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Biomedical subjects

Patrick H Maxwell

Publications and source records attributed to Patrick H Maxwell.

46 records · Page 3Linked to original sources
PubMed2002

The use of dioxygen by HIF prolyl hydroxylase (PHD1).

The hypoxic response in animals is mediated by hydroxylation of proline residues in the alpha-subunit of hypoxia inducible factor (HIF). Hydroxylation is catalysed by prolyl-4-hydroxylases (PHD isozymes in humans) which are iron(II) and 2-oxoglutarate dependent oxygenases. Mutation of the arginine proposed to bind 2-oxoglutarate and of the 2His-1-carboxylate iron(II) binding motif in PHD1 dramatically reduces its activity. The source of the oxygen of the product alcohol is (>95%) dioxygen.

Amino Acid Sequence↗
PubMed2002

Structural basis for the recognition of hydroxyproline in HIF-1 alpha by pVHL.

Hypoxia-inducible factor-1 (HIF-1) is a transcriptional complex that controls cellular and systemic homeostatic responses to oxygen availability. HIF-1 alpha is the oxygen-regulated subunit of HIF-1, an alpha beta heterodimeric complex. HIF-1 alpha is stable in hypoxia, but in the presence of oxygen it is targeted for proteasomal degradation by the ubiquitination complex pVHL, the protein of the von Hippel Lindau (VHL) tumour suppressor gene and a component of an E3 ubiquitin ligase complex. Capture of HIF-1 alpha by pVHL is regulated by hydroxylation of specific prolyl residues in two functionally independent regions of HIF-1 alpha. The crystal structure of a hydroxylated HIF-1 alpha peptide bound to VCB (pVHL, elongins C and B) and solution binding assays reveal a single, conserved hydroxyproline-binding pocket in pVHL. Optimized hydrogen bonding to the buried hydroxyprolyl group confers precise discrimination between hydroxylated and unmodified prolyl residues. This mechanism provides a new focus for development of therapeutic agents to modulate cellular responses to hypoxia.

Amino Acid Motifs↗
PubMed2002

Expression of hypoxia-inducible factors in human renal cancer: relationship to angiogenesis and to the von Hippel-Lindau gene mutation.

The von Hippel-Lindau tumor suppressor protein acts as the substrate recognition component of a ubiquitin E3 ligase that targets hypoxia-inducible factor (HIF)-alpha subunits for proteolysis. Stabilization of HIF-alpha subunits has been described in VHL-defective cell lines, leading to HIF activation and up-regulation of hypoxia-inducible mRNAs. Mutations of the von Hippel-Lindau tumor suppressor protein are found in most clear cell renal cell carcinomas (CC-RCCs) but not other renal tumors, raising a question about the importance of activation of the HIF pathway in CC-RCC development. To address this question, we have examined the expression of HIF-alpha subunits in 45 primary renal tumors and related this to tumor subtype, the presence of VHL mutations, and measures of angiogenesis. We show that HIF-alpha is up-regulated in the majority of CC-RCCs, and that the pattern of expression is biased toward the HIF-2alpha isoform. Expression of HIF-alpha proteins was associated significantly with up-regulation of VEGF mRNA and protein and increased microvessel density. Up-regulation of HIF-alpha in CC-RCC was found to involve increased mRNA as well as protein expression, suggesting that both VHL-dependent and VHL-independent mechanisms are involved. These results suggest that activation of the HIF pathway is functionally important in CC-RCC development and might provide a new therapeutic target.

Adenocarcinoma, Clear Cell↗
PubMed2002

Oxygen sensors and angiogenesis.

