Hypoxia, HIF-1, and the pathophysiology of common human diseases.

Hypoxia plays a fundamental role in the pathophysiology of common causes of mortality, including ischemic heart disease, stroke, cancer, chronic lung disease, and congestive heart failure. In these disease states, hypoxia induces changes in gene expression in target organs that either fail to result in adequate adaptation or directly contribute to disease pathogenesis. Hypoxia-inducible factor 1 (HIF-1) is a transcriptional activator that is expressed in response to cellular hypoxia and mediates multiple cellular and systemic homeostatic responses to hypoxia. Recent studies have provided evidence that important pathophysiological responses to hypoxia in pulmonary hypertension, myocardial ischemia, and cancer are mediated by HIF-1. Pharmacologic and gene therapy strategies designed to modulate HIF-1 activity may represent a novel and effective therapeutic approach to these common disorders.

Regulation of cardiovascular development and physiology by hypoxia-inducible factor 1.

Hypoxia is an essential pathophysiologic component of ischemic cardiovascular disease. A better understanding of the molecular mechanisms underlying adaptive responses to hypoxia may lead to novel therapeutic strategies. Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric basic-helix-loop-helix-PAS domain transcription factor that mediates changes in gene expression in response to changes in O2 concentration. Genes that are transcriptionally activated by HIF-1 in hypoxic cells encode proteins that increase O2 delivery or allow metabolic adaptation to limited O2 availability. HIF-1 target genes include those encoding vascular endothelial growth factor (VEGF), erythropoietin, glucose transporters, and glycolytic enzymes. In anemic fetal sheep, increased myocardial vascularization was associated with concomitant increases in the expression of HIF-1 and VEGF. Expression of HIF-1 target genes was not induced by hypoxia in embryonic stem cells lacking expression of the O2-regulated HIF-1 alpha subunit. Mouse embryos lacking HIF-1 alpha expression arrested in their development by E9.0 and died by E10.5 with cardiovascular malformations and massive cell death throughout the embryo. These studies indicate that HIF-1 functions as a master regulator of O2 homeostasis that controls the establishment of essential physiologic systems during embryogenesis as well as their subsequent utilization during fetal and postnatal life.

Defective vascularization of HIF-1alpha-null embryos is not associated with VEGF deficiency but with mesenchymal cell death.

Hypoxia-inducible factor 1 (HIF-1) is a dimeric transcription factor composed of HIF-1alpha and HIF-1beta subunits that plays an essential role in mammalian O2 homeostasis. In Hif1a-/- knockout mice, complete deficiency of HIF-1alpha resulted in cardiac and vascular malformations and embryonic lethality at E10.5. Between E8. 75 and E9.25 striking vascular regression and abnormal remodeling occurred in the cephalic region concomitant with marked mesenchymal cell death. Similar vascular defects were observed in HIF-1alpha- and VEGF-deficient embryos and VEGF mRNA expression was not induced by hypoxia in Hif1a-/- embryonic stem cells. Surprisingly, Hif1a-/- embryos demonstrated increased VEGF mRNA expression compared to wild-type embryos. In tissue culture cells, VEGF mRNA expression was induced by glucose deprivation independent of HIF-1alpha, providing a mechanism for increased VEGF mRNA expression in Hif1a-/- embryos, in which absence of adequate tissue perfusion resulted in both O2 and glucose deprivation. Rather than being associated with VEGF deficiency, the vascular defects in Hif1a-/- embryos were spatially correlated with cell death, the onset of which preceded vascular regression.

Cellular and developmental control of O2 homeostasis by hypoxia-inducible factor 1 alpha.

