New strategies in renal cell carcinoma: targeting the genetic and metabolic basis of disease

Abstract

The development of new forms of treatment of advanced renal cell carcinoma over the past two decades has been primarily focused on targeting the VHL/HIF pathway. The recent identification of mutations of chromatin-remodeling genes in clear-cell renal carcinoma (ccRCC), of genomic heterogeneity, and of a Warburg-like metabolic phenotype in advanced disease has had a profound effect on our understanding of the evolution of ccRCC and on potential approaches to personalized therapy. Early approaches to therapy for patients with advanced type I papillary RCC that have centered around the MET/HGF pathway will expand as more genomic information becomes available. Sporadic and familial type II papillary renal cell carcinoma are characterized by enhanced aerobic glycolysis and share an antioxidant response phenotype. In fumarate hydratase-deficient RCC, fumarate-induced succination of KEAP1 activates Nrf2 signaling. CUL3 and Nrf2 mutations as well as an Nrf2 activation phenotype are found in sporadic type II papillary RCC. Therapeutic approaches designed to target the Nrf2 pathway as well as to impair blood flow and glucose delivery in these cancers that are highly dependent on a robust tumor vasculature and on ready availability of glucose for energy production and glycolysis are in development.

Figure 1. Potential therapeutic targets: clear cell renal cell carcinoma

Potentially drug targetable pathways associated with clear cell RCC include chromatin remodeling and altered tumor metabolism. A, mutations in several chromatin remodeling genes such as PBRM1, SETD2 and BAP1 are common in clear cell RCC and result in altered transcription profiles due to the specific effects of loss of each of these genes. As knowledge of these pathways increases, potential therapies could target downstream transcriptional gain of function or loss of function events. B, higher grade, high stage low survival clear cell tumors are associated with a metabolic shift consistent with a suppression of oxidative phosphorylation and a subsequent dependence upon glycolysis for energy. This would result in a dependence on glucose for ATP and for the conversion of pyruvate to lactate for excretion while suppressing AMP-activated protein kinase (AMPK) to aid fatty acid production and growth. The enzymes and transporters involved in these processes, such as Hexokinase 2 (HK2), lactate dehydrogenase A (LDHA) and glucose transporters 1/4 (GLUT1/4), are potential targets (highlighted in red). Additionally, the increased need for fatty acids for growth requires an alternative carbon metabolite to enter the citric acid cycle to produce the necessary citrate. These tumors would increase their glutamine uptake and conversion to glutamate for entry into the Krebs cycle and both the glutaminase (GLS) and fatty acid synthesis enzymes, such as ATP citrate lyase (ACLY), acetyl-CoA carboxylase (ACC) and Fatty acid synthase (FAS), are potential therapeutic targets. Due to the suppression of the normal flow in the citric acid cycle the alternative reductive carboxylation pathway in used to convert α-ketoglutarate (α-KG) to citrate via the isocitrate dehydrogenase (IDH) enzymes that provide further potential targets.

Figure 2. Potential therapeutic targets: papillary type 2 renal cell carcinoma

Loss of fumarate hydratase (FH) is associated with the familial form of type 2 papillary RCC and results in the activation of several pathways. The loss of FH activity suppresses flow through the citric acid cycle in the normal direction and impairs the cell’s ability to use oxidative phosphorylation, forcing the cells to be dependent upon glucose and glycolysis for energy production. The loss of canonical tricarboxylic acid cycle function results in usage of the reductive carboxylation pathway and increased glutamine uptake for fatty acid synthesis and, in a similar manner to clear cell RCC, provides a series of potential targets for therapy (highlighted in red) involved in the conversion of glutamine to fatty acids. The highly increased levels of fumarate (Fum) result in inhibition of α-ketoglutarate depenedent enzymes such as the prolyl hydroxylases (PHDs). In normoxia, the PHDs hydroxylate the HIF-α transcription factors to allow for VHL-dependent degradation. In fumarate hydratase deficient RCC the HIF1α accumulates and activates downstream targets. This results in increased levels of vascular epithelial growth factor (VEGF), lactate dehydrogenase A (LDHA) and the glucose transporter, GLUT1, that support the high levels glycolysis required by this aggressive form of renal cell carcinoma. A therapeutic approach utilizing the combination of bevacizumab and erlotinib is currently being evaluated. In addition, the elevated levels of fumarate result in succination of multiple proteins including the KEAP1 protein, which, as part of an E3 ubiquitin ligase complex with CUL3, targets the NRF2 transcription factor for degradation. Succination of KEAP1 inactivates KEAP1 and inhibits NRF2 degradation, resulting in activation of the NRF2 antioxidant response element (ARE) transcription pathway. While mutation of fumarate hydratase is rarely seen in sporadic type 2 papillary RCC, recent studies have reported activation of the NRF2 pathway in sporadic type 2 PRCC due to inactivating mutations of CUL3 or activating mutations of NRF2. An intense effort is underway to develop therapeutic agents that target the NRF2 ARE pathway.