BHD and apoptosis: a review

Last week’s blog described a recent paper by Sano et al., which showed that FNIP2 is required for apoptosis in human cells. Additional studies have shown that FLCN and FNIP1 also function in apoptotic pathways, thus we have decided to review the current literature describing the role of FLCN and its associated proteins in apoptosis.

Apoptosis is the process of programmed cell death and is required for the correct development and tissue maintenance of an organism. It is a tightly controlled process and its dysregulation is observed in many diseases, including cancer (Favaloro et al., 2012). The Bcl-2 family of proteins are key regulators of apoptosis. When an apoptotic signal is received, pro-apoptotic Bcl-2 genes increase the permeability of mitochondrial membranes, releasing mitochondrial proteins such as cytochrome c and SMAC/Diablo into the cytoplasm (Shimizu et al., 1999; Sun et al., 2002). These activate the Caspase proteins, which degrade proteins, causing the cell to die (Du et al., 2000). This pathway is known to be dysregulated in renal cell carcinomas; high expression of the anti-apoptotic Bcl-2 and/or Bcl-XL genes is often observed in renal tumours that are resistant to treatment (Gobé et al., 2002).

Cash et al. (2011) reported that FLCN-null ES cells were resistant to cell-intrinsic apoptosis, as discussed in this blog post. This was due to disruption of TGF-B signalling and consequently a reduction in the expression of Bim, a pro-apoptotic Bcl-2 protein. Additionally, Reiman et al. (2012) used mRNA and protein array analysis to find targets of FLCN (as described here), and found the mitochondrial apoptotic genes SMAC/Diablo and HtrA2, and Caspase 1 were all upregulated by FLCN. Whether these genes are upregulated directly by FLCN, or via intermediate proteins, is unknown. Together, these findings demonstrate a tumour suppressor role for FLCN, whereby it activates apoptosis via the TGF-B and Bcl-2 signalling pathways.

Conversely, Baba et al. (2012) used a FNIP1 constitutive mouse knock out and a conditional FLCN mouse knock out to show that both caused a block in B cell development due to increased apoptosis of pro-B cells, as discussed here. The phenotype was reversed by ectopic expression of Bcl-2. These findings indicate that both FNIP1 and FLCN interact with the Bcl2 family in order to inhibit apoptosis in bone marrow.

FNIP2 (called MAPO1 in the paper) was identified in a gene trap screen to identify clones that were resistant to MNU-induced apoptosis (Komori et al., 2009). Combining the observations from two further studies conducted by the same group in both murine and human cells (Lim et al., 2012; Sano et al., 2012 and described in blog posts here and here) it is tempting to suggest the following model: under normal conditions, the FNIP2 protein is maintained in equilibrium at a low level by FLCN, which stabilises FNIP2, and AMPK which phosphorylates FNIP2, causing it to be degraded by the proteasome and thus preventing apoptosis (Sano et al., 2012). Upon treatment with MNU, AMPK is phosphorylated by both FLCN and FNIP2 (Lim et al., 2012), AMPK dissociates from the FLCN-FNIP2-AMPK complex, thus stabilising FNIP2 and allowing apoptosis to proceed (Sano et al., 2012). Analogously, mutations in FH, which cause the related syndrome HLRCC, lead to the inactivation of BAD, another pro-apoptotic Bcl-2 protein, though overactive AMPK kinase activity (Bardella et al., 2012). Thus it would be interesting to determine whether FNIP2 similarly regulates any of the Bcl-2 proteins, as the precise mechanism through which it promotes apoptosis is currently unknown.

Given that the common feature of all three BHD symptoms is hyperplasia, it seems likely that the inhibition of apoptosis is an important step in the pathogenesis of BHD. Indeed, the signalling pathways and proteins found to be dysregulated in these studies have all been previously implicated in cancer (TGF-B signalling – Yin et al., 2010; the Bcl2 family – Gobé et al., 2002; SMAC/Diablo – Martinez-Ruiz et al., 2008; and the Caspase family – Olsson and Zhivotovsky, 2011) and interestingly, autophagy and RhoA signalling, which have already been linked to FLCN function (Behrends et al., 2010; Medvetz et al., 2012; Nahorski et al., 2012) have been shown to regulate apoptosis (Esteve et al., 1998; Luo and Rubinsztein, 2013). Therefore, it seems that determining the role of FLCN and its interacting partners in apoptosis will likely prove to be an important endeavour in order to understand how FLCN mutations cause BHD.


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