Metabolic alterations in kidney cancer

Primo N. Lara, Jr. Eric Jonasch (Editors). kidney cancer (2 ed). Springer International Publishing (2015) → Русский

Although metabolic reprogramming is a common theme in the different subtypes of kidney cancer, there are significant differences in the nature of the metabolic alterations and the genetic and molecular mechanisms driving these alterations between individual RCC variants. Elucidating the precise changes underlying each entity is crucial for developing specific targeting strategies against a given subtype.

Kidney cancers harboring mutations in Krebs cycle enzyme genes

Inactivating germ line mutations or deletions of fumarate hydratase and subunits of succinate dehydrogenase are associated with distinct forms of RCC that offer classic examples of the Warburg effect in human cancer. In both cases, the basic underlying defect is an enzymatic deficiency that impairs the Krebs cycle and oxidative phosphorylation, with a consequent glycolytic shift.

Hereditary leiomyomatosis and renal cell carcinoma (HLRCC)

HLRCC is an autosomal dominant hereditary cancer syndrome characterized by the presence of potentially painful cutaneous leiomyomas occurring on the arms or trunk and development of early-onset uterine fibroids in affected women (often necessitating hysterectomy in the third or fourth decade of life) [29]. In addition, affected patients are at risk for developing a particularly aggressive form of renal cancer with potential for rapid growth and early metastasis [30]. HLRCCassociated kidney cancer presents as early-onset, unifocal or bilateral, multifocal renal cysts or type 2 papillary tumors that have a characteristic histologic appearance that differentiates it from other forms of RCC [31]. Localized renal tumors are managed surgically, with even small tumors being removed due to a heightened risk of metastases. Once metastatic, this form of kidney cancer is almost always fatal.

The underlying genetic alteration is a germ line mutation/deletion of the FH gene; a “second hit,” i.e., loss of the normal somatic allele, precedes development of kidney cancer. Inactivation of FH has several consequences. Loss of fumarate hydratase activity leads to accumulation of its substrate, fumarate, interrupting a key oxidative metabolic pathway critical for meeting cellular bioenergetic and biosynthetic needs. Impairment of the Krebs cycle imposes a metabolic shift characterized by a reliance on aerobic glycolysis for generating ATP and by the use of glutamine as the major source of carbon for fatty acid synthesis.

The metabolic shift in FH-/cells is initiated and sustained by a number of factors. First, excess fumarate inhibits hydroxylation of HIF-1α, leading to the accumulation of this key driver of aerobic glycolysis. Additionally, FH–/cells are characterized by decreased activity of activated protein kinase adenosine monophosphate kinase (AMPK), the master energy sensor of the cell, with consequent upregulation of mTOR activity [32]. Decreased AMPK activity also leads to decreased cytosolic iron levels by downregulating iron transporter activity (DMT1), which further diminishes the catalytic activity of prolyl hydroxylases, complementing the competitive inhibition of these enzymes imposed by fumarate accumulation. Lastly, excess fumarate leads to activation of an oxidative stress response mediated by nuclear factor E2-related factor 2 (Nrf2) [33]. Nrf2 activity is normally regulated tightly by an E3 ubiquitin ligase complex composed of Kelch-like ECH-associated protein 1(KEAP1) and cullin 3 (cul3). In FH-/cells, KEAP1 undergoes a posttranslational modification (succination at cysteine residues) disrupting its interaction with Nrf2, thus allowing the latter to accumulate in the nucleus. Nrf2 is an essential transcriptional regulator mediating the cellular antioxidant response which is critical for neutralizing the effect of reactive oxygen species during oxidative stress. Activation of Nrf2 has been shown to redirect glucose and glutamine into anabolic pathways including glycolysis and the pentose phosphate pathway. The Nrf2 pathway is also activated in some forms of sporadic papillary RCC; mutations in KEAP1, Nrf2, and cul3 leading to disruption of protein– protein interactions have been described and are thought to mediate the oxidative stress response in these tumors [34]. The reliance of FH-/cells on glycolysis and glutamine catabolism offers rational therapeutic targets. Approaches directed against these pathways are subjects of ongoing preclinical and clinical investigation.

