Below are some of the papers we've published which are central to our lab's understanding of hypoxia and its pathophysiologic consequences.
The Cardiovascular and Metabolic Effects of Chronic Hypoxia in Animal Models: A Mini-Review
Animal models are useful to understand the myriad physiological effects of hypoxia. Such models attempt to recapitulate the hypoxemia of human disease in various ways. In this mini-review, we consider the various animal models which have been deployed to understand the effects of chronic hypoxia on pulmonary and systemic blood pressure, glucose and lipid metabolism, atherosclerosis, and stroke. Chronic sustained hypoxia (CSH)-a model of chronic lung or heart diseases in which hypoxemia may be longstanding and persistent, or of high altitude, in which effective atmospheric oxygen concentration is low-reliably induces pulmonary hypertension in rodents, and appears to have protective effects on glucose metabolism. Chronic intermittent hypoxia (CIH) has long been used as a model of obstructive sleep apnea (OSA), in which recurrent airway occlusion results in intermittent reductions in oxyhemoglobin saturations throughout the night. CIH was first shown to increase systemic blood pressure, but has also been associated with other maladaptive physiological changes, including glucose dysregulation, atherosclerosis, progression of nonalcoholic fatty liver disease, and endothelial dysfunction. However, models of CIH have generally been implemented so as to mimic severe human OSA, with comparatively less focus on milder hypoxic regimens. Here we discuss CSH and CIH conceptually, the effects of these stimuli, and limitations of the available data.
Combined intermittent and sustained hypoxia is a novel and deleterious cardio-metabolic phenotype
Study objectives: Chronic obstructive pulmonary disease and obstructive sleep apnea overlap syndrome is associated with excess mortality, and outcomes are related to the degree of hypoxemia. People at high altitude are susceptible to periodic breathing, and hypoxia at altitude is associated with cardio-metabolic dysfunction. Hypoxemia in these scenarios may be described as superimposed sustained plus intermittent hypoxia, or overlap hypoxia (OH), the effects of which have not been investigated. We aimed to characterize the cardio-metabolic consequences of OH in mice.
Methods: C57BL/6J mice were subjected to either sustained hypoxia (SH, FiO2=0.10), intermittent hypoxia (IH, FiO2=0.21 for 12 hours, and FiO2 oscillating between 0.21 and 0.06, 60 times/hour, for 12 hours), OH (FiO2=0.13 for 12 hours, and FiO2 oscillating between 0.13 and 0.06, 60 times/hour, for 12 hours), or room air (RA), n=8/group. Blood pressure and intraperitoneal glucose tolerance test were measured serially, and right ventricular systolic pressure (RVSP) was assessed.
Results: Systolic blood pressure transiently increased in IH and OH relative to SH and RA. RVSP did not increase in IH, but increased in SH and OH by 52% (p<0.001) and 20% (p=0.001). Glucose disposal worsened in IH and improved in SH, with no change in OH. Serum LDL and VLDL increased in OH and SH, but not in IH. Hepatic oxidative stress increased in all hypoxic groups, with the highest increase in OH.
Conclusions: Overlap hypoxia may represent a unique and deleterious cardio-metabolic stimulus, causing systemic and pulmonary hypertension, and without protective metabolic effects characteristic of sustained hypoxia.
Hepatocyte HIF-1 and Intermittent Hypoxia Independently Impact Liver Fibrosis in Murine NAFLD
Obstructive sleep apnea is associated with insulin resistance, lipid dysregulation, and hepatic steatosis and fibrosis in nonalcoholic fatty liver disease (NAFLD). We have previously shown that hepatocyte HIF-1 (hypoxia-inducible factor-1) mediates the development of liver fibrosis in a mouse model of NAFLD. We hypothesized that intermittent hypoxia (IH) modeling obstructive sleep apnea would worsen hepatic steatosis and fibrosis in murine NAFLD, via HIF-1. Mice with hepatocyte-specific deletion of Hif1a (Hif1a-/-hep) and wild-type (Hif1aF/F) controls were fed a high trans-fat diet to induce NAFLD with steatohepatitis. Half from each group were exposed to IH, and the other half were exposed to intermittent air. A glucose tolerance test was performed just prior to the end of the experiment. Mitochondrial efficiency was assessed in fresh liver tissue at the time of death. The hepatic malondialdehyde concentration and proinflammatory cytokine levels were assessed, and genes of collagen and fatty acid metabolism were examined. Hif1a-/-hep mice gained less weight than wild-type Hif1a mice (-2.3 g, P = 0.029). There was also a genotype-independent effect of IH on body weight, with less weight gain in mice exposed to IH (P = 0.003). Fasting glucose, homeostatic model assessment for insulin resistance, and glucose tolerance test results were all improved in Hif1a-/-hep mice. Liver collagen was increased in mice exposed to IH (P = 0.033) and was reduced in Hif1a-/-hep mice (P < 0.001), without any significant exposure/genotype interaction being demonstrated. Liver TNF-α and IL-1β were significantly increased in mice exposed to IH and were decreased in Hif1a-/-hep mice. We conclude that HIF-1 signaling worsens the metabolic profile and hastens NAFLD progression and that IH may worsen liver fibrosis. These effects are plausibly mediated by hepatic inflammatory stress.
