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T. Shinohara, C.F. Madson, M.L. Mulhern, A. Danford, K. Ikesugi, P.F. Kador; Hypoglycemic and Hyperglycemic Conditions Induce the Unfolded Protein Response (UPR) in Galactosemic Rat Lenses and Human Lecs in Culture . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4117.
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Diabetes induces a number of ocular complications including cataracts. Diabetic complications are tightly associated with hyperglycemia; however, the incidence of hypoglycemia is often increased as tight control of hyperglycemia is established. The purpose of this study was to investigate whether glucose imbalances in lens epithelial cells (LECs) can induce endoplasmic reticulum (ER) stress which, in turn, induces the unfolded protein response (UPR) and results in the production of reactive oxygen species (ROS) and apoptosis.
Human LECs were cultured in DMEM with 5–125 mM of glucose or glucose–free DMEM with 10% of BFS. Sprague Dawley rats were fed a chow containing 50% galactose. Calcein, ethidium homodimer–1 (EthD) and TUNEL staining were used to evaluate live and apoptotic cells. Protein blot analysis was used to identify the UPR specific proteins. Dichlorodihydrofluorescin diacetate (DCF) was used to evaluate levels of ROS.
In 5–9 day galactose–fed rats, a significant increase in LEC mortality was observed. In these LECs, increased levels of the UPR specific proteins Bip/GRP78, CHOP, and LEDGF were observed and caspases–12 and –3 were activated, along with the production of ROS and apoptosis. The UPR was only observed in LECs but not fiber cells. Since hyperglycemia is associated with osmotic and oxidative stress and protein glycation, LECs were cultured with 125 mM of mannitol and NaCl (osmotic stress), in 30–50 µM of H2O2 (oxidative stress), and 10–500 µM of methylglyoxal (protein glycation). The UPR activation was only observed under osmotic and oxidative stress but not with glycation. In addition, strong UPR induction was observed with glucose and galactose deprivation (hypoglycemia) in LECs.
These findings indicate that osmotic stress, oxidative stress, and sugar deprivation can induce the UPR in LECs and that this induction produces ROS and induces apoptosis. Modulation of the UPR pathway may offer a novel method of reducing LEC mortality associated with glucose imbalances.
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