Overview
Authors: Thomas O. Daniel
Journal: Journal of the American Society of Nephrology (2003)

The emerging picture surrounding hypoxia-inducible factors (HIF) provides an outstanding example of the integrative value that converging information from different sources provides to define interlinked pathways. In this case, an intricate tapestry of information has been assembled from clinical observations of a human genetic cancer syndrome (von Hippel-Lindau [VHL] disease), transcriptional mechanism studies in cell culture, genetic and biochemical studies of protein degradation in Saccharomyces cerevisiae, oxygen-dependent development in Caenorhabditis elegans, and developmental phenotypes in mouse gene deletion studies. The kidney plays a central role in a regulatory control loop in which tissue and enzyme access to molecular O2 control the abundance and function of HIF (1). HIF regulate (1) erythropoietin production and erythron mass, (2) vascular endothelial growth factor (VEGF) and other angiogenic factors regulating neovascularization, (3) oncogenesis in specific tissue sites, (4) developmental programs interlinking vascularization with organogenesis, (5) ischemic preconditioning, and (6) metabolic enzymes that participate in inflammatory cell responses (2). As many of the molecular components regulating HIF are now defined, understanding the mechanics of their regulation provides a new platform from which to identify and address therapeutic targets in cancer, ischemia, and inflammation.Thomas O. Daniel, MD: Chief Scientific Officer Ambrx, Inc., San Diego, CaliforniaThis Frontiers miniseries provides an up-to-date look at the biochemical and physiologic controls imposed at tissue and cellular levels in the kidney through systems that intersect on HIF. The overriding goal is to provide the reader complementary but distinct perspectives on the regulation imposed through HIF, by focusing on renal regulation of erythropoietin production, renal development, and renal carcinogenesis and its therapy, while pointing out important questions for further investigation. Two highly productive and concurrent lines of inquiry converge biochemically at the molecular interaction between HIF-1α and the protein product of the VHL gene, pVHL, as described in the accompanying reviews. The hypoxic response concepts were developed by Semenza and colleagues (3,4) more than a decade ago, as they defined molecular mechanisms regulating transcription of erythropoietin in response to hypoxia and then used this cell-based response to identify hypoxia response elements (HRE) and the transacting factors that bind them, the HIF (reviewed in 3). HIF are now appreciated to be a multicomponent system of transcription factors that function as critical points of regulation for a wide range of growth factors, metabolic enzymes, and other targets. Maxwell’s (4) review frames critical roles for HIF in the kidney as they regulate the mass of the erythron through erythropoietin, and the effect of VHL gene inactivation to activate HIF-1 in renal cells. He outlines current understanding of the biochemical mechanisms through which oxygen availability regulates HIF abundance and function, highlighting observations in C. elegans that advanced definition of molecular interrelationships and comparing this simple system with the multicomponent family of homologous protein isoforms found in mammals. Important questions surrounding the biochemical and physiologic coupling between tissue pO2 and HIF activation are addressed in contexts of renal ischemic susceptibility and erythropoietin production. He describes the critical link that he and his colleagues made between HIF-1α regulation and molecular function of the VHL gene product, pVHL, to target HIF-1α for degradation (5), reviews the key observations linking covalent modification (hydroxylation) of HIF-1α with degradation (6,7), and explains the utility of those observations to provide in situ molecular surrogates for VHL inactivation. The second line of convergent inquiry is reviewed by Kaelin (8). His article highlights the historically important clinical observations that led to definition of the VHL syndrome at the beginning of the last century, anticipating today’s picture that links the molecular pathogenesis of hemangioblastomas with that of renal cell and other restricted tissue cancers. Following up on definition of the VHL gene locus (9), the Kaelin and collaborator laboratories have advanced our understanding of the function of the encoded protein pVHL, including definition of pVHL interactive protein complex components with homology to S. cerevisiae proteins known to regulate protein turnover. Those observations, coupled with functional insights of Maxwell and others, led to definition of the role of pVHL as a substrate-selective E3 ubiquitin ligase that targets HIF-1α for destruction in settings of adequate substrate oxygen (6,7). The correlation between functions of pVHL mutant proteins and the distinct clinical syndromes of patients with VHL disease from whom they are derived promises to identify additional targets for pVHL interaction and to answer questions about the tissue-restricted specificity of VHL-related oncogenesis. The rationale for directing therapies against HIF target genes is clarified, and challenges expected of therapies directed at HIF stabilization are defined. The developmental requirements for HIF axis function in early development and organogenesis are discussed by Freeburg and Abrahamson, in a review that focuses on the molecular linkage between HIF-α isoforms and VEGF expression (10). They emphasize the cellular type restricted expression patterns seen for different HIF-α isoforms and describe implications of the coupled function of HIF regulating expression of VEGF, PDGF, and angiopoietin system components in the developmental vascularization of the mammalian glomerulus. Their synthesis of currently available information poses new questions about hypoxia-dependent and hypoxia-independent regulation of renal organogenesis. The final article, by Sosman, provides an up-to-date review of the therapy of renal cell carcinoma and frames current approaches in light of the high prevalence of VHL mutations accompanying HIF dysregulation in clear cell carcinoma (11). As he outlines, treatment concepts coupled to nephrectomy have included a number of immunotherapy approaches in this chemotherapy-resistant tumor, ranging from enhancement of endogenous tumor immunity with cytokines interferon-α or IL-2, to more recent adoptive transfer of immunity using nonablative mini-allograft transplantation. The striking efficacy of immunotherapy in subsets of patients has focused attention on what distinguishes responsive from nonresponsive tumors. Accumulating molecular profile data from tumors can now be classified by correlation with specific VHL mutations. Hypoxia-induced or other pVHL target gene products may, in the future, be defined as tumor cell targets of endogenous or engrafted tumor immunity. Promising early evidence is accumulating showing clinical efficacy in patients with renal cell carcinoma treated with an anti-VEGF antibody, heightening attention on and providing rationale for therapeutic combinations that simultaneously target multiple hypoxia-induced target systems, including EGFR, TGF-α, and PDGF. These therapeutic concepts continue to be substantially enhanced by definition of the underlying molecular pathogenesis. Although study of HIF and VHL have to date provided an integrated map of the biochemical O2-dependent regulatory control loops that regulate erythron mass, vascularization, and oncogenesis, further work promises to define additional specific points of control amenable to therapeutic intervention in neoplastic, ischemic, and inflammatory diseases. The range of potential impact is great.