Overexpression of AR in a transgenic
Overexpression of AR in a transgenic mouse model leads to a capsular cataract phenotype involving proliferation and formation of a fibrotic plaque of cells reminiscent of cells at the posterior KB SRC 4 in PCO . To investigate the molecular mechanism that could link AR expression to this phenotype, we examined Smad-dependent and Smad-independent signaling pathways. The Smad signaling pathway downstream from the receptor has been shown to play a crucial role in PCO development , . TGF-β binding to its cell surface receptor induces a recruitment of Smad-2 and Smad-3 to the receptor complex where they are phosphorylated by receptor kinases and subsequently associate with Smad-4 and translocate to the nucleus to activate the transcription of a variety of genes involved in EMT. In our comparison of epithelial cells from lenses of WT and AR-Tg mice, we observed no significant difference in p-Smad2 activation (Fig. 3A). Thus, overexpression of AR was not sufficient to increase basal levels of p-Smad2 even though the basal level of the EMT markers aSMA and fibronectin were increased over WT (Fig. 2A and D). Thus, AR overexpression resulted in the activation of a Smad-independent pathway that caused the upregulation of some EMT markers. Other than the TGF-β2-stimulated Smads signaling pathway, PCO development could be mediated by MAP signaling kinases (RAS/MEK/ERK) , . We observed that AR overexpression induces ERK activation (Fig. 3B) indicating that AR-induced EMT elevation may involve a Smad-independent pathway. To understand the effect of AR on ERK activation, lens epithelial cells from AR-Tg mice were incubated in the presence or absence of Sorbinil. We observed that Sorbinil treatment reduced AR-induced ERK activation (Fig. 3C). To further confirm that AR-induced EMT process is mediated by ERK activation, we observed that AR-Tg LECs treated with the ERK inhibitor U0126 expressed less fibronectin as compared to WT (Fig. 3D). These results suggest that AR may influence EMT progression in a process mediated not only by Smad signaling downstream from the TGF-β2- receptor  but also through a process involving ERK activation. AR plays a role in the conversion aldehydes such as 4-hydroxy-trans-2-nonenal (4-HNE) into 1,4-dihydroxynonene (DHN) , which induces more oxidative stress. Consequently, it is possible that the AR effects observed here could be due to the aldehyde reducing capacity of the enzyme, rather than its role in glucose metabolism. Understanding the complexity of these mechanisms may provide clues to alternative therapeutic strategies, such as blockade of ERK signaling, for PCO prevention after cataract surgery.
Conflict of interest statement
Acknowledgements This study is supported by NIH grants EY005856 and EY021498 and by a Challenge Grant from Research to Prevent Blindness.
Introduction Alcoholic liver disease is a major cause of morbidity and mortality in the world. There are 3.3 million alcohol-related deaths annually . The effect of ethanol on liver structural and functional capacity depends on the drinking pattern (excessive or not, acute or chronic). However, a dose-response relationship exists between alcohol consumption and risk of liver damage . The progression of alcoholic fatty liver can be characterized by steatosis, alcoholic hepatitis, fibrosis, cirrhosis and hepatocellular carcinoma. Steatosis is an early pathological stage and can be diagnosed from number and size of cytoplasmic lipid droplets [3,4]. Lipid droplets consist of a neutral lipid core surrounded by a monolayer of phospholipids and many proteins. Excess accumulation of lipid droplets makes hepatocytes vulnerable to reactive oxygen species, adipocytokines and gut-derived endotoxins . Therefore, it is necessary to find effective drug to reverse steatosis. Polyol pathway converts glucose to fructose in the liver. This process needs aldose reductase (AR) and sorbitol dehydrogenase. AR converts glucose to sorbitol with cofactor nicotinamide adenine dinucleotide phosphate. Sorbitol dehydrogenase metabolizes sorbitol to fructose with cofactor nicotinamide adenine dinucleotide . AR is a rate-limiting step of polyol pathway. The levels of AR mRNA and protein are stimulated in alcoholic fatty liver . The inhibition of AR attenuates gut bacterial endotoxin-mediated cytokine overproduction . These allow AR to be a candidate target protein for treating alcoholic fatty liver. AR inhibitors include carboxylic acid derivatives, spirohydantoins and related cyclic amides, phenolic derivatives and so on . It has been previously reported that higher levels of myristic acid, palmitic acid and oleic acid are detected in mice with alcoholic liver injury [10,11]. Interestingly, other studies show that palmitic acid and oleic acid modulate steatosis in primary mouse hepatocytes [12,13]. These studies suggest steatosis-associated metabolites are considered as a contributory factor to alcoholic steatosis, but whether AR inhibitor ameliorates steatosis by steatosis-associated metabolites is largely unclear.