Local oxygen tension has a profound effect on the vasculature, which compensates vascular insufficiency through the induction of angiogenesis. An important mediator in this process is the hypoxia-inducible factor (HIF) complex, which is activated in hypoxic cells and increases transcription of a broad range of genes including angiogenic growth factors such as VEGF. HIF is primarily regulated through oxygen-dependent proteasomal destruction of the regulatory subunit, HIF-1 alpha or HIF-2 alpha. Regulation is through the modification of specific prolines in HIF- alpha chains which are hydroxylated by a recently identified family of enzymes which require molecular oxygen and 2-oxoglutarate as cosubstrates, and iron as a cofactor. Following modification HIF- alpha chains are captured by a ubiquitin ligase E3 complex containing the von Hippel-Lindau (VHL) tumour suppressor protein. The HIF prolyl hydroxylases (PHD enzymes) act as oxygen sensors regulating HIF, and hence angiogenesis. The PHD-HIF-VHL system provides a range of opportunities for therapeutic manipulation.

Animals↗
PubMed2002

HIF activation identifies early lesions in VHL kidneys: evidence for site-specific tumor suppressor function in the nephron.

Mutations in the von Hippel-Lindau (VHL) gene are associated with hereditary and sporadic clear cell renal carcinoma. VHL acts in a ubiquitin ligase complex regulating hypoxia-inducible factor-1 (HIF-1), but the link between this function and cancer development is unclear. Here we show that in the kidneys of patients with VHL disease, HIF activation is an early event occurring in morphologically normal single cells within the renal tubules. In comparison, dysplastic lesions, cystic lesions, and tumors showed evidence of additional mechanisms that amplify HIF activation. Detection of cells with constitutive HIF activation identified a large number of previously unrecognized foci of VHL inactivation. In proximal tubules these were almost entirely unicellular, whereas multicellular foci were almost exclusively seen in the distal nephron.

Adenocarcinoma, Clear Cell↗
PubMed2002

Expression of hypoxia-inducible factor-1alpha and -2alpha in hypoxic and ischemic rat kidneys.

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-1alpha and -2alpha 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. High-amplification immunohistochemical analyses were used to study the expression of HIF-1alpha and -2alpha 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-1alpha and -2alpha in different renal cell populations. HIF-1alpha 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-1alpha colocalized with inducible expression of the target genes heme oxygenase-1 and glucose transporter-1. HIF-2alpha 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-1alpha and HIF-2alpha 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-2alpha in peritubular fibroblasts suggests a role in erythropoietin regulation.

Animals↗
PubMed2002

Delivery of erythropoietin by encapsulated myoblasts in a genetic model of severe anemia.

BACKGROUND: Existing animal models of anemia inadequately reflect the hematocrit usually present in chronic renal failure (CRF) patients and do not permit long-term treatment studies. The transgenic mouse strain 134.3LC (Epo-TAg(H)) displays a severe chronic anemia resembling that observed clinically during CRF, while displaying an active, normal life span. This phenotype makes it a particularly interesting mouse model for testing erythropoietin (Epo)-based gene transfer strategies. METHODS: Ex vivo gene therapy was employed to administer mouse Epo to homozygous anemic Epo-TAg(H) mice. Encapsulated C(2)C(12) myoblasts genetically engineered to secrete 163 IU mouse Epo/10(6) cells/day were subcutaneously transplanted on the dorsal flank of the mice. Efficacy of delivered Epo was monitored by weekly measurements of animal hematocrit. RESULTS: Most treated homozygous Epo-TAg(H) mice displayed only a transient rise in hematocrit before eventually decreasing to levels as low as 3%. Administering the immunosuppressor anti-CD4+ monoclonal antibody (mAb) to homozygous Epo-TAg(H) mice, beginning at the time of implantation, permitted a rise in hematocrit that remained stable at elevated levels in cases of continued immunosuppression. CONCLUSIONS: Mice having the T antigen insertion in both Epo alleles appeared to develop an immune response to the natural mouse Epo delivered by encapsulated cells. By preventing this reaction using immunosuppression, we demonstrate that encapsulated myoblasts can deliver therapeutic doses of mouse Epo systemically and restore hemopoiesis in a genetic model of severe anemia.

