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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Gene. Author manuscript; available in PMC Aug 17, 2009.
Published in final edited form as:
PMCID: PMC2727720
NIHMSID: NIHMS66289

Identification and characterization of a novel folliculin-interacting protein FNIP2

Abstract

Birt-Hogg-Dube’ syndrome characterized by increased risk for renal neoplasia is caused by germline mutations in the BHD/FLCN gene encoding a novel tumor suppressor protein, folliculin(FLCN), which interacts with FNIP1 and 5′-AMP-activated protein kinase(AMPK). Here we report the identification and characterization of a novel FNIP1 homolog FNIP2 that also interacts with FLCN and AMPK. C-terminally-deleted FLCN mutants, similar to those produced by naturally-occurring germline mutations in BHD patients, were unable to bind FNIP2. These data taken together with our previous results that demonstrated FNIP1 binding to the C-terminus of FLCN suggest that FLCN tumor suppressor function may be facilitated by interactions with both FNIP1 and FNIP2 through its C-terminus. Furthermore, we demonstrate that FNIP1 and FNIP2 are able to form homo- or heteromeric multimers suggesting that they may function independently or cooperatively with FLCN. Differential expression of FNIP1 and FNIP2 transcripts in some normal tissues may indicate tissue specificity for these homologs. Interestingly FNIP1 and FNIP2 were oppositely expressed in human clear cell renal cell carcinoma(RCC), and coordinately expressed in chromophobe RCC and oncocytoma, suggesting their differential function in different histologic variants of RCC.

Keywords: BHD, FLCN, AMPK, FNIP1, renal cell carcinoma(RCC), tumor suppressor

1. Introduction

Birt-Hogg-Dubé (BHD) syndrome is an inherited genodermatosis with an increased risk of renal neoplasia (Birt et al., 1977; Toro et al, 1999; Zbar et al., 2002) caused by germline mutations in the BHD(FLCN) gene located at chromosome 17p11.2 (Nickerson et al., 2002). Nearly all BHD mutations identified to date, which include frameshift, splice-site or nonsense mutations, are predicted to prematurely truncate the BHD protein, folliculin (FLCN) (Nickerson et al., 2002; Khoo et al., 2002; Schmidt et al., 2005; Leter et al., 2007). Renal tumors, which develop most frequently in BHD patients, include chromophobe renal cell carcinoma(RCC) and renal oncocytic hybrid tumors (Pavlovich et al., 2002; Pavlovich et al., 2005; Murakami et al., 2007). BHD may function as a tumor suppressor gene since somatic mutations in the remaining wild-type copy of BHD or loss of heterozygosity at chromosome 17p11.2 have been identified in BHD-associated renal tumors (Vocke et al., 2005). Moreover in the Nihon rat model of BHD inactivation, restoration of BHD expression in the rat suppresses renal tumorigenesis (Togashi et al., 2006).

Folliculin is a novel 64-kDa protein without characteristic domains to suggest function (Nickerson et al., 2002). We recently identified FNIP1, a novel folliculin-interacting protein, which also binds to 5′-AMP-activated protein kinase (AMPK) (Baba et al., 2006), an important energy sensor in cells that negatively regulates the master switch for cell growth and proliferation, mammalian target of rapamycin (mTOR) (Inoki et al., 2005). FLCN and FNIP1 phosphorylation levels were affected by AMPK and mTOR activities suggesting a functional relationship with the AMPK-mTOR pathway. Mutations in several other tumor suppressor genes have been shown to result in dysregulation of mTOR signaling leading to the development of other hamartoma syndromes. Here we report the identification of another novel FLCN binding protein FNIP2 (KIAA1450, GenBank accession no. NM 020840) with homology to FNIP1 (49% identity, 74% similarity), which is conserved across species, and also binds to AMPK. Interestingly, FNIP1 and FNIP2 were able to form homo- and heteromeric multimers suggesting a coordinated functional relationship between these proteins. We evaluated expression patterns of FLCN, FNIP1 and FNIP2 in normal human tissues, and compared their expression in sporadic RCC and normal kidney.

2. Materials and methods

2.1. FNIP2 identification and bioinformatic analysis

KIAA1450 (GenBank accession no. NM 020840) was identified as a FNIP1 homolog by bioinformatic searching of available sequence databases using BLAST (Altschul et al., 1990). ClustalX (1.8) interface with pairwise gap openings and gap extension penalties set at 10x and 0.2x, respectively, was used for the ClustalW multiple sequence alignment program to prepare multiple alignments of FNIP1, FNIP2 and their homologs. The distances between all pairs of sequences (percent divergence) were calculated from the multiple alignments using ClustalX and the neighbor-joining method (Saitou and Nei, 1987) was applied to the distance matrix. The unrooted phylogenetic tree was represented with the help of the Treeview 1.6.6 software (Page, 1996).

