Tauroursodeoxycholic acid attenuates endoplasmic reticulum stress and protects the liver from chronic intermittent hypoxia induced injury

Abstract

Obstructive sleep apnea that characterized by chronic intermittent hypoxia (CIH) has been reported to associate with chronic liver injury. Tauroursodeoxycholic acid (TUDCA) exerts liver-protective effects in various liver diseases. The purpose of this study was to test the hypothesis that TUDCA could protect liver against CIH injury. C57BL/6 mice were subjected to intermittent hypoxia for eight weeks and applied with TUDCA by intraperitoneal injection. The effect of TUDCA on liver histological changes, liver function, oxidative stress, inflammatory response, hepatocyte apoptosis and endoplasmic reticulum (ER) stress were investigated. The results showed that administration of TUDCA attenuated liver pathological changes, reduced serum alanine aminotransferase and aspartate aminotransferase level, suppressed reactive oxygen species activity, decreased tumor necrosis factor-α and interleukin-1β level and inhibited hepatocyte apoptosis induced by CIH. TUDCA also inhibited CIH-induced ER stress in liver as evidenced by decreased expression of ER chaperone 78 kDa glucose-related protein, unfolded protein response transducers and ER proapoptotic proteins. Altogether, the present study described a liver-protective effect of TUDCA in CIH mice model, and this effect seems at least partly through the inhibition of ER stress.

Keywords: apoptosis; chronic intermittent hypoxia; endoplasmic reticulum stress; liver; obstructive sleep apnea; tauroursodeoxycholic acid.

Figure 1.

Liver histological changes and serum biochemical indicator levels. In total, 8 weeks after CIH, (A) the histological changes of liver tissues were analyzed by hematoxylin and eosin staining and (B) the biochemical indicators of liver function were detected by commercial kits. Scale bar, 50 µm. **P<0.01 vs. NC group, ^##P<0.01 vs. CIH group. CIH, chronic intermittent hypoxia; NC, negative control; TUDCA, tauroursodeoxycholic acid; ALT, alanine aminotransferase; AST, aspartate aminotransferase; ALP, alkaline phosphatase.

Figure 2.

Liver ROS activity and proinflammatory cytokine level. Relative (A) ROS activity, (B) TNF-α and (C) IL-1β level in liver tissues. **P<0.01 vs. NC group, ^##P<0.01, ^#P<0.05 vs. CIH group. CIH, chronic intermittent hypoxia; NC, negative control; TUDCA, tauroursodeoxycholic acid; ROS, reactive oxygen species; TNF, tumor necrosis factor; IL, interleukin.

Figure 3.

Apoptosis of hepatocytes. Apoptosis of hepatocytes were analyzed by (A) TUNEL, (B) the expression of cleaved caspase-3 and (C) cleaved caspase-9 in the liver were detected by western blotting. Scale bar, 50 µm. **P<0.01 vs. NC group, ^##P<0.01 vs. CIH group. CIH, chronic intermittent hypoxia; NC, negative control; TUDCA, tauroursodeoxycholic acid.

Figure 4.

UPR activation in liver tissues. Expression of (A) GRP78 and three major transducers of UPR, (B) PERK, (C) ATF6 and (D) IRE1 were measured by western blotting. **P<0.01 vs. NC group, ^##P<0.01, ^#P<0.05 vs. CIH group. CIH, chronic intermittent hypoxia; NC, negative control; TUDCA, tauroursodeoxycholic acid; GRP78, 78 kDa glucose-related protein; PERK, eukaryotic translation initiation factor 2 α kinase 3; ATF, activating transcription factor; IRE, inositol-requiring enzyme; UPR, unfolded protein response.

Figure 5.

ER stress-associated apoptosis in the liver. Expression of ER proapoptotic molecules, (A) CHOP, (B) cleaved caspase-12, (C) p-eIF2α and (D) p-JNK were detected by western blotting. **P<0.01 vs. NC group, ^##P<0.01, ^#P<0.05 vs. CIH group. CIH, chronic intermittent hypoxia; NC, negative control; TUDCA, tauroursodeoxycholic acid; CHOP, C/EBP homologous protein; eIF2α, eukaryotic translation initiation factor 2 subunit α; JNK, janus kinase; ER, endoplasmic reticulum.