Vascular adaptive response in chronic liver hypoxia

Abstract

INTRODUCTION: Cardiovascular adaptation is well recognized in the form of vascular remodeling in the lungs and hypertrophy of the heart that are necessary for relieving the state of hypoxemia. In chronic lung hypoxia, there is a shift in the production metabolites away from vasodilators to vasoconstrictors. The vascular response for adaptation in chronic liver hypoxia is poorly understood. Furthermore, prolonged hypoxia is a common underlying event in most chronic liver diseases such as alcoholic liver disease, cirrhosis and malignancy. Studying the events related to the gene expression in the liver during hypoxia could lead to a better understanding on how to modulate new treatment approaches in the management of liver diseases in which hypoxia is a major factor. Hypoxia-inducible factor 1 (HIF-1) activates transcription of genes encoding proteins that mediate the adaptive response to hypoxia. These genes include erythropoietin, VEGF and glycolytic enzymes. In the present study, we determined the hepatic expression of hypoxia-inducible factor 1 (HIF-1) in hypoxia as well as the expression of downstream genes that carry the hypoxic response element (HRE) such as iNOS, VEGF and ET-1. These genes modulate the vascular response through nitric oxide. We also evaluated the role of other nuclear transcription factors such as nuclear factor kappa B (NF-kB) and activator protein–1 (AP-1). eNOS is regulated by AP-1. MATERIALS AND METHODS: Blood and liver samples from adult SD rats were collected at various time-points. Samples from normoxic and hypoxic rats were analyzed for different genes and proteins using histological analysis, immunohistochemistry, serum ALT, hematocrit, lipid peroxidation (8-isoprostane), RT-PCR, Western Blotting and EMSA. RESULTS: Our results showed that there was a significant increase in the hematocrit levels from day 7 to day 28 in the hypoxic rats when compared with normoxic controls. A significant weight loss was also observed from day 7 to day 28 in the hypoxic group. Right ventricular hypertrophy occurred in hypoxic rats. The liver morphology and serum ALT were within normal range at all time-points suggesting the absence of significant necroinflammation. Also, 8-isoprostane levels were not elevated in either group at all time-points. iNOS mRNA peaked in hypoxic liver at day 21 and then decreased by day 28 but was higher than normoxic controls. Nitrotyrosine protein levels were not significantly increased at any of the time-points. eNOS, VEGF and ET-1 mRNAs progressively increased from day 7 to day 28 in hypoxic liver when compared with normoxic controls. The trends observed at the transcription level were confirmed at the protein level as determined by Western blot analysis. HIF-1a, NK-kB and AP-1 were upregulated in hypoxic liver when compared to the normoxic control. Our data suggest that the vascular adaptive ability of the liver in a prolonged state of oxygen deprivation triggers compensatory mechanisms for survival towards a vasodilatory response by expressing significant levels of nitric oxide through HIF-1a, NK-kB, AP-1, HRE and eNOS genes. There was no significant necroinflammatory change or increased oxidative stress in the hypoxic liver.