Tumor Suppressor Folliculin Regulates mTORC1 through Primary Cilia

Abstract

Folliculin (FLCN) is the tumor suppressor associated withBirt-Hogg-Dubé (BHD) syndrome that predisposes patients to incident of hamartomas and cysts in multiple organs. Its inactivation causes deregulation in the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. However, the underlying mechanism is poorly defined. In this study, we show that FLCN is a ciliary protein that functions through primary cilia to regulate mTORC1. In response to flow stress, FLCN associates with LKB1 and recruits the kinase to primary cilia for activation of AMPK resided at basal bodies, which causes mTORC1 down-regulation. In cells depleted of FLCN, LKB1 fails to accumulate in primary cilia and AMPK at the basal bodies remains inactive, thus nullifying the inhibitory effect of flow stress on mTORC1 activity. Our results demonstrate that FLCN is part of a flow sensory mechanism that regulates mTORC1 through primary cilia. Birt-Hogg-Dubé (BHD)4 syndrome is a rare autosomal dominant genetic disorder characterized by development of hamartomas and cysts in multiple organs, including skin, lung, colon, and kidney (1, 2). The syndrome is caused by germ-line mutations in the BHD gene, which encodes the folliculin protein (FLCN), a 64-kDa polypeptide that shares little sequence similarity with any other known proteins (3). FLCN is found to complex with AMPK and FNIP1, although the significance of the complex for FLCN function remains unclear (4). In mouse models FLCN deficiency leads to development of polycystic kidneys and renal cell carcinoma that are characteristically similar to those found in BHD patients (5, 6). Inactivation of FNIP1, together with its homolog FNIP2, in mice also produces similar phenotypes as does by FLCN deficiency (7), suggesting that FNIP1 may be required for FLCN function. Analyses of tumors derived from BHD patients and FLCN deficient animals have revealed deregulation in mammalian target of rapamycin complex 1 (mTORC1) signaling, a key event in tumorigenesis (5, 8 -12). This abnormality in mTORC1 signaling is believed to be a major contributor to the pathological conditions in BHD, as inhibition of mTORC1 with rapamycin has been found to reduce the BHD tumor growth in animal models (5, 6). However, the mechanism by which FLCN regulates mTORC1 remains poorly understood.At non-cycling resting state, most eukaryotic cells possess a microtubule-based membranous protrusion from cell surface termed as primary cilium (13). This unique structure plays a critical role in maintaining tissue homeostasis by functioning as a sensor for extracellular fluidic shear stress and chemicals (14, 15). Many signaling pathways involved in cell growth and proliferation are regulated by this environmental sensor, among which is the mTORC1 pathway (16 -18). Several upstream regulators of mTORC1 have been found to localize to primary cilia, including the tuberous sclerosis complex proteins, LKB1 and AMPK (19 -21). A recent study has shown that primary cilia are able to act through a LKB1-and AMPK-dependent mechanism to dow...

Comprehensive transcriptome analysis of fluid shear stress altered gene expression in renal epithelial cells

“…Also in endothelial cells shear stress exposure decreased expression of genes involved in glycolysis (Doddaballapur et al, ; Kim, Lee, Kawata, & Park, ), lipid metabolism (Fisslthaler & Fleming, ; Mun, An, Park, Jo, & Boo, ; Yamamoto & Ando, ) and cholesterol biosynthesis (Fisslthaler, Fleming, Keseru, Walsh, & Busse, ; Yamamoto & Ando, ). This was dependent on AMPK, which is an important kinase in energy metabolism (Carling, ; Fisslthaler & Fleming, ) and plays a central role in fluid flow induced primary cilium bending and down‐regulation of mTORC1 activity in renal epithelial cells (Boehlke et al, ; Zhong et al, ). Overall, the data show that increased shear stress reduces metabolic activity in renal epithelial cells.…”

“…Other cilia‐dependent signaling cascades affected by fluid flow include the canonical Wnt‐signaling pathway, which is restrained by fluid‐flow induced ciliary signaling in favor of non‐canonical Wnt signaling (Simons et al, ). Furthermore, mTOR signaling and cell‐size control, as well as STAT6/p100‐regulated transcription are thought to be negatively regulated upon flow‐induced bending of the cilium, independent from flow‐induced Ca 2+ influx (Boehlke et al, ; Low et al, ; Weimbs, ; Zhong et al, ). Cilia‐independent shear‐induced alterations in renal signaling include increased Na + and HCO 3 − reabsorption and autocrine TGF‐β/ALK5 signaling (Kotsis, Boehlke, & Kuehn, ; Kunnen et al, ).…”

Control of B lymphocyte development and functions by the mTOR signaling pathways

“…In contrast, various tissues and cells from patients with BHDS, or from mice with deletion of Flcn or deletion of both Fnip1 and Fnip2 , exhibit hyperactive mTORC1 signaling [47–52]. In addition, pre-B cells and skeletal muscle from mice deficient in Fnip1 , as well as cardiac tissue from Flcn knockout mice also exhibit increased mTOR activation [53–55].…”

Lysosome: The metabolic signaling hub

“…A possible explanation for this paradox may be that the function of FLCN as a tumor suppressor is context dependent. One recent study reports that FLCN is a ciliary protein that recruits LKB to activate AMPK – an inhibitor of mTORC1 signaling – in response to flow stress 70 . Thus, cell type, local environmental conditions, and possibly, the subcellular compartment being investigated may impact the net effect of FLCN loss on mTORC1 activity.…”