2.2. Cell lines and cell culture

HEK293 cells expressing doxycycline-inducible HA-FNIP2 were established using the Flp-In T-Rex System (Invitrogen, Carlsbad, CA) according to the manufacturer’s protocol. HEK293 cells were cultured in Dulbecco’s modified Eagle medium (DMEM; Invitrogen) supplemented with 10% tet-screened fetal bovine serum (Hyclone, Logan, UT) and antibiotics (penicillin/streptomycin).

2.3. Antibodies

Normal rabbit immunoglobulin (IgG; Santa Cruz Biotechnology, Santa Cruz, CA), HRP-labeled anti-mouse and anti-rabbit secondary antibodies (Vector Laboratories, Burlingame, CA), AMPKα mouse monoclonal(F6), AMPKα, and AMPKβ1 rabbit polyclonal antibodies (Cell Signaling, Beverly, MA), AMPKγ1 rabbit polyclonal antibody (Zymed, San Francisco, CA), HA and Flag rabbit polyclonal antibodies (Santa Cruz Biotechnology), V5 rabbit polyclonal antibody (Sigma Aldrich, St. Louis, MO), and HA rat monoclonal(3F10) antibody (Roche Applied Science, Indianapolis, IN) were used according to manufacturer’s protocol. FNIP1-189 rabbit polyclonal antibody was generated against a His6X- tagged recombinant protein corresponding to codons 765-929 of FNIP1. FNIP2-3G and FNIP2-4G rabbit polyclonal antibodies were raised against FNIP2 peptide CSRDLGLKPDKEANR and FNIP2 peptide CDKGFAEDRGSRND, respectively. We chose non-homologous regions to use as antigens to generate FNIP1 and FNIP2 antibodies, and these antibodies were confirmed not to cross react with each other. FLCN-105 rabbit polyclonal antibody was raised against GST-FLCN and FLCN monoclonal antibody (FLCN-mAb) was raised against full length GST-FLCN in the mouse. Culture medium from a single clone hybridoma cell line was used as the antibody source.

2.4. Immunoprecipitation and western blotting

Cells were lysed in lysis buffer [20mM Tris-HCl, 150mM NaCl, 5% glycerol, 0.1% TritonX-100, Complete Protease Inhibitor Cocktail, Phostop Phosphatase Inhibitor Cocktail (Roche Applied Science)] and immunoprecipitated at 4°C overnight with protein G-Sepharose (GE Healthcare Bioscience, Piscataway, NJ) bound antibodies, anti-HA affinity matrix (Roche Applied Science), anti-V5-Agarose or anti-Flag M2-Agarose(Sigma Aldrich). For sequential immunoprecipitation, cell lysates were immunoprecipitated with anti-HA affinity matrix overnight. Immunoprecipitates were washed five times followed by elution with HA-peptide (Roche Applied Science) competition as described previously (Baba et al., 2006). Eluted protein was diluted with lysis buffer and immunoprecipitated with anti-Flag M2-Agarose overnight. The resin was washed five times with lysis buffer and proteins were eluted with sodium dodecyl (SDS)-containing sample buffer followed by boiling and subjected to SDS-polyacrylamide gel (SDS-PAGE) electrophoresis. ReliaBlot (Bethyl Laboratories, Montgomery, TX) was used for rabbit polyclonal western blotting to detect proteins immunoprecipitated by rabbit polyclonal antibodies.

2.5. Immunofluorescence microscopy

Immunofluorescence microscopy of MCF7 cells transfected with FNIP2-Flag and HA-FLCN was performed as previously described (Baba et al., 2006). Alexa488 conjugated goat anti-rabbit antibody and Alexa594 conjugated goat anti-rat antibody (Invitrogen) were used as secondary antibodies. Images were obtained by confocal microscopy.

2.6. Plasmids, transfection, recombinant protein expression and purification

The Gateway® Protein Expression System (Invitrogen) was used to produce a variety of expression vectors encoding full-length FLCN, FNIP1 and FNIP2. GST-fusion FLCN deletion mutants were produced in bacmid-infected Sf9 cells as previously described (Baba et al., 2006). Transfections were performed using Lipofectamine 2000 (Invitrogen) according to the manufacturer’s protocol.