Hypoxia is an essential developmental and physiological stimulus that plays a key role in the pathophysiology of cancer, heart attack, stroke, and other major causes of mortality. Hypoxia-inducible factor 1 (HIF-1) is the only known mammalian transcription factor expressed uniquely in response to physiologically relevant levels of hypoxia. We now report that in Hif1a-/- embryonic stem cells that did not express the O2-regulated HIF-1alpha subunit, levels of mRNAs encoding glucose transporters and glycolytic enzymes were reduced, and cellular proliferation was impaired. Vascular endothelial growth factor mRNA expression was also markedly decreased in hypoxic Hif1a-/- embryonic stem cells and cystic embryoid bodies. Complete deficiency of HIF-1alpha resulted in developmental arrest and lethality by E11 of Hif1a-/- embryos that manifested neural tube defects, cardiovascular malformations, and marked cell death within the cephalic mesenchyme. In Hif1a+/+ embryos, HIF-1alpha expression increased between E8.5 and E9.5, coincident with the onset of developmental defects and cell death in Hif1a-/- embryos. These results demonstrate that HIF-1alpha is a master regulator of cellular and developmental O2 homeostasis.

Dose-related growth deficits in LS but not SS mice prenatally exposed to alcohol.

Genetically based alcohol sensitivity may influence the severity of alcohol-related birth defects. To examine this question, measures of growth and survival were examined in offspring of the alcohol sensitive Long-Sleep (LS) and alcohol-resistant Short-Sleep (SS) mouse lines following prenatal ethanol exposure. Pregnant LS and SS mice received an ethanol dose of either 6 or 8 g/kg/day from days 7 through 18 of pregnancy. Control groups received a maltose-dextran solution made isocaloric to the 8 g/kg/day dose. Ethanol and maltose-dextrin solutions were administered as split doses, 6 h apart, via gavage. Nonintubated lab chow control groups were also included for both mouse lines. Offspring were fostered at birth to lactating mice of an outbred stock. Pregnancy was longer for ethanol-treated LS dams compared to maltose-dextrin and lab chow LS control groups, whereas pregnancy length for ethanol-treated SS dams was similar to SS controls. Prenatal ethanol exposure resulted in dose-related growth deficits in LS but not in SS litters. Line differences in postnatal growth deficits in response to prenatal alcohol exposure suggest maternal or fetal alcohol sensitivity influence alcohol-related birth defects.

Effects of cocaine administration during early organogenesis on prenatal development and postnatal growth in mice.

Cocaine use has been associated with adverse developmental effects in humans. However, clinical reports both confirm and deny an association between cocaine use and malformations. Similarly, differences in species and strain, as well as route and timing of cocaine administration, have added to the difficulties in determining the teratogenicity of cocaine in animal models. This study was undertaken to compare the effects of dose, route, and timing of cocaine administration in ICR mice during early organogenesis. A single intraperitoneal (ip) administration of cocaine ( > or = 60 mg/kg) on Day 9 of gestation (plug day = 1) produced maternal lethality. The predominant developmental effect of cocaine administration was an increase in the percentage of litters exhibiting an enlarged renal pelvis. Despite a high incidence of affected pups at these doses, the enlargement was not severe. These results, in agreement with previous reports, provide further evidence that the developing urogenital system is sensitive to cocaine administration. When cocaine was administered using a subcutaneous route, pup weights were greater and the incidence of enlarged renal pelvis was lower than when an ip route was used. To better mimic human binge cocaine abuse, the toxicity of a "split dose" was determined. A 60 mg/kg dose was administered using one administration of 60 mg/kg, two treatments of 30 mg/kg, or three administrations of 20 mg/kg with 1 hr separating the treatments. The incidence of enlarged renal pelvis was similar when cocaine was administered as one or two but was decreased when cocaine was administered as three treatments. Both the route and split-dose studies suggest that high-peak serum concentrations are required to perturb development. There were no differences in the incidence or severity of enlarged renal pelvis when cocaine was administered on Day 8, 9, or 10 or on all 3 days of gestation. This suggested that the increase in enlarged renal pelvis may not be a specific teratogenic effect of cocaine administration but may be a delay of normal development induced by cocaine exposure during this early period of organogenesis. To address this hypothesis, cocaine was administered on Day 9 using an ip route and the pups were allowed to be naturally born. In pups whose mothers received cocaine there was an increase in postnatal deaths and a trend toward a reduction in pup body weight/litter at Postnatal Day 21. However, when renal morphology was assessed on Postnatal Day 21 no abnormal kidneys were seen. This supports the hypothesis that enlarged renal pelvis produced by cocaine administration during early organogenesis represents a developmental delay and not a persistent teratogenic defect. These studies suggest that high peak cocaine concentrations are required to delay normal kidney morphogenesis in mice.