Anemia↗
PubMed2002

Hypoxia-induced, perinecrotic expression of endothelial Per-ARNT-Sim domain protein-1/hypoxia-inducible factor-2alpha correlates with tumor progression, vascularization, and focal macrophage infiltration in bladder cancer.

Endothelial Per-ARNT-Sim (PAS) domain protein-1 (EPAS-1)/hypoxia-inducible factor-2alpha (HIF-2alpha) is a member of the basic helix-loop-helix/PAS domain protein family and is considered to be an endothelial-specific, hypoxia-inducible transcription factor. Because hypoxia is a fundamental element of tumor biology determining clinical outcome, we performed an immunohistochemical study of EPAS-1 expression in a cohort of bladder cancer cases and assessed the possible correlation of EPAS-1 expression with tumor hypoxia and growth. In the 67 cases (37 radical cystectomy and 30 transurethral resection) studied, overexpression of EPAS-1/HIF-2alpha protein was not found in cancer cells or in normal tissues but was mostly found in stroma around cancer cells, and strong positive staining was noted in perinecrotic regions. The perinecrotic/tumorous expression of EPAS-1/HIF-2alpha was correlated statistically with higher histological grade (P < 0.001), advanced pathological T stage (P < 0.001), and presence of necrosis (P < 0.001). A parallel immunohistochemical analysis of a marker gene of vascular endothelial growth factor demonstrated its positive correlation with tumor grade, stage, and EPAS-1/HIF-2alpha overexpression, supporting the correlation of EPAS-1/HIF-2alpha up-regulation with tumor angiogenesis. To further clarify the relationship between hypoxia and vascularity in the perinecrotic/tumorous area with EPAS-1/HIF-2alpha expression, tissue microvessel density (MVD) was assessed. No significant correlation (P = 0.442) was found between EPAS-1/HIF-2alpha expression and MVD if the 67 tumors of different stages were all included. However, EPAS-1/HIF-2alpha-positive cases had lower MVD than EPAS-1/HIF-2alpha-negative cases (P = 0.001) if only invasive cancer cases were analyzed. In addition, in all EPAS-1/HIF-2alpha-positive staining cases, EPAS-1/HIF-2alpha-positive foci had lower MVD than EPAS-1/HIF-2alpha-negative foci (P < 0.001). Finally, using serial sections, the location of EPAS-1/HIF 2alpha expression was identified mainly in tumor-associated macrophage (TAM) as well as in some fibroblast cells. Focal TAM infiltration was identified at a higher level in EPAS-1-positive cases than EPAS-1-negative cases (P < 0.001). This is the first clinical report suggesting that hypoxia-induced, perinecrotic EPAS-1/HIF-2alpha expression is correlated with tumor progression and angiogenesis at higher grade and stage through focal TAM infiltration in invasive bladder cancer.

Adult↗
PubMedDate not supplied

Oxygen homeostasis and cancer: insights from a rare disease.

Many aspects of physiology and anatomy are precisely adjusted so that the right amount of oxygen reaches cell throughout the body. Hypoxia-inducible factor-1 (HIF-1) is activated by low oxygen tension in all mammalian cells and underpins many aspects of the impressive ability to match oxygen supply and demand. As examples, HIF-1 regulates: local capillary architecture via angiogenic signalling red cell production via erythropoietin cellular metabolism via increased expression of glucose transporters and glycolytic enzymes. HIF-1 is also important in disease, for example in cancer where it is involved in angiogenesis. This review describes how HIF-1 is regulated by oxygen and the central role played by the von Hippel-Lindau tumour suppressor protein. The underlying oxygen sensor is provided by a family of enzymes which oxidize specific proline residues in HIF alpha subunits. Inhibiting these newly discovered enzymes provides a way of activating HIF-1 in the presence of oxygen--an exciting prospect for therapeutic intervention in ischaemic diseases.

Animals↗