according to an estimation of the world health organization ( who ) , more than 366 million people worldwide will have diabetes mellitus in 2030 ; the urban population in developing countries is projected to double between 2000 and 2030 . diabetes mellitus ( dm ) is an endocrine and metabolic disease which affects almost all organs in our body . although the lungs are not thought to be primarily affected by diabetes and lung physiological and structural abnormalities are happening in both type 1 and type 2 diabetes , the lung should be considered as a  target organ  . chronic hyperglycemia is considered as the main reason for diabetes complications . in the process of prolonged increased glucose concentration , reactive oxygen species ( ros ) overproduction may be one of the key factors which induce organ injury . decreased antioxidative function and increased oxidative stress were seen in the lung of diabetic rats and rabbits [ 2 , 3 ] . mitochondria dysfunction will cause oxidative stress injury which is one of the underlying factors for neurodegenerative diseases , diabetes , cardiovascular diseases , cancer , and so on . physiological hypoxia triggers the changes of mitochondrial redox and increases superoxide generation at complex iii in alveolar epithelial cells . mitochondrial aldehyde dehydrogenase 2 ( aldh2 ) is a nuclear - encoded mitochondrial enzyme that localizes in mitochondrial matrix ; aldh2 protein had been detected in liver , lung , heart , kidney , testis , esophagus , stomach , colon , and pancreas . our previous study had reported that , in diabetic rats , ethanol ( etoh ) at low concentration offered myocardial protection through activation of aldh2 expression , and upregulation of aldh2 also plays the protective effect in myocardial ischemia and reperfusion injury , diabetes cardiomyopathy , and kidney and brain injury [ 912 ] ; meanwhile , some researchers reported the association between aldh2 and lung disease . xu et al . reported that , in neonatal rat lung , after prolonged hyperoxic exposure , aldh2 was downregulated . in lung epithelial cells , overexpression of aldh2 attenuated hyperoxia - induced cell death through reduction of ros , activation of erk / mapk , and pi3k - akt signaling pathways . but it remains unknown whether activation of aldh2 expression can prevent diabetes induced lung injury . so in this study , we mimic diabetes model by intraperitoneal injection of streptozotocin ( stz ) to observe the role of etoh at low concentration in diabetes induced lung injury and analyze the related mechanisms . male sprague - dawley ( sd ) rats ( 200250  g ) obtained from the animal center of bengbu medical college were selected for the study . all animal studies were approved by the animal ethics committee of bengbu medical college and performed in accordance with the ethical standards . streptozotocin ( stz ) was purchased from sigma ( st . louis , mo , usa ) . malondialdehyde ( mda ) and superoxide dismutase ( sod ) assay kits were from nanjing jiancheng bioengineering institute , china . ethanol ( etoh , figure 1 ) was purchased from bengbu new chemical reagent factory , china . the primers used were as follows : for aldh2 forward : 5-gtg ttc gga gac gtc aaa ga-3 and reverse : 5-gca gag ctt ggg aca ggt aa-3 and the product size was 187  bp ; for -actin forward : 5-gat ggt ggg tat ggg tca gaa gga c-3 and reverse : 5-gct cat tgc cga tag tga tga ct-3 and the product size was 630  bp . mouse anti - aldh2 and anti--actin monoclonal antibodies were from santa cruz biotechnology ( ca ) . stz at 55  mg / kg freshly dissolved in 0.1  mol / l sodium citrate buffer ( ph 4.5 ) was injected intraperitoneally to induce diabetic models in overnight fasted rats . the rats in control group were injected with a similar volume of sodium citrate buffer alone . the rats with plasma fasting blood glucose level higher than 16.7  mmol / l 72  h after injection were regarded as diabetic . animals were randomly divided into control , diabetes ( dm ) , and etoh+dm groups , respectively ( n = 6 ) . in etoh+dm group , dm rats were fed with 2.5% etoh in their drinking water for one week to initiate drinking and then it was changed to 5% etoh continuous access through the 8 weeks . the plasma fbg level and body weight ( bw ) were measured at the 8th week . the ratio of lw / bw was calculated . at the end of the experimental period the supernatant was collected after centrifugation for 20  min ( 2000  rpm ) . malondialdehyde ( mda ) content and superoxide dismutase ( sod ) activity were detected according to the instruction manual . for histological analysis by light microscope , the lung tissue was harvested and fixed in 4% paraformaldehyde for 24 hours , embedded in paraffin and cut into 5  m thick serial sections , and then stained with hematoxylin and eosin ( he ) for light microscope observation . lung injury degree was evaluated according to mikawa 's scoring standards : ( 1 ) alveolar congestion , ( 2 ) hemorrhage , ( 3 ) infiltration or aggregation of neutrophils in airspace or vessel wall , and ( 4 ) thickness of alveolar wall / hyaline membrane formation . each item was scored on a 5-point scale as follows : 0 : minimal damage , 1 : mild damage , 2 : moderate damage , 3 : severe damage , and 4 : maximal damage . the final lung injury score was the summation of the four items . lung tissue was dissected and small pieces were fixed with 2.5% glutaraldehyde in 0.1  mol / l cacodylate buffer for 2  h and postfixed in 1% osmium tetroxide in 0.1  mol / l cacodylate buffer for 1  h. ultrathin sections were cut and contrasted with uranyl acetate followed by lead citrate and observed with jem-1230 transmission electron microscope ( jeol , japan ) . briefly , total rna was extracted with trizol according to the manufacturer 's instructions . a total rna ( 2  mg ) quantification of the result was determined through measuring the optical density of the labeled bands ; the value was normalized to -actin intensity level . anti - aldh2 ( 1  :  500 ) antibody was used . mouse anti--actin antibody ( 