Ethanol-induced teratogenesis: free radical damage as a possible mechanism.

To investigate the possibility of a free radical mechanism for ethanol-induced teratogenesis, gestational day 8 mouse embryos were exposed for 6 hr in whole embryo culture to a teratogenic dosage of ethanol alone (500 mg%) or in conjunction with an antioxidant, superoxide dismutase (SOD; 300 U/ml). For subsequent analysis, some embryos were examined at the end of this 6-hr period, while others were removed to control medium and cultured for an additional time period. Ethanol exposure resulted in increased superoxide anion generation and increased lipid peroxidation (as noted 6 hr after initial ethanol exposure) and in excessive cell death (as noted 12 hr after initial exposure) in the embryos. Following a total of 36 hr in culture, a high incidence of malformation, including failure of the anterior neural tube to close in 63% of the ethanol-exposed embryos, was noted. The ethanol-induced superoxide anion generation, lipid peroxidation, excessive cell death, and dysmorphogenesis were diminished in embryos co-treated with SOD, suggesting that the teratogenicity of ethanol is mediated, at least in part, by free radical damage.

Pathogenesis of ethanol-induced limb reduction defects in mice.

Acute administration of dosages of 2.5, 2.8, or 2.9 g/kg of ethanol to pregnant C57BL/6J mice on gestational day 9 1/4 resulted in major malformations of the forelimb including postaxial ectrodactyly, preaxial syndactyly, and reduction defects involving intermediate digits. The incidence and severity of these defects was positively correlated with dosage. Sidedness of the defects was also dose-dependent. In affected embryos, excessive amounts of cell death were notable within 5-9 hr of treatment initiation in selected cell populations. Cell death was primarily distributed in two regions of the developing limb bud--a ventrodistal ectodermal cell population (apical ectodermal ridge) and a proximal mesenchymal cell population. The patterns of cell death observed appear to be pathogenically related to the limb defects noted at later stages. In particular, it would appear that the deficiencies in the apical ectodermal ridge resulting from ethanol-induced cell death can account for virtually all the subsequent limb defects.

Experimental fetal alcohol syndrome: proposed pathogenic basis for a variety of associated facial and brain anomalies.

Acute teratogenic exposure of C57Bl/6J mouse embryos to ethanol in vivo results, within 12 hours of initial insult, in excessive cell death in selected cell populations. The patterns of excessive cell death observed following exposure of gestational day 8 embryos (late presomite--approximately 5 somite pair stages) vary somewhat temporospatially, but primarily involve the cell populations at the rim of the anterior neural plate. The cell death patterns appear to be pathogenically correlated with subsequently observed malformations including exencephaly (anencephaly), arhinencephaly, pituitary dysplasia, bilateral or unilateral cleft lip, maxillary hypoplasia, and median facial deficiencies and clefts. The association of these brain and facial malformations in this model, and perhaps in humans, may be accounted for by early insult to the selected cell populations identified in the current investigation.

Patterns of ethanol-induced cell death in the developing nervous system of mice; neural fold states through the time of anterior neural tube closure.

