HLRCC, caused by mutations in FH, predisposes patients to develop papillary renal cell carcinoma. FH-deficient cells have an accumulation of fumarate, which leads to the stabilisation of HIF-α subunits and therefore an increase in HIF-α levels. An increased expression of HIF target genes is believed to drive tumourigenesis. Along with FH, increased levels of HIF are also seen in renal cell carcinomas with VHL or SDH mutations. However, unlike HLRCC, these tumours are predominantly of the clear cell histology, suggesting a different mechanism of tumourigenesis in HLRCC.
Two independent studies, published recently in the same journal, have now identified a HIF-independent pathway for FH-mediated tumourigenesis. Adams et al. (2011) used mice models with both Fh1 and either Hif-1α or Hif-2α inactivation, to demonstrate that cyst formation in HLRCC is HIF independent. Interestingly, specifically deleting both Fh1 and Hif-1α actually accelerated cyst formation.
In the other study, Ooi et al. (2011) performed gene expression profiling using FH-/- fibroids to identify additional pathways deregulated in HLRCC. The most highly upregulated gene was the aldo-keto reductase family 1 member B10 gene (AKR1B10). Ooi and colleagues next set out to identify the transcription factors regulating this gene. Promoter sequence analysis identified the antioxidant response element as a regulator of AKR1B10, which is bound by the transcription factor NRF2. NRF2 levels were shown to increase with the accumulation of fumarate, suggesting NRF2 drives the expression of AKR1B10.
Adams et al. also performed gene expression profiling and found that the NRF2-mediated antioxidant pathway was the most upregulated in Fh1-/- mice. NRF2 target genes also had increased expression in Fh1-/- mouse kidneys. Interestingly, Hmox1, the haem oxygenase gene shown to be synthetically lethal in HLRCC (Frezza et al., 2011 – see this previous blog post) is a target gene of NRF2 and was shown to be upregulated in this study. Adams et al. used mouse models to confirm that the upregulation of NRF2 is HIF-independent.
Both studies found that the increase in NRF2 levels was due to inactivation of KEAP1, a negative regulator of NRF2 which binds to NRF2 and mediates its ubiquitination. It was found that in FH-deficient cells, KEAP1 is succinated by fumarate, leading to its inactivation and therefore the stabilisation of NRF2 and the expression of antioxidant genes.
How the increased level of NRF2 drives tumourigenesis in HLRCC is still unclear and warrants further investigation. These studies also highlight the need for more research into the effects of increased HIF activity in BHD syndrome: although HIF-1α activity has been shown to be increased in FLCN-null cells (Preston et al., 2011), does it contribute to tumourigenesis or are HIF-independent mechanisms involved?
- Adam J, Hatipoglu E, O’Flaherty L, Ternette N, Sahgal N, Lockstone H, Baban D, Nye E, Stamp GW, Wolhuter K, Stevens M, Fischer R, Carmeliet P, Maxwell PH, Pugh CW, Frizzell N, Soga T, Kessler BM, El-Bahrawy M, Ratcliffe PJ, & Pollard PJ (2011). Renal cyst formation in Fh1-deficient mice is independent of the Hif/Phd pathway: roles for fumarate in KEAP1 succination and Nrf2 signaling. Cancer cell, 20 (4), 524-37 PMID: 22014577
- Ooi A, Wong JC, Petillo D, Roossien D, Perrier-Trudova V, Whitten D, Min BW, Tan MH, Zhang Z, Yang XJ, Zhou M, Gardie B, Molinié V, Richard S, Tan PH, Teh BT, & Furge KA (2011). An antioxidant response phenotype shared between hereditary and sporadic type 2 papillary renal cell carcinoma. Cancer cell, 20 (4), 511-23 PMID: 22014576
- Frezza C, Zheng L, Folger O, Rajagopalan KN, MacKenzie ED, Jerby L, Micaroni M, Chaneton B, Adam J, Hedley A, Kalna G, Tomlinson IP, Pollard PJ, Watson DG, Deberardinis RJ, Shlomi T, Ruppin E, & Gottlieb E (2011). Haem oxygenase is synthetically lethal with the tumour suppressor fumarate hydratase. Nature, 477 (7363), 225-8 PMID: 21849978
- Preston RS, Philp A, Claessens T, Gijezen L, Dydensborg AB, Dunlop EA, Harper KT, Brinkhuizen T, Menko FH, Davies DM, Land SC, Pause A, Baar K, van Steensel MA, & Tee AR (2011). Absence of the Birt-Hogg-Dubé gene product is associated with increased hypoxia-inducible factor transcriptional activity and a loss of metabolic flexibility. Oncogene, 30 (10), 1159-73 PMID: 21057536