Induction of T-type Calcium Channel Gene Expression by Chronic Hypoxia

Abstract

Cellular responses to hypoxia can be acute or chronic. Acute responses mainly depend on oxygen-sensitive ion channels, whereas chronic responses rely on the hypoxia-inducible transcription factors (HIFs), which upregulate the expression of enzymes, transporters, and growth factors. It is unknown whether the expression of genes coding for ion channels is also influenced by hypoxia. We report here that the ␣ 1H gene of T-type voltage-gated calcium channels is highly induced by lowering oxygen tension in PC12 cells. Accumulation of ␣ 1H mRNA in response to hypoxia is time-and dose-dependent and paralleled by an increase in the density of Ttype calcium channel current recorded in patch clamped cells. HIF appears to be involved in the response to hypoxia, since cobalt chloride, desferrioxamine, and dimethyloxalylglycine, compounds that mimic HIF-regulated gene expression, replicate the hypoxic effect. Moreover, functional inhibition of HIF-2␣ protein accumulation using antisense HIF-2␣ oligonucleotides reverses the effect of hypoxia on T-type Ca 2؉ channel expression. Importantly, regulation by oxygen tension is specific for T-type calcium channels, since it is not observed with the L-, N-, and P/Q-channel types. These findings show for the first time that hypoxia induces an ion channel gene via a HIF-dependent mechanism and define a new role for the T-type calcium channels as regulators of cellular excitability and calcium influx under chronic hypoxia.Maintaining optimal oxygen homeostasis is of paramount importance for cells. Reductions of oxygen supply trigger cell adaptive responses that minimize the deleterious effects of hypoxia. Cellular responses to hypoxia can be acute, occurring over a time scale of seconds or minutes, or chronic, with time courses of hours to days (1-5). The major effectors of the acute cellular responses to hypoxia are oxygen-sensitive ion channels. These channels mediate the fast adaptive changes in cell excitability, contractility, and secretory activity that occur in response to low ambient oxygen tension (PO 2 ) (1, 5). On the other hand, chronic cellular responses to hypoxia, studied in great detail in the past few years, are mediated by ubiquitously expressed hypoxia-inducible transcription factors (HIF-1␣ 1 and isoforms). Stabilization and transcriptional activity of HIF depend on oxygen-regulated hydroxylases (6 -8). Hypoxia-inducible factors regulate the expression of a wide repertoire of oxygen-sensitive genes with roles in diverse cellular functions such as angiogenesis, red blood cell production, glucose and energy metabolism, apoptosis, and cell proliferation (1-5).Despite progress in the understanding of the role of ion channels and gene expression in the cellular responses to hypoxia, long term regulation of ion channel expression by maintained low PO 2 is poorly known. There are previous reports showing that prolonged hypoxia down-regulates various voltage-gated K ϩ (Kv) channel genes in pulmonary artery smooth muscle cells (9), and the opposite has been observed with the ...

Decreased Expression of Maxi-K + Channel β 1 -Subunit and Altered Vasoregulation in Hypoxia

“…Altogether, these results support the view that reduced expression of the maxi-K ϩ channel ␤ 1 -subunit contributes to vascular pathophysiology in hypoxia. However, it is important to keep in mind that we have studied downregulation of the maxi-K ϩ channel ␤ 1 -subunit within the first 24 to 48 hours of exposure to hypoxia (a protocol normally used for the study of hypoxia-regulated genes [25][26][27] ), whereas the establishment of chronic hypertension in humans results from exposures to hypoxia lasting weeks or months.…”

“…We assayed the effect of dimethyloxalylglycine (DMOG), a competitive inhibitor of prolyl hydroxylases that stabilizes HIF in normoxia. 5,26,27 DMOG induced HO-1 by approximately 2.5-fold but did not alter the mRNA levels of the ␤ 1 -subunit. However, downregulation of the ␤ 1 -subunit was more pronounced in cells exposed sequentially to hypoxia (6 hours, 1% O 2 ), reoxygenation (18 hours, 20% O 2 ), and hypoxia (6 hours, 1% O 2 ) compared with those exposed to sustained hypoxia (30 hours, 1% O 2 ) (Figure 2).…”

Cellular and Molecular Mechanisms Associated with Carotid Body Adaptations to Chronic Hypoxia

“…Based on the analysis of the rate of mRNA production and stability, these authors concluded that posttranslational mechanisms contribute to the upregulation of the ␤-subunit. Using a clonal cell line (PC12 cells), del Toro et al (2003) examined the effects of chronic hypoxia on T-type Ca 2ϩ channel expression. They observed robust upregulation of the ␣ 1H subunit of the T-type Ca 2ϩ channel and increased current density of this channel in response to prolonged hypoxia.…”

Chronic continuous hypoxia decreases the expression of SLC4A7 (NBCn1) and SLC4A10 (NCBE) in mouse brain

“…Chronic hypoxia induces a decrease in GLUT1 expression in the adult rat brain but an increase in the fetal as well as in the developing brain (73). Chronic hypoxia also raises the mRNA and channel currents of T-type voltage-gated Ca 2ϩ channels but not of other types of Ca ϩϩ channels in pheochromocytoma-derived PC12 cells (21).…”