Succinate dehydrogenase deficiency kidney cancer

SDH is the only enzyme that participates in both the TCA cycle and the electron transport chain; SDH is composed of four subunits that are bound to the inner mitochondrial membrane. Germ line mutations in genes encoding the SDH-C, SDHB, and SDH-D subunits are associated with hereditary paraganglioma of the head and neck in addition to hereditary pheochromocytoma [35, 36]. Alteration of these subunits has been found to be associated with familial kidney cancer that can present with or without paragangliomas or pheochromocytoma [37, 38]. Loss of SDH activity leads to disruption of the TCA cycle and accumulation of succinate, with many biochemical and metabolic consequences similar to those seen with FH inactivation. Elevated levels of succinate inhibit the activity of prolyl hydroxylases, preventing the hydroxylation of HIF with resultant accumulation [11]. SDH-RCC presents with an aggressive phenotype associated with early onset and presentation with local symptoms or systemic manifestations associated with metastatic disease [39]. Treatment strategies similar to those employed in HLRCC are under investigation.

Von Hippel–Lindau syndrome and clear cell renal cell carcinoma

Patients with VHL syndrome present with a constellation of manifestations including central nervous system hemangioblastomas (cerebellum and spine), retinal angiomas, endolymphatic sac tumors of the inner ear, pheochromocytoma, pancreatic serous cystadenomas, neuroendocrine tumors, and epididymal cystadenomas, as well as renal cysts and clear cell kidney cancer. VHL patients often develop numerous, bilateral renal cysts, solid tumors, and mixed lesions. Renal tumors in VHL patients rarely metastasize when less than 3 cm in greatest dimension. Active surveillance of these cancers until they reach this size threshold has allowed for renal preservation while minimizing the number of surgical interventions needed to maintain oncologic outcomes [40, 41].

Genetic linkage analysis in VHL families led to the identification of the VHL tumor suppressor gene, located on the short arm of chromosome 3 [42, 43]. As a well-studied hereditary kidney cancer syndrome, it has provided profound insight into the behavior of both VHL-associated tumors and of nonfamilial clear cell kidney cancer. Indeed, somatic VHL gene alterations have been identified in up to 90% of patients with sporadic ccRCC [44]. The VHL protein product is a component of an E3 ligase that targets several proteins for ubiquitin-mediated degradation [20]. The most widely studied consequence of VHL inactivation is its effect on the alpha subunits of the hypoxia-inducible factors. Loss of functional VHL protein prevents the ubiquitin-mediated degradation of HIF-a, leading to intracellular accumulation of HIFs [21, 22]. The stabilization of HIFs independent of ambient oxygen tension leads to the transcriptional activation of a number of genes including those encoding proangiogenic factors (VEGF, PDGF), glucose transporters (GLUT1), and growth factors (TGF-a/EGFR) which are necessary for tumor growth and survival. Elucidation of the critical role played by dysregulated VHL/HIF activity in the genesis of ccRCC has been instrumental in the development of clinically effective targeted strategies in this disease; antagonists of the VEGF pathway are currently a mainstay of therapy for patients with metastatic ccRCC.

The metabolic alterations underlying ccRCC are not as well characterized as those in tumors with impaired Krebs cycle activity. It is, however, clear that ccRCC is characterized by several features that promote a glycolytic phenotype. HIF-1 is often upregulated in VHL-deficient ccRCC, usually in conjunction with an activated Akt/ mTOR pathway. The Cancer Genome Atlas recently conducted a comprehensive molecular characterization of over 400 primary ccRCC tumors [45]. In addition to VHL mutations, activation of the PI3K/Akt pathway was a recurrent theme, as was amplification of MYC. This analysis also uncovered evidence of metabolic reprogramming reminiscent of the Warburg effect, particularly in high-grade/high-stage tumors associated with poor survival; Krebs cycle enzymes and AMPK were downregulated, while glycolytic enzymes and enzymes catalyzing lipid synthesis were upregulated in these tumors. While attempts to target metabolic aberrations in ccRCC are still in the early stages of development, it is notable

that patients with poor prognosis appear to derive clinical benefit from temsirolimus, an inhibitor of mTOR activity. Given the central role played by mTOR in regulating cellular metabolism, it is tempting to speculate that the activity of mTOR inhibitors in this context might relate to their ability to reverse the altered metabolic phenotype.