2.7. In vitro binding assay

GST-fusion proteins were immobilized on glutathione-Sepharose beads (GE Healthcare) and incubated with 35S-labeled in vitro translated FNIP2 prepared using TNT T7 Quick kit (Promega, Madison, WI). After washing, beads were boiled with SDS-containing sample buffer and subjected to SDS-PAGE, followed by Coomassie Brilliant Blue (CBB) staining and autoradiography.

2.8. Quantitative Real Time-PCR(qRT-PCR)

qRT-PCR of human normal tissue was performed using TissueScan Real-Time Human-48 tissues (Origene, Rockville, MD), Power SYBR Green PCR Master Mix and 7300 Real Time PCR System (Applied Biosystems, Foster City, CA) according to the manufacturer’s protocol. Sequence specific primers were designed using Primer Express computer software (Applied Biosystems). The primer sequences were: FLCN forward, 5′-TGC AGG TGC TGG TGA AGG TAA CCT-3′and FLCN reverse, 5′-GGG ATT GGG CAA GTC AGA TGC TTG-3′; FNIP1 forward, 5′-TTT GTT CCT CCC CAC TGC TTC CCA-3′and FNIP1 reverse, 5′-AGT AGC AGC AGC TCA TTC CTT GGG-3′; FNIP2 forward, 5′-CAC CCC ACA TCC GTT TCT CAT TAC G-3′ and FNIP2 reverse, 5′-CCC TGG AAG GTG GTG AAT GAG GGA A-3′. All quantities were normalized to β actin measured with β actin primer provided by the manufacturer.

To investigate the expression levels of FLCN, FNIP1 and FNIP2 in renal tumors, qRT-PCR analysis was carried out with 106 sporadic renal tumor specimens and 37 normal kidney specimens obtained under an institutional review board-approved clinical protocol with patient informed consent. The isolation of total RNA, cDNA preparation and qRT-PCR with Taq-Man fluorescent probes for the measurement of gene expression were performed essentially as previously described (Yao et al., 2005; Yao et al., 2007). Primer sequences are: FNIP1 forward, 5′-AGA TTG GGC AGA AGA GGA TGA G-3′, FNIP1 reverse, 5′-TTG GCA ATG TTG GGC TGA-3′, FNIP1 TaqMan probe, 5′-CCT GGG TCA AAG TTA ATC GAA GTG AGT GCT G-3′; FNIP2 forward, 5′-GGT CCT TGG AAG TGG AGC TG-3′, FNIP2 reverse, 5′-GTG AGC GGC CAA AGT TCC T-3′, FNIP2 TaqMan probe 5′-CAA GGT CTC AGA GCA TCA GCA CCC AGA-3′.

2.9. Statistical analysis

The Mann—Whitney U-test was used to test for differences in distribution between groups. All statistical analysis was performed with SPSS software (version 10.1, SPSS, Inc., Chicago, IL). All statistical tests were two-sided and were considered to be statistically significant for p < 0.05.

3. Results and Discussion

3.1. Identification of FNIP2 and evolutionary divergence among FNIP1 and FNIP2 homologs

We have identified a human uncharacterized cDNA, KIAA1450, that encodes a human FNIP1 homolog by bioinformatic searching of available sequence databases using BLAST (Altschul et al., 1990) and designated it FNIP2 (folliculin interacting protein 2). The KIAA1450 transcript (GenBank accession no. NM 020840) had a 177bp sequence with a remarkably high GC content upstream of the ATG initiation codon, and we found an in-frame TGA termination codon, which resided in the 5′ end of the GC rich region. This finding enabled us to confirm the ATG initiation codon. Human FNIP1 and FNIP2 display high homology in the N-terminal half of the proteins and in a 158 residue block within the C-terminus, but low similarity in the other regions, suggesting that the regions of high homology may be important for critical functions (e.g., FLCN interaction), and the regions of low homology may contribute to the unique characteristics of each homolog. Overall, FNIP1 and FNIP2 proteins show 49% identity and 74% similarity, respectively (Fig.1A). Vertebrates from human to Xenopus have both FNIP1 and FNIP2 transcripts [human FNIP1(GenBank accession no. DO145719), mouse FNIP1 (GenBank accession no. NM 173753), X.tropicalis FNIP1(Ensembl ENSXETP00000036209), human FNIP2 (GenBank accession no. NM 020840), mouse FNIP2 (GenBank accession no. XM 907047), X. tropicalis FNIP2 (Ensembl ENSXETG00000016579)]. D. melanogaster (GenBank accession no. NM 140686) and C.elegans (Wormbase T04C4.1) each have a single transcript encoding a protein that shows homology to both FNIP1 and FNIP2. Seven amino acid sequence blocks in FNIP2 that were conserved across all species from human to C. elegans with similarity >30% were identified. Although there are no known functional domains in FNIP2, the conserved sequence blocks may represent regions important to FNIP2 function (Fig.1 B).

