Cash et al. (2011) reported that FLCN-null ES cells were resistant to cell-intrinsic apoptosis due to disruption of TGF-β signalling and consequently a reduction in the expression of Bim, a pro-apoptotic Bcl-2 protein. Microarray analysis has demonstrated that FLCN upregulates the expression of a number of apoptosis genes (Reiman et al., 2012): CASP1, which induces apoptosis; and HtrA2 and SMAC/Diablo which are both part of the mitochondrial apoptotic pathway (Verhagen et al., 2002; Martinez-Ruiz et al., 2008). Interestingly, decreased SMAC/Diablo expression has been linked to poor prognosis in RCC (Mizutani et al., 2005). Furthermore, loss of flcn-1 in C.elegans causes increased stress resistance due to apoptosis being repressed (Possik et al., 2014).
Together, these findings demonstrate a tumour suppressor role for FLCN, whereby it activates apoptosis. Conversely, Baba et al. (2012) used a FNIP1 constitutive mouse knock out and a conditional FLCN mouse knock out to show that both FNIP1 and FLCN interact with the Bcl2 family in order to inhibit apoptosis in B cells. Furthermore, FNIP1 protects iNKT cells from apoptosis during proliferation and maturation (Park et al., 2014).
FNIP2 was identified in a gene trap screen to identify clones that were resistant to MNU-induced apoptosis (Komori et al., 2009). Further studies from the same group showed that AMPK phosphorylation is required for MNU-apoptosis, and occurs in a FLCN and FNIP2 dependent manner (Lim et al., 2012) and that FLCN and AMPK act in opposition to regulate FNIP2 protein stability (Sano et al., 2013). Taken together, it is possible to deduce 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., 2013). 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., 2013).