FLCN has been previously reported to regulate both mTOR and RhoA signalling. Although currently considered to be two unrelated functions of FLCN, a recently published study suggest that the two pathways may form one larger signalling pathway.
Following the finding that Rho1 is able to bind TORC1 and inhibit its function in yeast (Yan et al., 2012), Gordon et al. (2013) sought to determine whether mammalian RhoA similarly inhibits mTORC1. In agreement with the yeast study, transfection of HEK293 cells with a constitutively active form of RhoA lead to mTORC1 inhibition, as measured by reduced p70S6K1, 4E-BP1 and ULK1 phosphorylation, despite mTORC1 stimulation with both insulin and leucine.
Contrary to the results of the yeast study, co-immunoprecipitation studies using the mTORC1 protein RAPTOR as bait did not pull down RhoA, suggesting that RhoA does not appear to bind mTORC1 directly. Phosphorylation status analysis of the proteins in the signalling pathways upstream of mTORC1 showed that RhoA exerts its affect upstream of the AKT–TSC1/2–Rheb signalling axis, indicating that it controls an early step in insulin signalling.
In 2012, two independent studies reported converse effects of FLCN on RhoA signalling and, consequently, cell-cell adhesion and cell migration (Medvetz et al., 2012, Nahorski et al., 2012). Similarly, several studies have reported conflicting effects of FLCN on mTOR signalling, with one study suggesting that FLCN’s role might be cell-type dependent (Hudon et al., 2010). More recently, FLCN was found to activate mTORC1 signalling at the lysosome via the Rag proteins upon amino acid stimulation following serum starvation.
The results of the Gordon et al. study suggests that in addition to activating mTORC1 via the Rag proteins, FLCN may also control mTORC1 signalling via Rheb through FLCN’s regulation of RhoA signalling. In fact, RhoA activation has also shown to be dependent on mTORC1 activity (Gulhalti et al., 2011, Liu et al., 2010), suggesting that a feed-back loop exists between the two pathways. This may explain why studies investigating the role of FLCN in RhoA and mTOR signalling have yielded conflicting results – RhoA signalling was not assessed or controlled in the experiments investigating the relationship between FLCN and mTOR and vice versa. Thus, experimental conditions used in the different studies, such as amino acid concentrations in cell culture medium or cell confluence which are known to affect mTOR and RhoA signalling respectively, could make a significant difference to the outcomes of a study.
It would be interesting to explore this link between RhoA and mTORC1 in FLCN-deficient cells to conclusively determine to what extent RhoA’s control on mTOR signalling is mediated by FLCN and how this contributes to BHD symptoms, if at all. Indeed, dual inhibition of RhoA and mTOR using simvastatin and Rapamycin partially reversed the lung damage seen in a mouse model of the cystic lung disease LAM (Goncharova et al., 2012), suggesting that both pathways contribute to lung cyst formation in LAM. If both pathways are found to contribute to BHD pathology, similar combined simvastatin and Rapamycin therapy may prove effective for BHD symptoms.
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