Figure 1
Identification of FNIP2 and evolutionary divergence among FNIP1 and FNIP2 homologs. (A) Amino acid sequence alignment of human FNIP1 and FNIP2 with 49% identity and 74% similarity. Asterisk, identity; dots, similarity. (B) Evolutionarily conserved sequence ...

To gain insight into the evolutionary relationships among FNIP1, FNIP2 and their homologs, we compared the amino acid sequences of these proteins and constructed a phylogenetic tree (Fig.1C; sequence alignment available as Supplemental Figure 1). Human FNIP1 and its homologs formed a cluster distinct from human FNIP2 and its homologs. Within each FNIP cluster, the mammalian proteins clustered separately from the Xenopus proteins. As expected, the Drosophila and Caenorhabditis species, each of which has a single FNIP protein, displayed the greatest divergence from the FNIP1 and FNIP2 branching clusters.

3.2. Analysis of expression patterns of FLCN, FNIP1 and FNIP2 in normal human tissues

To analyze gene expression patterns of FLCN, FNIP1 and FNIP2 in human tissues, quantitative Real Time PCR (qRT-PCR) was performed (Fig.2A). Expression patterns of FLCN, FNIP1 and FNIP2 were generally similar, and coordinately high in certain tissues including muscle, nasal mucosa, salivary gland and uvula, suggesting that FLCN, FNIP1 and FNIP2 may cooperate together in those organs. FLCN expression was rather ubiquitous compared to FNIP1 and FNIP2. Interestingly FNIP2 expression was higher relative to FNIP1 in fat, liver and pancreas, which may suggest that FNIP2 has a specific function in those metabolic tissues.

Figure 2
Relative mRNA expression levels of FLCN, FNIP1 and FNIP2 in human tissues analyzed by qRT-PCR. (A) FLCN (black), FNIP1 (red) and FNIP2 (blue) expression levels. Mean+standard deviation (SD). (B) FNIP2 immunohistochemistry staining in tubules of normal ...

3.3. Interaction of FNIP2 with FLCN and AMPK and subcellular localization

Based on the structural similarity between FNIP1 and FNIP2, FNIP2 binding to FLCN and AMPK was evaluated. HEK293 cells expressing HA-FNIP2 in a doxycycline-dependent manner were lysed and immunoprecipitated with anti-HA antibody followed by western blotting to identify co-immunoprecipitated endogenous FLCN and AMPK subunits. Endogenous FLCN and all of the AMPK subunits (α,β and γ) were co-immunoprecipitated with HA-FNIP2 (Fig.3A). Endogenous FLCN and AMPK were also co-immunoprecipitated with endogenous FNIP2 in an anti-FNIP2 immunoprecipitation (Fig.3B). Furthermore co-transfected HA-FLCN and FNIP2-Flag colocalized in the cytoplasm in a reticular pattern, which was identical to the FLCN/FNIP1 colocalization pattern (Baba et al., 2006). FLCN localized both in the cytoplasm and nucleus in a reticular pattern whereas FNIP2 localized only in the cytoplasm (Fig3C). In order to show a direct interaction between FLCN and FNIP2 and to determine the FNIP2 binding domain in FLCN, we performed an in vitro binding assay with full length GST-FLCN or GST-FLCN deletion mutants, and in vitro translated 35S- labeled FNIP2. Full-length GST-FLCN and N-terminal GST-FLCN deletion mutants that retained the C-terminus were able to bind to FNIP2. However, C-terminally deleted GST-FLCN mutants could not bind to FNIP2 (Fig.3D). Interestingly the majority of BHD mutations found in BHD patients result in an altered reading frame predicted to prematurely truncate FLCN thereby abrogating its ability to bind FNIP2. The finding that FLCN binding to FNIP2 is lost as a consequence of BHD mutations resulting in the BHD phenotype suggests that the interaction between FLCN and FNIP2 is functionally important.

Figure 3
FNIP2 interaction with FLCN and AMPK and its subcellular localization. (A) Co-immunoprecipitation of endogenous FLCN and AMPK subunits with HA-FNIP2. Doxycycline-inducible HA-FNIP2 expressing HEK 293 cell lysates were immunoprecipitated with anti-HA antibody ...