1  :  500 ) was used as an internal control . the immunoblots were exposed to x - ray film and analyzed with a digital image system . statistical comparisons were carried out by one - way variance analysis and the newman - keuls test . in contrast to control group , fbg levels in dm and etoh+dm groups were increased significantly and fbg level in etoh+dm group was lower than in dm group ( table 1 ) . compared with control animals , body weight ( bw ) and lung weight ( hw ) were significantly decreased in dm and etoh+dm groups , and lw / bw was increased in dm group . in contrast to dm group , bw was increased and lw / bw was decreased in etoh+dm group ( table 1 ) . in contrast to control rats , pulmonary sod activity was decreased in dm group while mda content was increased in dm and etoh+dm groups . in etoh+dm group , sod activity was higher while mda content was lower than in dm group ( table 2 ) . in control group , alveolar walls in lung tissue had unique shape and size , and hemorrhage and inflammatory infiltration were rare . in dm group , alveolar septum was thickened widely , infiltration of inflammatory cells and hemorrhage appeared , and the lung injury score was higher than in control group . the injury was ameliorated in etoh+dm group than in dm group ( figure 2 and table 1 ) . in control group , many regular structure lamellar bodies appeared in type ii alveolar cell cytoplasm . in dm group , the structure of lamellar bodies was collapsed , discontinuous , and vacuolated . in etoh+dm group , the expressions of pulmonary aldh2 mrna and protein were decreased in dm group ; compared with dm group , pulmonary aldh2 mrna and protein expressions were increased in etoh+dm group ( figures 4 and 5 and table 3 ) . in the present study , we observed that lung oxidative stress injury occurred in diabetic rats , which was indicated by the increase of pulmonary mda content and decrease of sod activity ; meanwhile , the decrease of pulmonary aldh2 mrna and protein expressions happened accompanying the happening of lung swelling , the destruction of pulmonary tissue , and type ii alveolar cell structure . when the diabetic rats were treated with etoh at low concentration which was reported to activate aldh2 expression [ 8 , 10 ] , lung aldh2 mrna and protein expressions were increased , lung oxidative stress injury and swelling degree were attenuated , the destruction of pulmonary tissue and type ii alveolar cell structure was alleviated , suggesting that downregulation of pulmonary aldh2 was likely to be correlated with oxidative stress injury in diabetic rats , and activation of aldh2 with etoh at low concentration attenuated diabetes induced lung injury and oxidative stress overload . in the development of diabetes , oxidative stress induced by chronic hyperglycemia plays a key role in the pathogenesis of diabetes - related complications including lung diseases . diabetes induces impairment of defense system , which is associated with reduced antioxidant capacity , abnormal activity , or expression of antioxidant enzymes . an increase of mda , a lipid peroxidation marker , accompanied by the depressing of sod activity ( one of the most important endogenous antioxidase enzymes ) , is assessed to imply oxidant damage . oxidative stress can destroy the pulmonary structure . in our study , we observed that , in diabetic rat , pulmonary mda content was increased with the decrease of sod activity , and the structures of lung tissue and type ii alveolar cell were destroyed , suggesting the unbalance of lung antioxidative system induced lung injury in diabetes rats . when the diabetic rat was treated with etoh at low concentration , pulmonary mda content was decreased and sod activity was increased accompanying the recovery of lung tissue and type ii alveolar cell structures in contrast to diabetic rat , suggesting that etoh protected the lung tissue through antioxidative stress role . activation of aldh2 with alda-1 alleviated cardiac ischemic and reperfusion injury with the decrease of 4-hne in rodent models . aldh2 overexpression rescued neuronal survival against 4-hne treatment in pc12 cells . in male c57bl/6 mice , activation of aldh2 with etoh prevented renal ischemia and reperfusion injury with suppressed lipid peroxidation and increased sod activity . but few reports focus on the relationship of pulmonary aldh2 and diabetes induced lung injury . aldh2 overexpression attenuated hyperoxia - induced cell death in lung epithelial cells through reduction of ros , activation of erk / mapk , and pi3k - akt signaling pathways . since diabetes induced lung injury accompanied with oxidative stress , while increasing aldh2 expression could decrease oxidative stress , so we investigated whether activating pulmonary aldh2 expression by etoh could decrease lung oxidative stress injury in diabetic rats . the results displayed pulmonary aldh2 mrna and protein expressions were decreased in diabetic rats , while etoh treatment increased aldh2 expression and , meanwhile , decreased oxidative stress injury ; it suggested that , in diabetes induced lung injury , the aggravation of oxidative stress may be derived from the decrease of pulmonary aldh2 expression and improving pulmonary aldh2 expression could be against the happening of hyperglycemia induced oxidative stress . it is worthwhile to note that , in our study , we selected etoh to promote aldh2 expression because there had been many papers reporting that etoh at a suitable concentration could activate aldh2 expression and play the protective effect [ 8 , 10 , 12 , 18 ] . but in clinic , etoh is difficult to apply for patients because of its toxicity and addiction . so selecting an appropriate drug to activate pulmonary aldh2 expression may be beneficial for diabetic patients who had suffered from lung injury . in conclusion , our results indicated that , in diabetes induced lung injury in rat model , pulmonary aldh2 expression was decreased . treatment with etoh at low concentration can decrease diabetes induced lung injury through activating aldh2 expression .