Decreased skin-mediated detoxification contributes to oxidative stress and insulin resistance

Abstract

The skin, the body's largest organ, plays an important role in the biotransformation/detoxification and elimination of xenobiotics and endogenous toxic substances, but its role in oxidative stress and insulin resistance is unclear. We investigated the relationship between skin detoxification and oxidative stress/insulin resistance by examining burn-induced changes in nicotinamide degradation. Rats were divided into four groups: sham-operated, sham-nicotinamide, burn, and burn-nicotinamide. Rats received an intraperitoneal glucose injection (2 g/kg) with (sham-nicotinamide and burn-nicotinamide groups) or without (sham-operated and burn groups) coadministration of nicotinamide (100 mg/kg). The results showed that the mRNA of all detoxification-related enzymes tested was detected in sham-operated skin but not in burned skin. The clearance of nicotinamide and N(1)-methylnicotinamide in burned rats was significantly decreased compared with that in sham-operated rats. After glucose loading, burn group showed significantly higher plasma insulin levels with a lower muscle glycogen level than that of sham-operated and sham-nicotinamide groups, although there were no significant differences in blood glucose levels over time between groups. More profound changes in plasma H(2)O(2) and insulin levels were observed in burn-nicotinamide group. It may be concluded that decreased skin detoxification may increase the risk for oxidative stress and insulin resistance.

Figure 1

RT-PCR analysis of mRNA expression of xenobiotic/drug- and ROS-metabolizing enzymes in rat skin. S: sham-operated skin. B: burned skin. The data shown are representative of three separate experiments.

Figure 2

Plasma concentrations of nicotinamide and N ¹-methylnicotinamide in different rat groups. Sham: sham-operated group; Sham + NM: sham-nicotinamide group; Burn: burn group; Burn + NM: burn-nicotinamide group. *P < 0.01. Bar graph indicates mean ± SEM.

Figure 3

Serum H₂O₂ and insulin levels and blood glucose concentrations in different rat groups. Sham: sham-operated group; Sham + NM: sham-nicotinamide group; Burn: burn group; Burn + NM: burn-nicotinamide group. *P < 0.05 and **P < 0.01 versus sham-nicotinamide group, ^# P < 0.05 versus burn-nicotinamide group and ⁺ P < 0.01 versus burn group at the same time point. Data were presented as means ± SEM.

Figure 4

Glycogen contents of liver and skeletal muscle in different rat groups. Sham: sham-operated group; Sham + NM: sham-nicotinamide group; Burn: burn group; Burn + NM: burn-nicotinamide group. *P < 0.01. Bar graph indicates mean ± SEM.

Figure 5

RT-PCR analysis of mRNA expression of NNMT and AOX1 in the muscle from sham-operated rat. The data shown are representative of three separate experiments.

Figure 6

Expression of NNMT and location of AOX1 in human liver and skin. (a) western blotting results. ((b)–(f)) immunohistochemistry analysis of AOX1 expression. (b) positive control of AOX1 expression (human liver). ((d) and (f)), showing the location of AOX1 in sweat glands and sebaceous glands, respectively. ((c) and (e)) negative control for ((d) and (f)) respectively (without primary antibody). AOX1 was stained using a streptavidin-biotin-peroxidase complex method with diaminobenzidine substrate. Nuclei were counterstained with hematoxylin. Magnification: ×200.

Figure 7

Proposed role of skin in oxidative stress and IR. The body's total antioxidant capacity, consisting of biotransformation system, excretion system, and reactive oxygen species clearance system, depends on the functions of body's tissues/organs. A decrease in the skin contribution, such as induced by severe burn and cold ambient environment, decreases the body's total antioxidant capacity and thus increases the risk for oxidative stress and subsequent IR. OK: the body's total antioxidant capacity > ROS generation; OS: oxidative stress, that is, ROS generation > the body's total antioxidant capacity.