Cellular responses to oxygen are increasingly recognized as critical in normal development and physiology, and are implicated in pathological processes. Many of these responses are mediated by the transcription factors HIF-1 and HIF-2. Their regulation occurs through oxygen-dependent proteolysis of the alpha subunits HIF-1alpha and HIF-2alpha, respectively. Both are stabilized in cell lines exposed to hypoxia, and recently HIF-1alpha was reported to be widely expressed in vivo. In contrast, regulation and sites of HIF-2alpha expression in vivo are unknown, although a specific role in endothelium was suggested. We therefore analyzed HIF-2alpha expression in control and hypoxic rats. Although HIF-2alpha was not detectable under baseline conditions, marked hypoxic induction occurred in all organs investigated, including brain, heart, lung, kidney, liver, pancreas, and intestine. Time course and amplitude of induction varied between organs. Immunohistochemistry revealed nuclear accumulation in distinct cell populations of each tissue, which were exclusively non-parenchymal in some organs (kidney, pancreas, and brain), predominantly parenchymal in others (liver and intestine) or equally distributed (myocardium). These data indicate that HIF-2 plays an important role in the transcriptional response to hypoxia in vivo, which is not confined to the vasculature and is complementary to rather than redundant with HIF-1.

Abstract. Oxygen tensions in the kidney are heterogeneous, and their changes presumably play an important role in renal physiologic and pathophysiologic processes. A family of hypoxia-inducible transcription factors (HIF) have been identified as mediators of transcriptional responses to hypoxia, which include the regulation of erythropoietin, metabolic adaptation, vascular tone, and neoangiogenesis. In vitro, the oxygen-regulated subunits HIF-1␣ and -2␣ are expressed in inverse relationship to oxygen tensions in every cell line investigated to date. The characteristics and functional significance of the HIF response in vivo are largely unknown. Highamplification immunohistochemical analyses were used to study the expression of HIF-1␣ and -2␣ in kidneys of rats exposed to systemic hypoxia bleeding anemia, functional anemia (0.1% carbon monoxide), renal ischemia, or cobaltous chloride (which is known to mimic hypoxia). These treatments led to marked nuclear accumulation of HIF-1␣ and -2␣ in different renal cell populations. HIF-1␣ was mainly induced in tubular cells, including proximal segments with exposure to anemia/carbon monoxide, in distal segments with cobaltous chloride treatment, and in connecting tubules and collecting ducts with all stimuli. Staining for HIF-1␣ colocalized with inducible expression of the target genes heme oxygenase-1 and glucose transporter-1. HIF-2␣ was not expressed in tubular cells but was expressed in endothelial cells of a small subset of glomeruli and in peritubular endothelial cells and fibroblasts. The kidney demonstrates a marked potential for upregulation of HIF, but accumulation of HIF-1␣ and HIF-2␣ is selective with respect to cell type, kidney zone, and experimental conditions, with the expression patterns partly matching known oxygen profiles. The expression of HIF-2␣ in peritubular fibroblasts suggests a role in erythropoietin regulation.Sufficient oxygenation is a prerequisite for organ function. However, oxygen delivery to organs and tissue oxygen tensions within organs vary considerably. The kidney is characterized by an interesting paradox with respect to its oxygen supply. Although blood flow is high in relation to organ weight and the arteriovenous oxygen difference is small, shunt diffusion of oxygen and heterogeneous utilization lead to marked oxygen gradients (1,2). Oxygen supply to the renal medulla barely exceeds demand, and medullary oxygen tensions are approximately 10 mmHg (3-6). Cortical oxygen tensions are more heterogeneous but are also frequently less than the venous oxygen tensions (4,6 -8).The effects of oxygen on cellular functions of the kidney are poorly understood. High rates of oxygen consumption in the proximal tubule and thick ascending limb, together with limited oxygen supply, are thought to be responsible for the high sensitivity to ischemic injury (2,9,10). A physiologic function directly related to renal oxygen tensions is the production of erythropoietin (EPO) by peritubular cortical fibroblasts (11-13). Regulation of EPO occurs at the mRN...

Regular use of sunscreens, as part of a consequent UV-protection strategy, may prevent the development of further AK and invasive SCC and, to a lesser degree, BCC in immune-compromised organ transplant recipients.

Hypoxia-inducible transcription factors (HIF) mediate complex adaptations to reduced oxygen supply, including neoangiogenesis. Regulation of HIF occurs mainly through oxygen-dependent destruction of its alpha subunit. In the presence of oxygen, two HIFalpha prolyl residues undergo enzymatic hydroxylation, which is required for its proteasomal degradation. We therefore tested whether pharmacological activation of HIFalpha by hydroxylase inhibitors may provide a novel therapeutic strategy for the treatment of ischemic diseases. Three distinct prolyl 4-hydroxylase inhibitors-l-mimosine (L-Mim), ethyl 3,4-dihydroxybenzoate (3,4-DHB), and 6-chlor-3-hydroxychinolin-2-carbonic acid-N-carboxymethylamid (S956711)-demonstrated similar effects to hypoxia (0.5% O2) by inducing HIFalpha protein in human and rodent cells. L-Mim, S956711, and, less effectively, 3,4-DHB also induced HIF target genes in cultured cells, including glucose transporter 1 and vascular endothelial growth factor, as well as HIF-dependent reporter gene expression. Systemic administration of L-Mim and S956711 in rats led to HIFalpha induction in the kidney. In a sponge model for angiogenesis, repeated local injection of the inhibitors strongly increased invasion of highly vascularized tissue into the sponge centers. In conclusion, structurally distinct inhibitors of prolyl hydroxylation are capable of inducing HIFalpha and HIF target genes in vitro and in vivo and induce adaptive responses to hypoxia, including angiogenesis.

Despite a high overall oxygen supply, the tissue oxygen tensions in the kidney are comparatively low and render the kidneys prone to hypoxic injury. However, the role of hypoxia in the pathogenesis of different types of renal disease remains incompletely understood. The importance of hypoxic cell injury is most obvious in renal vascular disease, in which occlusion of the renal artery or one of its branches can induce tissue necrosis. In acute renal failure, circumstantial evidence suggests that hypoxic injury to the renal medulla plays a significant role. In addition, chronically impaired oxygenation may also be an important factor in the progression of chronic renal disease. Destruction of the glomerular capillaries leads to hypoperfusion of the peritubular interstitium. Moreover, in focal disease, a compensatory increase in perfusion of other glomeruli may increase flow and pressure in peritubular capillaries derived from their vasa efferentia which could be a cause of microvascular injury. The interstitial capillary density is reduced in chronic renal disease, and results of animal experiments suggest that this is due to an imbalance in the expression of pro- and antiangiogenic factors. Besides its essential role in energy generation, oxygen is increasingly recognized as an important regulator of cellular functions. Hypoxia induces specific genes through increased expression of hypoxia-inducible transcription factors (HIF). Different HIF isoforms have recently been shown to be inducible in glomerular, tubular, and interstitial cells of the kidney. While the majority of HIF-dependent genes confer protection against hypoxia, hypoxia-inducible gene expression has been suggested to contribute also to increased interstitial matrix deposition.