Vital staining and routine histological analyses of mouse embryos 12 h after acute maternal ethanol administration (2.9 g/kg) illustrated that selected neuronal cell populations are killed. At the time of treatment, embryos had 5-15 somite pairs, corresponding to the developmental stages occurring in humans during the fourth week of post-fertilization; i.e. when neural folds are present and neural tube fusion begins. Affected cell populations in embryos having 6-26 somite pairs (up to the stage of anterior neuropore closure) were in discrete locations in the alar and basal plates of the rhombencephalon, in the otic placode/vesicle, and in the regions of the epibranchial placodes, olfactory placodes and trigeminal ganglion. The potential basis for the vulnerability of these cell populations to ethanol-induced cell death is discussed. Our understanding of the scope of ethanol-induced CNS damage is dependent upon further defining ethanol-sensitive cell populations at all stages of CNS development.

Developmental thermoregulatory deficits in prenatal ethanol exposed long- and short-sleep mice.

Sensitivity to alcohol may influence the severity of prenatal alcohol effects. To examine this hypothesis, the ontogeny of thermoregulation was measured in prenatal ethanol exposed offspring of mice selected for differences in alcohol sensitivity. Pregnant long-sleep (LS) and short-sleep (SS) mice were exposed to 3 or 4 g/kg ethanol or an isocaloric amount of maltose-dextrin twice per day from day 7 through 18 of pregnancy. Doses were given six hours apart via gavage. Nonintubated lab chow controls were included for both genotypes. Offspring were fostered at birth to lactating mice of an outbred strain. Offspring temperatures were measured at 0, 60, and 120 min away from the nest on alternating days from 7 through 21 days of age. LS and SS offspring prenatally exposed to the high ethanol dose showed lower temperatures at the 60 and 120 min time points on each day of testing compared to all other treatment groups. Temperatures of offspring prenatally exposed to the low ethanol dose did not differ from controls. These results suggest a relatively steep dose-response curve for thermoregulatory deficits in LS and SS offspring prenatally exposed to alcohol. Genetically-based alcohol sensitivity did not influence the effects of prenatal alcohol exposure on this response.

Alcohol-related birth defects in long- and short-sleep mice: postnatal litter mortality.

Alcohol sensitivity may influence the severity of alcohol-related birth defects (ARBD). To examine this hypothesis, pregnancy outcome and offspring development were examined in alcohol-sensitive Long-Sleep (LS) mice and alcohol-resistant Short-Sleep (SS) mice following prenatal ethanol exposure. Dams were intragastrically intubated twice per day (6 hr apart) with either 4.5 g/kg (20% w/v) ethanol (E) or an isocaloric amount of sucrose (S) on days 7 through 18 of pregnancy. An untreated control group (C) was maintained for each line. Results showed litter mortality at 10 days of age was greater for LS-E litters compared to both LS-S and LS-C litters. Litter mortality for SS-E litters did not differ from either SS-S or SS-C litters. Maternal weight gain, blood ethanol levels, and birth weight deficits were similar for ethanol-exposed LS and SS groups. These results suggest genetically based alcohol sensitivity influences the severity of ARBD.

Alterations in gait following ethanol exposure during the brain growth spurt in rats.

Walking patterns were assessed in rats that had been exposed to alcohol neonatally during a period encompassing the brain growth spurt. Rat pups were exposed via an artificial rearing technique to either a 2.50% (w/v) or 2.15% (w/v) EtOH-milk formula on Days 26-32 postconception. An artificially reared control group and a suckle-control group were also included in the experiment. Gait patterns were assessed in animals from each of the neonatal treatment groups at 43, 67, and 87 days postconception. No differences in gait patterns were evident on Day 43 postconception; however, on Days 67 and 87 animals exposed to alcohol during the neonatal period displayed an abnormal gait. These animals had a shortened stride length and an increased angle of placement of the hindfeet relative to artificially reared and suckle-control animals. The altered gait pattern may be the result of alcohol-induced hippocampal and cerebellar damage during the brain growth spurt.

Neonatal ethanol exposure: functional alterations associated with cerebellar growth retardation.