Hereditary papillary renal cell carcinoma and type 1 papillary renal cell carcinoma

HPRC is an autosomal dominant condition resulting from activating mutations of the MET protooncogene located on 7q31. Affected individuals are at risk for developing bilateral, multifocal type 1 papillary RCC. Somatic MET mutations are seen in approximately 10–15% of papillary kidney cancers; additionally, chromosome 7, on which both MET and its ligand HGF are located, is duplicated in a significant percentage of papillary RCC, and it has been suggested that this may represent an alternative means of activating the HGF/MET pathway [46].

MET encodes the hepatocyte growth factor receptor which exhibits tyrosine kinase activity [47]. In normal cells, MET is activated by binding to HGF; however, mutations in the tyrosine kinase domain, as seen in HPRC, render MET constitutionally active. Activation of MET engages multiple signal transduction pathways including the phosphatidylinositol 3-kinase pathway (PI3K). PI3K-RAS and PI3K-AKT activation play a role in increased expression of nutrient transporters resulting in additional uptake of glucose and amino acids. Mutations leading to constitutive activation of HGF/MET can also overcome the negative regulation of AMPK in response to nutrient starvation and/or low cellular ATP (which in normal conditions will promote fatty acid oxidation and ketogenesis).

Clinical management of HPRC is similar to that of ccRCC in VHL syndrome, with active surveillance of lesions until growth of at least one tumor to 3 cm triggers nephron-sparing surgical intervention. A small molecule kinase inhibitor of MET and VEGF receptors (Foretinib) demonstrated an overall response rate of 13.5%, with the presence of a germ line MET mutation being highly predictive of response (50% response rate in those with activating germ line MET mutations) [48].

Birt–Hogg–Dubé syndrome and chromophobe renal cell carcinoma

Germ line mutations of the tumor suppressor FLCN, located on the short arm of chromosome 17 at position 11.2, have been noted in a large proportion of Birt–Hogg–Dubé (BHD) families with up to 70% of BHD-associated tumors demonstrating loss of heterozygosity or sequence alterations in the somatic copy of FLCN [49, 50]. BHD is inherited in an autosomal dominant fashion and can be associated with pulmonary cysts predisposing to spontaneous pneumothorax, fibrofolliculomas, and renal tumors including chromophobe, oncocytic, and clear cell RCC in addition to oncocytomas and hybrid chromophobe/oncocytic tumors.

Folliculin, the protein product of FLCN, interacts with two other cellular proteins, folliculin interacting proteins 1 and 2 (FNIP1 and FNIP2), which aid in folliculin localization to lysosomes during amino acid starvation [51]. FNIP1 and FNIP2 are both phosphorylated by AMPK and bind to the ?-subunit of AMPK which responds to low levels of ATP and nutrients by inhibiting mTOR activity and thereby downregulating cellular metabolism, protein synthesis, and growth [52]. Both in vitro studies and animal models of FLCN inactivation suggest that loss of folliculin is associated with activation of both mTORC1 and mTORC2 activity [26]. The metabolic changes resulting from FLCN loss are yet to be fully characterized. Despite the presence of increased mTOR activity, the preponderance of evidence suggests that oxidative phosphorylation is active in BHD-associated tumors. In a mouse model where FLCN was selectively knocked out in skeletal muscle, FLCN-null cells appeared to have increased mitochondrial activity and demonstrated a shift toward oxidative phosphorylation [53]. The metabolic environment in BHD-associated tumors is likely to be more complex than simply a preference for oxidative phosphorylation and is the subject of ongoing investigation. A sporadic counterpart of BHD-associated tumors has not yet been identified. However, sporadic forms of chromophobe RCC (one of the histologic subtypes seen in BHD patients) appear to be characterized by genetic alterations that lead to increased mTOR activity (including PTEN loss, inactivating mutations in TSC1/2, and activating mTOR mutations) [54]. Additionally, mutations in mitochondrial genes that may promote a glycolytic phenotype have also been identified in these tumors, although their precise role in tumorigenesis and metabolic regulation remains to be determined [44].

Tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder resulting from germ line mutations in the TSC1 or TSC2 genes which act as tumor suppressor genes. Affected individuals are at risk for involvement of multiple organs, and CNS, skin, pulmonary, cardiac, ocular, and renal manifestations have been described. Although the most common renal manifestation is the presence of angiomyolipomas, clear cell RCC is not uncommon [55].

Tuberin and hamartin, the protein products of TSC1 and TSC2, respectively, form a complex with a GTPase-activating protein complex which acts to inhibit mTOR activity [56]. TSC loss is associated with increase in mTORC1 activity and elevated HIF-1α levels; in vitro, treatment of TSC2-deficient cells with rapamycin has been shown to restore normal levels of HIF, supporting the role of TSC2 in regulation of HIF translation by mTOR inhibition [57].

TFE-fusion renal cell carcinoma

Transcription factor E (TFE)3 and TFEB are transcription factors that belong to the microphthalmiaassociated (MiT) family of transcription factors. Gene translocations involving both TFE3 and TFEB have been described in a variant of kidney cancer commonly referred to as “translocation RCC” characterized histologically by the presence of papillary and clear cell features and comprising around 1–5% of all kidney cancers. TFE3 and TFEB are located on chromosome Xp11.2 and 6p21.2, respectively. A number of fusion partners for these transcription factors have been identified in kidney cancer (PRCC–TFE3, ASPSCR1–TFE3, SFPQ–TFE3, NONO–TFE3, CLTC–TFE3, and MALAT1–TFEB). Unique characteristics of this group include its predilection for presenting earlier in life, with up to 15% of those with kidney tumors less than 45 years of age possessing a TFE gene fusion [58, 59]. A recent study suggests that these tumors may have identifiable imaging characteristics although these findings should be validated in a larger study [60–62].

TFE3-fusion cancers are usually aggressive and often present with advanced disease with an increased incidence of regional nodal involvement [63]. Recently, a germ line missense mutation in MiTF, another member of the family, has been shown to increase the risk of RCC in addition to a predisposition to melanoma [64].

The MiT transcription factor family has been implicated in a host of functions, notably cellular differentiation. TFE3 appears to be involved in diverse signaling pathways, but the precise role of individual pathways in the genesis of translocation RCC remains unclear. TFE3-fusion cancers have higher levels of phosphorylated S6, a downstream target of MTORC1 and marker of mTOR activation [65]. TFE3 also plays a role in insulin signaling and glucose metabolism via upregulation of the IRS-2 and hexokinase enzymes in the liver, although this role may be tissue specific [66]. Furthermore, TFEB is a master gene for lysosomal biosynthesis with a role in extracellular nutrient sensing and autophagy.

Cowden’s syndrome

Cowden’s syndrome, or multiple hamartoma syndrome, is an autosomal dominant inherited disorder associated with mutations in the PTEN tumor suppressor gene. It presents with multisystem manifestations including benign trichilemmomas and acral keratosis of the skin, macrocephaly, intestinal hamartomas, and gangliocytoma of the cerebellum [67]. There is an increased risk of breast, thyroid, endometrial, and kidney cancer (30-fold increased risk according to some estimates) [68]. In one study, 4/24 patients with Cowden’s syndrome had kidney cancer; the histology of RCC associated with this condition is varied, and papillary, chromophobe, and clear cell subtypes have all been described [69].

Loss of PTEN function results in constitutive activation of AKT, which in turn results in upregulation of mTOR. In animal models of PTEN loss, mice treated with rapamycin have demonstrated regression of disease-associated mucocutaneous lesions, and prolonged survival in those treated before appearance of disease manifestations [70].