Renal cancer: oxygen meets metabolism

Abstract

Over the last two decades molecular studies of inherited tumor syndromes that are associated with the development of kidney cancer have led to the identification of genes and biochemical pathways, which play key roles in the malignant transformation of renal epithelial cells. Some of these findings have broad biological impact and extend beyond renal cancer. This review's focus is on the von Hippel-Lindau (VHL)/hypoxia-inducible factor (HIF) oxygen-sensing pathway and its role in physiology, energy metabolism and tumorigenesis.

Figure 1. HIF has a central role in the pathogenesis of RCC

Mutations in TCA cycle enzymes fumarate hydratase (FH) and succinate dehydrogenase (SDH) result in HIF stabilization, and predispose to certain subtypes of familial RCC. Other RCC-predisposing gene products, c-MET tyrosine kinase receptor, TSC1 and TSC2, and folliculin signal through mTOR, which regulates HIF-α translation. The most common genetic event associated with sporadic RCC is loss of pVHL function. The pathogenesis of VHL-associated ccRCC involves both HIF-dependent and HIF- independent functions of pVHL, as well as other genetic events that ultimately lead to malignant transformation. (A) gross photography of ccRCC; (B) clear cell histology (arrows), magnification ×400. Images were kindly provided by Dr. John Tomaszewski, Department of Pathology, University of Pennsylvania, Philadelphia, PA.

Figure 2. pVHL regulates HIF

Under normoxia, HIF-α is hydroxylated by prolyl-4-hydroxylases and targeted for proteasomal degradation by the pVHL-E3 ubiquitin ligase complex. Binding to hydroxylated HIF-α occurs through pVHL’s β-domain, which spans amino acid residues 64–154. The C-terminal α-domain of pVHL (which functions as a substrate recognition component) connects via elongin C to the other components of the E3-ubiquitin ligase complex (Elongin B, Cullin 2 and RBX1). When prolyl-4-hydroxylation is inhibited, e.g. in the absence of molecular oxygen, HIF-α is not degraded and translocates to the nucleus where it heterodimerizes with ARNT. HIF-α/ARNT heterodimers bind to the HIF consensus-binding site, RCGTG, and recruit co-transactivators, followed by transactivation of target genes (e.g. VEGF, PDK-1, EPO). Nitric oxide, reactive oxygen species, TCA cycle metabolites succinate and fumarate, cobalt chloride and iron chelators such as desferrioxamine inhibit HIF prolyl-4-hydroxylases in the presence of oxygen. Aside from targeting HIF-α for proteasomal degradation, pVHL has multiple other functions. These include regulation of primary cilium maintenance, regulation of microtubule stability and interactions with several other signaling pathways. Abb.: CoCl₂, cobalt chloride; EPO, erythropoietin; NO, nitric oxide; PDK-1, pyruvate dehydrogenase kinase 1; ROS, reactive oxygen species; ub, ubiquitin; VEGF, vascular endothelial growth factor.

Figure 3. Metabolic reprogramming by HIF

Overview of HIF-regulated enzymatic steps in glycolysis and mitochondrial metabolism. Green arrows depict promoting effects; red arrows indicate inhibition. Green letters highlight HIF-regulated transcriptional targets (increased expression). HIF-1 activation increases the expression of several glycolytic enzymes (see main text). Increased glycolytic flux results in increased production of lactate, which is excreted via MCT-4. HIF-controlled CA IX and NHE-1 maintain intracellular pH. PDK-1 inhibits the enzymatic activity of pyruvate dehydrogenase, which converts pyruvate to acetyl-CoA. As a result mitochondrial O₂ consumption is diminished. The mitochondrial electron transport chain is depicted by rectangles. SDH is part of ETC complex II (yellow rectangle) and the TCA cycle. Furthermore, HIF regulates the subunit composition of mitochondrial complex IV (blue rectangle). HIF-1 increases COX4-2 expression directly and diminishes COX4-1 expression by enhancing COX4-1 degradation through up-regulation of LON. In contrast to HIF-1, HIF-2 is responsible for neutral fat accumulation. HIF-2 increases the expression of ADFP (direct transcriptional target), inhibits fatty acid synthesis and β-oxidation in VHL-deficient hepatocytes. Abb.: ADFP, adipose differentiation-related protein; CA IX, carbonic anhydrase 9; COX4, cytochrome c oxidase subunit 4; ETC, electron transport chain; GLUT-1, glucose transporter 1; HK, hexokinase; LON; mitochondrial LON protease; MCT-4, monocarboxylic acid transporter 4; NHE-1; Na⁺/H⁺ exchanger 1; PEP, phosphoenol pyruvate; PKM2, pyruvate kinase isoform M2.