The effects of alcohol exposure during the brain growth spurt on development and on behavioral assessments of functional alterations in the cerebellum were examined in the rat. Rat pups were exposed via an artificial rearing technique to either a 2.50% w/v or 2.15% w/v EtOH-milk formula during a period encompassing the brain growth spurt. An artificially reared control group and a suckle control group were also included. Peak blood alcohol concentrations for animals in the high and low dose alcohol exposure groups were approximately 300 mg/dl and 180 mg/dl, respectively. Reductions in brain minus cerebellum to body weight (BR-C/BD) and cerebellum to body weight (C/BD) ratios were noted in animals from each of the alcohol-treated groups. Some catch-up growth in terms of brain mass was noted in animals from each of the alcohol-exposed groups. Animals exposed to alcohol during the neonatal period displayed deficits on several tests of balance and motor ability. Alcohol-exposed animals performed more poorly than controls when traversing two parallel horizontal rods and on tests of hindlimb and head elevation. No differences were noted in the ability to remain on a rotating drum. These results suggest that some of the behavioral consequences of neonatal ethanol exposure might be due to ethanol's actions on the cerebellum.

Ethanol teratogenesis in selectivity bred long-sleep and short-sleep mice: a comparison to inbred C57BL/6J mice.

Sensitivity to alcohol may influence the severity of ethanol teratogenesis. To examine this hypothesis, the teratogenic effects of ethanol were compared in Long-Sleep (LS) and Short-Sleep (SS) mice, selectively bred for differences in ethanol-induced narcosis. Inbred C57BL/6J (B6) mice were included to confirm previously reported teratogenic effects using our own treatment regimen and standard assessment technique. Intragastric administration of ethanol (5.8 g/kg) on Days 9 and 10 of pregnancy resulted in growth retardation and an increase in prenatal mortality in LS litters but not in SS litters. Therefore, alcohol sensitivity plays a role in the severity of prenatal alcohol effects. B6 mice showed more ethanol teratogenicity than either LS or SS mice, even though maternal blood ethanol levels were similar across genotypes. This result suggests genetic variations other than alcohol sensitivity also influence ethanol teratogenesis.

Ethanol teratogenesis in mice selected for differences in alcohol sensitivity.

Long-Sleep (LS) and Short-Sleep (SS) mice, selectively bred for differences in ethanol-induced narcosis, were administered ethanol (2.9, 4.0, 4.5, or 5.0 g/kg) twice per day during the period of organogenesis. On gestation day 18, the dams were sacrificed and the uterine horns were examined for live, dead, and resorbed fetuses. Live fetuses were weighed and assessed for either skeletal or soft tissue anomalies. The 5.8 g/kg/day dose had no effect on prenatal mortality, litter size, body weight, or number of physical anomalies in either line. However, the alcohol-sensitive LS mice exposed to ethanol doses of 8.0 g/kg/day or more evidenced decreased body weights while weights for the alcohol-insensitive SS mice differed from controls at only the highest dose tested. The incidence of skeletal variants was increased in the LS mice exposed to the 10 g/kg/day ethanol dose. These results indicate genetically-mediated alcohol sensitivity increases susceptibility to some of the fetotoxic effects of in utero alcohol exposure.

The effects of prenatal alcohol exposure on odor associative learning in rats.

Alcohol was administered to pregnant females via a liquid diet that contained either 35% ethanol-derived calories (35% EDC) or 0% EDC on gestation days 6-20. An ad lib lab chow group (LC) was also included. In Experiment 1, odor-aversion learning was examined in 10-day-old offspring. While both the 0% EDC and LC groups displayed odor aversions, the 35% EDC offspring did not. In Experiment 2, learning was assessed in an appetitive paradigm in three-day-old offspring. Once again, the 35% EDC offspring showed no evidence of learning. Experiment 3 examined odor-aversion learning in adults. Both alcohol-exposed offspring and controls learned the odor association equally well. These findings suggest that odor associative learning is a sensitive indicator for alcohol-related learning deficits in rat pups although these deficits may dissipate as the offspring matures. Since odor associations play a critical role in neonatal behaviors, these deficits may help explain other behavioral anomalies noted following prenatal alcohol exposure.