Renal cell carcinoma (RCC), as well as other solid tumours, can prove resistant to standard cancer treatments such as chemotherapy. One mechanism likely to play a role in this resistance is activation of HIF signalling either as a result of hypoxia within the tumour or as the result of mutations as in BHD, VHL and TSC. Increased HIF activity, as well as altered PI3K/AKT/mTOR, MEK/ERK and TGFβ signalling, can shift the balance of anti- and pro-apoptotic factors enabling tumour cells to survive in unfavourable conditions.
In response to cell death signals BH3-only family members, such as BIM, bind to and sequester the anti-apoptotic BCL-2 family members, liberating the pro-apoptotic BAX and BAK proteins that induce apoptosis (Figure 1). However, tumour-specific loss of BIM, and associated resistance to apoptosis, has been reported in several tumour types: in clear cell RCC samples (35/45) with a correlation between BIM expression and apoptosis-susceptibility identified in RCC lines (Zantl et al., 2007); in BHD tumour samples (Cash et al., 2011) and VHL-deficient cell lines (Guo et al., 2009) suggesting a role for FLCN and pVHL in BIM expression and stability respectively; and in breast cancer tumour lines with aberrant PI3K/AKT/mTOR or MEK/ERK signalling (Faber et al., 2011).
Figure 1: BIM binds to and sequesters Bcl-2 and Mcl-1 to induce apoptosis via dimerisation of BAX and BAK.
One potential treatment for such tumours is the use of BH3-mimetics – small molecule inhibitors, such as ABT-737 (or the orally bioavailable formulation ABT-263) and Obatoclax (OBX), which bind to BCL-2 and induce apoptosis (Figure 2, Oltersdorf et al., 2005, Tse et al., 2008, Nguyen et al., 2007). OBX has a broader specificity that ABT-737 binding MCL-1 as well as BCL-2, making it a more potent antagonist. Additionally it has been shown that OBX can reduce mTOR activity in melanoma cells (Espona-Fiedler et al., 2012) and both disrupt HIF-1-α protein synthesis and enhance proteosomal degradation to lower HIF-1α protein levels under hypoxic conditions (Gariboldi et al., 2015). As HIF signalling is often perturbed in RCC this suggests that the use of BH3-mimetic may also be useful in these cases.
Figure 2: BH3-mimetics can bind Bcl-2 and Mcl-1 to induce apoptosis in the absence of BIM.
BH3-mimetics could be of particular interest in BHD as several of the pathways known to be altered in FLCN-deficient cells have been implicated in the control of BIM expression: increased HIF-signalling plays a role in BHD renal and pulmonary pathology (Preston et al., 2011, Nishii et al., 2013); increased mTOR activity has been seen in patient samples and BHD models (Baba et al., 2008, Nishii et al., 2013); and increased activation of ERK1/2 signalling was identified in FLCN-null kidneys (Baba et al., 2008).
In addition to these pathways Cash et al., (2011) reported a TGFβ-dependent reduction in BIM expression and increased apoptotic resistance in Flcn-/- ES cells, with Flcn-restored cells showing rescued apoptotic response. A role for FLCN in the regulation of TGFβ signalling had previous been suggested (Hong et al., 2010) following the reduction in TGFβ-target expression in FLCN-null cells (including SMAD7 as discussed in a previous blog regarding a role of MMPs in BHD). Cash et al. determined that the deregulation of TGFβ signalling resulted in hypoacetylation of target promotors, including BIM, and reduced expression. Treatment of these cells with either ABT-737 or an HDAC inhibitor increased susceptibility to apoptosis identifying them as potential therapies in BHD.
BH3-mimetics have proven successful in a range of cancers and there are several ongoing trials assessing the efficiency of ABT-263 and OBX in the treatment of solid tumours. The reduction in BIM expression in BHD tumours and the links between FLCN-associated signalling and BIM expression suggest that BH3-mimetics could be a valid treatment in BHD. Interestingly Cash et al. reported a reduction in BIM levels in a fibrofolliculoma sample raising the possibility for BH3-mimetics to be used for multiple BHD phenotypes.
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