Diabetic Vasculopathy and Alcohol Tolerance Trait in Type 2 Diabetes

In 1978, Pyke and Leslie (1,2) first proposed the hypothesis that clinical pictures of patients with type 2 diabetes can be characterized by two clinical presentations in response to chlorpropamide use and tolerance to alcohol. Chlorpropamide alcohol flushing (CPAF) is often observed in diabetic patients with a family history of diabetes, but among those patients without CPAF, there is a high probability of developing severe diabetic retinopathy. Subsequently, Barnett et al. (3) has reported that persistent proteinuria is also more commonly observed in patients without CPAF. However, the role and significance of CPAF and diabetic vasculopathy still remains controversial.

Aldehyde dehydrogenase-2 (ALDH2) and alcohol dehydrogenase-2 (ADH2) are the key enzymes for alcohol metabolism. Many Asians lack enzyme activity of ALDH2 and have superactive enzyme activity of ADH2, attributed to point mutations within both structural genes (4). Hence, the expression of these two enzyme mutations could determine the alcohol tolerance among the Japanese population.

We have found an increase in the prevalence of nephropathy and advanced diabetic retinopathy among Japanese patients with diabetes and a specific ADH2 and ALDH2 genotype. A total of 158 patients with type 2 diabetes (114 men and 44 women aged 17–81 years) were examined. The subjects were consecutively selected from our outpatient clinic and were all unrelated. After informed consent, a blood sample was obtained from each subject. Genotyping of ALDH2 and ADH2 was performed by the PCR-restriction fragment–length polymorphism (RFLP) method, details described elsewhere (4). The phenotype of ALDH2 inactivity is compatible with possession of the genotype ALDH2*1/ALDH2*2 or ALDH2*2/ALDH2*2, and the phenotype of ADH2 superactivity is compatible with possession of the genotype ADH2*2/ADH2*2 (4). Diabetic retinopathy was assessed and categorized by ophthalmologist examination. Nephropathy was diagnosed if proteinuria was found on testing with CLINITEK-200+ (Bayer Medical) on at least three consecutive clinic visits in the absence of other causes of proteinuria.

The results of this study showed that 41 subjects have active ALDH2 and superactive ADH2 genotypes, which was regarded as the alcohol tolerance (ATO) group. The other 117 subjects were regarded as the alcohol intolerance (AIT) group, because these patients had usual ADH2 and/or inactive ALDH2 genotypes, which accounts for delayed alcohol metabolism. There was no difference between the two groups in sex, age, age of diabetes onset, duration of diabetes, height, BMI, fasting plasma glucose, and HbA_1c level (for ATO vs. AIT, respectively, male/female 28/13 vs. 86/31, age 59.1 ± 9.6 vs. 57.7 ± 11.2 years, onset of diabetes 47.6 ± 10.4 vs. 46.5 ± 12.4 years, duration 11.7 ± 7.3 vs. 11.0 ± 7.7 years, height 161.3 ± 9.7 vs. 162.9 ± 7.9 cm, BMI 23.2 ± 3.9 vs. 22.8 ± 3.4 kg/m², fasting plasma glucose 149.5 ± 38.9 vs. 146.5 ± 42.4 mg/dl, and HbA_1c 7.8 ± 1.4 vs. 7.8 ± 1.3%). However, the ATO group had a higher frequency of having persistent proteinuria than the AIT group (ATO 15 of 41, 36.6%; AIT 24 of 117, 20.5%; P < 0.05 by χ² analysis). Among all, retinopathy was found in 31.7% (13 of 41) of the ATO group and in 32.5% (38/117) of the AIT group, showing no difference. However, among the patients with retinopathy, the frequency of proliferative retinopathy was three times higher in the ATO group (5 of 13, 38.5%) than in the AIT group (5 of 38, 13.2%) (P < 0.05). Thus, the ATO group had higher frequency of having nephropathy and of developing diabetic proliferative retinopathy than the AIT group.

It has been noted that activation of protein kinase C (PKC)-β under hyperglycemia can lead to a number of downstream sequelae, which are potentially damaging to the vascularity of glomerulus and retina in diabetes (5). ADH and ALDH are the enzymes not only for alcohol metabolism, but also for degradation of 4-hydroxy-2-nonenal (4-HNE) and other aldehydes (6–8). 4-HNE is a by-product of lipid peroxidation and has a role in pathophysiological conditions by acting as a signal molecule able to modulate relevant biological events, such as cell signaling, gene expression, cell proliferation, and cell differentiation. Interestingly, Chiarpotto et al. (9) has reported on differential regulation of protein PKC isoforms by a concentration of 4-HNE that is actually detectable in specific biological fluids or tissues. PKC-β1 and PKC-βII activities are markedly increased by 0.1 μmol/l 4-HNE, whereas they are unaffected or even inhibited by 1–10 μmol/l 4-HNE. Decreased tissue levels of 4-HNE could result from active ALDH2 and superactive ADH2 expression, as represented by subjects of the ATO group (6–8). Therefore, we speculate, in the ATO group, that the 4-HNE in the low micromolar range is able to have an influence on cell function through upregulation of PKC-β isoforms, which aggravates the damaging effects of PKC-β isoforms induced by hyperglycemia. Then, in the chronic situation, the lower concentration trait of 4-HNE in the ATO group may account for the long-term development of diabetic nephropathy and severe retinopathy.

In conclusion, we suggest that in Japanese individuals, the alcohol tolerance genetic trait is associated with the occurrence of diabetic vasculopathy. Our finding seemingly has a similar importance to that of the CPAF hypothesis, in terms of a suggestion for a relationship between alcohol tolerance and diabetic vasculopathy (2,3).