mTOR signalling controls the balance of anabolic and catabolic cell metabolism, in response to the cellular environment. A link between FLCN and mTOR signalling was first reported in 2006 by Baba et al., and subsequent research has shown FLCN to both activate and inhibit mTOR signalling, indicating that FLCN’s role in this process is not straightforward (Hudon et al., 2010). A recent study from researchers at Yale University has found the mechanism through which FLCN and its binding partner FNIP1 activate mTOR signalling in response to amino acid stimulation following serum starvation.
Using HeLa cells, Petit et al. showed that under basal cell conditions FLCN is broadly expressed in the cell, but under amino acid starvation, is quickly recruited to the cytosolic surface of lysosomes in a FNIP1-dependent manner. Upon restimulation with amino acids, FNIP1 and FLCN activate RagA or RagB, which in turn activates mTORC1 signalling, and both rapidly dissociate from the lysosome. Subsequently, Ser211 of the transcription factor TFEB is phosphorylated, precluding it from the nucleus and inhibiting its function. This suggests that when amino acid levels are low, FNIP1 and FLCN are recruited to the lysosome, meaning that they can activate the Rag proteins and mTOR signalling rapidly once amino acid levels are restored.
TFEB is a transcription factor that activates the transcription of both lysosomal and autophagy genes (Settembre et al., 2013). This suggests that in FLCN-null cells, lysosomal and autophagy proteins would be increased, increasing catabolic metabolism and potentially allowing cells to survive inappropriately, which may account for the tumour suppressor function of FLCN. Indeed, dysregulated autophagy is a common feature of a number of hereditary kidney cancer syndromes and has been observed in Drosophila lacking FLCN, and possibly in FLCN-null nematodes. In light of these new data, may be due to dysregulated TFEB function. Furthermore, TFEB is part of a family of helix-loop-helix transcription factors which includes TFE3, whose nuclear localisation is known to be controlled by FLCN in order to regulate exit from pluripotency. This suggests that FLCN may regulate the activity of a number of helix-loop-helix transcription factors by controlling their access to the nucleus.
mTOR activation is usually associated with tumour survival and tumorigenesis. FLCN is known to regulate apoptosis, and it could be that FLCN prevents cancer by activating mTORC1 signalling, thus using up available nutrients and activating apoptotic pathways, causing potentially cancerous cells to starve to death. However, FLCN’s effect on mTOR has been reported to be context dependent (Hudon et al., 2010), meaning that the relationship between FLCN, mTOR and BHD pathogenesis remains unclear.
FLCN carries a non-canonical DENN domain and has been shown to have GEF activity towards Rab35 in vitro. Petit et al. show that FLCN specifically binds the GTPase domain of RagA, suggesting that FLCN may act as a GEF towards these proteins in vivo. If this is found to be the case, GEF activity towards RagA and RagB will be the first elucidated molecular function of FLCN.
This study shows that FLCN activates mTORC1 signalling specifically during amino acid stimulation following serum starvation. Interestingly, dietary Leucine partially reverses the growth defect seen in FLCN-null flies, suggesting that FLCN activation of mTOR in response to amino acid levels may be evolutionarily conserved. Additionally, FLCN is known to interact with AMPK (Baba et al., 2006), a known energy sensor protein (Hardie et al., 2012). Thus it would be interesting to determine whether FLCN’s recruitment to lysosomes upon amino acid depletion also requires AMPK.
While it is clear from this study that FLCN activates mTOR in response to amino acids, more research is required to determine how these results relate to the pathogenesis of BHD. However, it does identify the Rag proteins and TFEB as targets of FLCN, thus providing new information about FLCN’s function and suggesting new avenues of research.
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Hudon V, Sabourin S, Dydensborg AB, Kottis V, Ghazi A, Paquet M, Crosby K, Pomerleau V, Uetani N, & Pause A (2010). Renal tumour suppressor function of the Birt-Hogg-Dubé syndrome gene product folliculin. Journal of medical genetics, 47 (3), 182-9 PMID: 19843504
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Settembre C, Fraldi A, Medina DL, & Ballabio A (2013). Signals from the lysosome: a control centre for cellular clearance and energy metabolism. Nature reviews. Molecular cell biology, 14 (5), 283-96 PMID: 23609508