Folliculin encoded by the BHD gene interacts with a binding protein, FNIP1, and AMPK, and is involved in AMPK and mTOR signaling

Abstract

Birt-Hogg-Dubé syndrome, a hamartoma disorder characterized by benign tumors of the hair follicle, lung cysts, and renal neoplasia, is caused by germ-line mutations in the BHD(FLCN) gene, which encodes a tumor-suppressor protein, folliculin (FLCN), with unknown function. The tumor-suppressor proteins encoded by genes responsible for several other hamartoma syndromes, LKB1, TSC1͞2, and PTEN, have been shown to be involved in the mammalian target of rapamycin (mTOR) signaling pathway. Here, we report the identification of the FLCN-interacting protein, FNIP1, and demonstrate its interaction with 5 AMP-activated protein kinase (AMPK), a key molecule for energy sensing that negatively regulates mTOR activity. FNIP1 was phosphorylated by AMPK, and its phosphorylation was reduced by AMPK inhibitors, which resulted in reduced FNIP1 expression. AMPK inhibitors also reduced FLCN phosphorylation. Moreover, FLCN phosphorylation was diminished by rapamycin and amino acid starvation and facilitated by FNIP1 overexpression, suggesting that FLCN may be regulated by mTOR and AMPK signaling. Our data suggest that FLCN, mutated in Birt-Hogg-Dubé syndrome, and its interacting partner FNIP1 may be involved in energy and͞or nutrient sensing through the AMPK and mTOR signaling pathways.hamartoma syndrome ͉ renal cancer ͉ Birt-Hogg-Dubé ͉ tumor suppressor B irt-Hogg-Dubé (BHD) syndrome predisposes patients to develop hair follicle hamartomas, lung cysts, and an increased risk for renal neoplasia (1-3). BHD patients develop bilateral, multifocal renal tumors with a variety of histologies (4). We mapped the BHD locus to chromosome 17p11.2 by linkage analysis in BHD kindreds (5, 6) and identified germ-line mutations in a gene with unknown function that is highly conserved (7,8). Twenty-two unique mutations predicted to truncate the BHD protein folliculin (FLCN), including a ''hot spot'' insertion͞deletion in a C 8 tract, were identified in 84% of BHD kindreds (9). Somatic ''second-hit'' mutations identified in BHD-associated renal tumors suggest a tumor-suppressor function for FLCN (10), underscored by loss of BHD mRNA expression in renal tumors from BHD patients (11).Recent studies suggest that several hamartoma syndromes may be linked through the convergent energy͞nutrient-sensing pathways involved in mammalian target of rapamycin (mTOR) regulation (12-15). These inherited syndromes are characterized by multiple hamartomas and an increased risk of cancer. Germ-line mutations have been identified in four causative genes: LKB1, responsible for Peutz-Jeghers syndrome (16-18), TSC1 and TSC2, responsible for tuberous sclerosis complex (TSC) (19), and PTEN, responsible for Cowden syndrome (20). Loss of gene function leads to dysregulation of mTOR, which regulates cell growth and size through stimulation of protein synthesis (15, 21, 22).BHD syndrome, also a hamartoma disorder, displays phenotypic similarities to TSC that have led to speculation that BHD may function in the pathway(s) signaling through mTOR (12,23). To ascertain FLCN function,...

Tumor Biology and Prognostic Factors in Renal Cell Carcinoma

“…Inactivation of the FLCN gene results in the development of polycystic kidneys and renal tumors in mouse models, and inactivating mutations have been identified in BHD-associated RCC, confirming the tumor suppressor function of the gene [18,19]. There are some indirect data suggesting an interaction between folliculin and adenosine monophosphate-activated protein kinase of the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) via an interaction with two novel proteins called folliculin-interacting protein 1 and folliculin-interacting protein 2 [18,20]. Based on its position in this pathway, folliculin could hold a nutrient-/energy-sensing role [19].…”

Birt-Hogg-Dube Syndrome with Multiple Cysts and Recurrent Pneumothorax: Pathological Findings

“…Type-I alveolar cells gradually become thinner. BHD mRNA is expressed on type-I alveolar cells, and the BHD gene is known to regulate cell growth (6). Mature alveoli develop after birth, but folliculin dysfunction obstructs this development, and immature alveoli may then result in cyst formation.…”

“…One hypothesis is that cyst formation is related to development of the lungs (6). In humans, the growth of bronchi and lungs continues for several years after birth.…”

A new locus-specific database (LSDB) for mutations in the folliculin (FLCN) gene

“…Folliculin and its interacting protein FNIP1 are thought to be involved in energy and/or nutrient sensing via the AMPK and mTOR (mammalian target of rapamycin) pathway (Baba et al, 2006).…”