Hypoxia Up-Regulates Expression of Hemoglobin in Alveolar Epithelial Cells

Abstract

Alveolar epithelial cells are directly exposed to acute and chronic fluctuations in alveolar oxygen tension. Previously, we found that the oxygen-binding protein hemoglobin is expressed in alveolar Type II cells (ATII). Here, we report that ATII cells also express a number of highly specific transcription factors and other genes normally associated with hemoglobin biosynthesis in erythroid precursors. Because hypoxia-inducible factors (HIFs) were shown to play a role in hypoxia-induced changes in ATII homeostasis, we hypothesized that the hypoxia-induced increase in intracellular HIF exerts a concomitant effect on ATII hemoglobin expression. Treatment of cells from the ATII-like immortalized mouse lung epithelial cell line-15 (MLE-15) with hypoxia for 20 hours resulted in dramatic increases in cellular levels of HIF-2a protein and parallel significant increases in hemoglobin messenger RNA (mRNA) and protein expression, as compared with that of control cells cultured in normoxia. Significant increases in the mRNA of globin-associated transcription factors were also observed, and RNA interference (RNAi) experiments demonstrated that the expression of hemoglobin is at least partially dependent on the cellular levels of globin-associated transcription factor isoform 1 (GATA-1). Conversely, levels of prosurfactant proteins B and C significantly decreased in the same cells after exposure to hypoxia. The treatment of MLE-15 cells cultured in normoxia with prolyl 4-hydroxylase inhibitors, which mimic the effects of hypoxia, resulted in increases of hemoglobin and decreases of surfactant proteins. Taken together, these results suggest a relationship between hypoxia, HIFs, and the expression of hemoglobin, and imply that hemoglobin may be involved in the oxygen-sensing pathway in alveolar epithelial cells.Keywords: hemoglobin; alveolar epithelial cells; hypoxia; hypoxiainducible factor Pulmonary hypoxia occurs under both physiologic and pathologic conditions. In fact, a hypoxic environment is necessary for proper embryonic lung development, by promoting the formation of microvasculature and epithelial branching morphogenesis (the average fetal blood O 2 fraction is z2-5%) (1-3). However, postnatal decreases in alveolar oxygen tension as a result of pulmonary disease disrupt alveolar homeostasis. High-altitude ascent, pathologic conditions resulting in inadequate respiration, pulmonary edema after acute lung injury, or congestive heart failure may all result in decreased oxygen tension. Alveolar Type II (ATII) cells represent approximately two thirds of epithelial cell numbers, and are of special clinical interest because of their role in the production, secretion, and recycling of pulmonary surfactant (4). In addition, ATII cells differentiate into Type I (ATI ) cells upon epithelial injury, and also act to clear fluid from the alveolar space. Although numerous studies evaluated the effects of hypoxia on the pulmonary endothelium, few sought to identify hypoxia-regulated genes in alveolar epithelial cells.Our previous stu...

Molecular and ultrastuctural changes of rat pre-implantation embryos during two-cell developmental arrest

“…Increase in hemoglobin mRNAs in invivo embryos suggest that oxygen level in culture conditions may affect genes that are sensitive to oxygen levels. It has been shown that hemoglobin may be involved in oxygen or nitric oxide sensing [35] and hemoglobin mRNA and protein were up-regulated during hypoxia [36]. Thus, lower levels hemoglobin mRNAs in cultured embryos in our study suggest that decrease in hemoglobin mRNAs may be the initial response against atmospheric oxygen levels in in-vitro culture.…”

Upregulation and Mitochondrial Sequestration of Hemoglobin Occur in Circulating Leukocytes during Critical Illness, Conferring a Cytoprotective Phenotype

“…For instance, hypoxia upregulates hemoglobin expression in alveolar epithelial cells in vitro (22) and bacterial lipopolysaccharide upregulates the β hemoglobin subunit in murine macrophages (23). H 2 O 2 increases Hb-α1 and Hb-β expression in HepG2 and HEK293 cells, which, in turn, protects the cells from oxidative stress (24).…”

“…; n = 3; *p < 0.05; **p < 0.01. stress, the rate of the gaseous transmitters produced, as well as many additional factors. Based on several sets of independent investigations, it appears that when oxidative stress is relatively low, hemoglobin (and its degradation products, for example, heme) increases oxidative stress, and scavenges physiologically necessary amounts of NO, but when the degree of oxidative stress is high, hemoglobin tends to protect from cellular damage (22)(23)(24)(25)(26)(27)31). One also has to keep in mind that hemoglobin decomposes into multiple products, including heme (which increases oxidative stress, but also acts as a substrate of the gasotransmitter carbon monoxide, produced by heme oxygenase), while other degradation products (for example, bilirubin) have significant antioxidant effects (31).…”

“…It remains to be elucidated if similar reactive responses can also be elicited by other stimuli (for example, hypoxia, oxidative stress, NO, CO and others) in PBMCs. In this context, it is worth mentioning that, in other cell types, Hb upregulation has been demonstrated in response to bacterial lipopolysaccharide stimulation in murine monocytemacrophages (23), in response to hypoxia in epithelial cells (22) or in response to oxidative stress in hepatocytes (24). (d) What is the exact functional role of Hbs in the mitochondria, and how does Hbs translocation occurs into the mitochondria?…”

Maternal administration of nanomaterials elicits hemoglobin upregulation in the neonatal brain of non-human primates

“…41 No. 2 in mRNA and protein in cells such as alveolar epithelial cells (Grek et al, 2011). In this condition, hemoglobin upregulation is considered a response to activation of hypoxia inducible factor (HIF) (Grek et al, 2011).…”