Hereditary Leiomyomatosis and Renal Cell Carcinoma (HLRCC) is a kidney cancer syndrome caused by mutation of Fumarate Hydratase (FH), a TCA cycle enzyme which catalyses the conversion of fumarate to malate. The accumulation of fumarate in FH-deficient cells results in the stabilisation of HIFα subunits, but it is unknown if this drives tumourigenesis.
A recent paper by Bardella et al. (2012) has found that FH-deficient cells are protected from apoptosis. When FH was knocked down by shRNA in human renal cells (both RPTEC and HK-2 cells), the amount of apoptosis was reduced when treated with stimuli inducing cell death (compared to FH-positive cells). The authors confirmed that HIF-1α and HIF-2α accumulate in FH-null cells, but by knockdown of both HIF subunits, it was found that the protection from apoptosis was independent of HIF-1α and HIF-2α.
The authors next considered whether kinases were involved in the protection from apoptosis. ERK1/2, MEK1 and AMPK were all more active in FH-deficient cells than FH-positive cells. When FH-deficient cells were treated with apoptotic stimuli, only AMPK was further activated. This activation of AMPK was shown to be HIF independent. Functional and knock down assays confirmed that AMPK activation protects cells from apoptosis. The increase in AMPK activation was found to be due to the accumulation of fumarate, with the authors suggesting that the increased fumarate could activate G-protein coupled receptors, which may lead to AMPK activation.
The mechanism by which AMPK activation leads to reduced apoptosis was investigated. The BCL2 family of apoptotic proteins was studied and it was seen that the expression and phosphorylation of these proteins were affected by FH suppression. In particular, the pro-apoptotic protein BAD was more phosphorylated in FH-deficient cells. Treatment with an AMPK activator confirmed that AMPK was directly responsible for the increase in phosphorylation, and therefore inhibition, of BAD.
Perhaps avoiding apoptosis could contribute directly to tumourigenesis in FH-null cells. The results in this paper suggest an oncogenic role for AMPK, which is independent of HIF. This correlates with results from Adam et al. (2011) who found that Fh1-associated cyst formation in mice is Hif independent (see this previous blog). The results in this recent paper do, however, differ from those presented by Tong et al. (2011, discussed here), who found that AMPK levels were reduced in FH-deficient cells. Bardella et al. suggest that more stringent conditions used in the Tong paper might lead to the selection of more aggressive cancer cells, which likely have AMPK suppression.
A connection between FLCN and apoptosis has been proposed in papers by Cash et al. (2011, discussed here) and Lim et al. (2012, discussed here). It is also known that FLCN has a role to play in the activation of AMPK during MNU-induced apoptosis (Lim et al., 2012). Understanding more about how AMPK can promote or inhibit tumourigenesis may further help in unravelling BHD syndrome and perhaps in the development of therapies for both BHD and HLRCC.
- Bardella C, Olivero M, Lorenzato A, Geuna M, Adam J, O’Flaherty L, Rustin P, Tomlinson I, Pollard PJ, & Di Renzo MF (2012). Cells Lacking the Fumarase Tumor Suppressor Are Protected from Apoptosis through a Hypoxia-Inducible Factor-Independent, AMPK-Dependent Mechanism. Molecular and cellular biology, 32 (15), 3081-94 PMID: 22645311
- 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
- Tong WH, Sourbier C, Kovtunovych G, Jeong SY, Vira M, Ghosh M, Romero VV, Sougrat R, Vaulont S, Viollet B, Kim YS, Lee S, Trepel J, Srinivasan R, Bratslavsky G, Yang Y, Linehan WM, & Rouault TA (2011). The glycolytic shift in fumarate-hydratase-deficient kidney cancer lowers AMPK levels, increases anabolic propensities and lowers cellular iron levels. Cancer cell, 20 (3), 315-27 PMID: 21907923
- Cash TP, Gruber JJ, Hartman TR, Henske EP, & Simon MC (2011). Loss of the Birt-Hogg-Dubé tumor suppressor results in apoptotic resistance due to aberrant TGFβ-mediated transcription. Oncogene, 30 (22), 2534-46 PMID: 21258407
- Lim TH, Fujikane R, Sano S, Sakagami R, Nakatsu Y, Tsuzuki T, Sekiguchi M, & Hidaka M (2012). Activation of AMP-activated protein kinase by MAPO1 and FLCN induces apoptosis triggered by alkylated base mismatch in DNA. DNA repair, 11 (3), 259-66 PMID: 22209521