathione known to be associated with diabetes. Although there was a small but noticeable increment in the sorbitol content of the diabetic lenses, this was osmotically insignificant. Even this increase was prevented by pyruvate. The magnitude of the elevation in the contents of glycated proteins and the depression in the level of glutathione were, on the contrary, highly pronounced, suggesting a more prominent role of the latter factors. In addition, the possibility of a direct metabolic support it could offer to the tissue is also imminent by its effect on the maintenance of ATP, as shown earlier. The present studies are therefore considered more relevant to the pathogenesis of cataract in human diabetics and its possible prevention by endogenous compounds with antiglycating and antioxidant properties. Inhibition of cataract formation by pyruvate in an animal model with low lens AR, similar to that in humans, has been shown for the first time.