Obstructive Sleep Apnea, Hypoxia, and Nonalcoholic Fatty Liver Disease
Recent studies have demonstrated that obstructive sleep apnea (OSA) is associated with the development and evolution of nonalcoholic fatty liver disease (NAFLD), independent of obesity or other shared risk factors. Like OSA, NAFLD is a prevalent disorder associated with major adverse health outcomes: Patients with NAFLD may develop cirrhosis, liver failure, and hepatocellular carcinoma. One major finding that has emerged from these studies is that the OSA-NAFLD association is related to the degree of nocturnal hypoxemia in OSA. Animal models have therefore largely focused on intermittent hypoxia, a key manifestation of OSA, to shed light on the mechanisms by which OSA may give rise to the complex metabolic disturbances that are seen in NAFLD. Intermittent hypoxia leads to tissue hypoxia and can result in oxidative stress, mitochondrial dysfunction, inflammation, and overactivation of the sympathetic nervous system, among many other maladaptive effects. In such models, intermittent hypoxia has been shown to cause insulin resistance, dysfunction of key steps in hepatic lipid metabolism, atherosclerosis, and hepatic steatosis and fibrosis, each of which is pertinent to the development and/or progression of NAFLD. However, many intriguing questions remain unanswered: Principally, how aggressively should the clinician screen for NAFLD in patients with OSA, and vice versa? In this review, we attempt to apply the best evidence from animal and human studies to highlight the relationship between these two disorders and to advocate for further trials aimed at defining these relationships more precisely.
Hepatocyte Hypoxia Inducible Factor-1 Mediates the Development of Liver Fibrosis in a Mouse Model of Nonalcoholic Fatty Liver Disease
Background: Obstructive sleep apnea (OSA) is associated with the progression of non-alcoholic fatty liver disease (NAFLD) to steatohepatitis and fibrosis. This progression correlates with the severity of OSA-associated hypoxia. In mice with diet induced obesity, hepatic steatosis leads to liver tissue hypoxia, which worsens with exposure to intermittent hypoxia. Emerging data has implicated hepatocyte cell signaling as an important factor in hepatic fibrogenesis. We hypothesized that hepatocyte specific knockout of the oxygen sensing α subunit of hypoxia inducible factor-1 (HIF-1), a master regulator of the global response to hypoxia, may be protective against the development of liver fibrosis.
Methods: Wild-type mice and mice with hepatocyte-specific HIF-1α knockout (Hif1a-/-hep) were fed a high trans-fat diet for six months, as a model of NAFLD. Hepatic fibrosis was evaluated by Sirius red stain and hydroxyproline assay. Liver enzymes, fasting insulin, and hepatic triglyceride content were also assessed. Hepatocytes were isolated from Hif1a-/-hep mice and wild-type controls and were exposed to sustained hypoxia (1% O2) or normoxia (16% O2) for 24 hours. The culture media was used to reconstitute type I collagen and the resulting matrices were examined for collagen cross-linking.
Results: Wild-type mice on a high trans-fat diet had 80% more hepatic collagen than Hif1a-/-hep mice (2.21 μg collagen/mg liver tissue, versus 1.23 μg collagen/mg liver tissue, p = 0.03), which was confirmed by Sirius red staining. Body weight, liver weight, mean hepatic triglyceride content, and fasting insulin were similar between groups. Culture media from wild-type mouse hepatocytes exposed to hypoxia allowed for avid collagen cross-linking, but very little cross-linking was seen when hepatocytes were exposed to normoxia, or when hepatocytes from Hif1a-/-hep mice were used in hypoxia or normoxia.
Conclusions: Hepatocyte HIF-1 mediates an increase in liver fibrosis in a mouse model of NAFLD, perhaps due to liver tissue hypoxia in hepatic steatosis. HIF-1 is necessary for collagen cross-linking in an in vitro model of fibrosis.