3.4. Identification of multimeric molecular complexes containing FNIP1 and FNIP2

Our results demonstrate that FNIP1 and FNIP2 share many characteristics. FNIP1 and FNIP2 are structurally similar, display identical interactions with FLCN and AMPK, and show similar subcellular localization patterns. However, there are also distinct differences between FNIP1 and FNIP2. With the exception of the last 158 amino acids, the homology between FNIP1 and FNIP2 in the C-terminal half of the proteins is low. These non-homologous regions of FNIP1 and FNIP2 may characterize their specific functions (Fig.1A). FNIP1 and FNIP2 mRNA expression levels in human tissues were similar but in some tissues these two mRNAs were expressed in an opposing fashion or at dramatically different levels (i.e., higher FNIP2 expression in pancreas and higher FNIP1 expression in parathyroid gland [Fig.2A]). These data may suggest that FNIP1 and FNIP2 each play a specific role in certain tissues or that the ratio of FNIP1 to FNIP2 has physiologic significance. To analyze the relationship between FNIP1, FNIP2 and FLCN, we tested the ability of FNIP1 and FNIP2 to interact in the presence and absence of FLCN overexpression. V5-FNIP1 and Flag-FNIP2 were co-transfected into HEK293 cells, which expressed HA-FLCN in a doxycycline-dependent manner. In both doxycycline-induced HA-FLCN-expressing cells and in uninduced cells, V5-FNIP1 and Flag-FNIP2 co-immunoprecipitated in a reciprocal fashion (Fig.4A). These data indicate that FNIP1 and FNIP2 were able to form hetero-multimers in a FLCN-independent manner. Endogenous FNIP1 and FNIP2 also co-immunoprecipitated from HEK 293 cells supporting the existence of a naturally-occurring FNIP1/FNIP2 hetero-multimer (Fig.4B). We performed an in vitro binding assay with GST-FNIP1 or GST-FNIP1 deletion mutants, and 35S-labeled in vitro translated FNIP2 to identify the FNIP2 binding region in FNIP1 and to confirm their direct interaction with each other. Full length GST-FNIP1 interacted with FNIP2, and all GST-FNIP1 deletion mutants that contained 2 of the 5 sequence blocks, which were conserved across species (Baba et al., 2006), were able to interact with FNIP2 (Fig. 4C). These findings suggest that these FNIP1 conserved sequences are important for recognition and binding to FNIP2.

Figure 4
FNIP1 and FNIP2 multimer formation. (A) Doxycycline-inducible HA-FLCN-expressing HEK293 cells co-transfected with V5-FNIP1 and Flag-FNIP2 were immunoprecipitated with anti-V5 or anti-Flag antibody followed by western blotting with indicated antibodies. ...

In order to characterize the FNIP1/FNIP2/FLCN/AMPK complex, we purified the FNIP1 homo-multimer, FNIP2 homo-multimer and FNIP1/FNIP2 hetero-multimer by sequential immunoprecipitation from HEK 293 cells expressing FNIP1 and FNIP2 with different epitope tags. The HA-immunoprecipitates were eluted by HA-peptide competition followed by immunoprecipitation with anti-Flag, which enabled us to purify protein complexes that had both HA- and Flag-tagged proteins. Immunoprecipitates were detected by antibodies against FLCN, AMPKγ, HA and Flag epitope tags, indicating that all of the possible multimers-FNIP1 homo-multimer, FNIP2 homo-multimer and FNIP1/FNIP2 hetero-multimer-had the ability to interact with FLCN and AMPK (Fig.4D). Further analysis to characterize the nature of each multimer will clarify the functional significance of these multimeric complexes.

3.5. FNIP1 and FNIP2 mRNA expression levels in sporadic renal cell carcinoma (RCC) and normal kidney tissue

BHD germline mutations predispose patients to develop kidney tumors, predominantly oncocytic hybrid tumors, chromophobe RCC and oncocytoma. Since FNIP1 and FNIP2 interact with FLCN, which is encoded by the BHD gene, these proteins may play a role in renal carcinogenesis. To investigate the relationship between FLCN, FNIP1 and FNIP2 expression and sporadic RCC, we performed quantitative RT-PCR with mRNA isolated from 106 cases of sporadic renal tumors and 37 cases of normal kidney tissue. There were no significant differences in FLCN mRNA expression levels between normal kidney tissue and sporadic renal tumor tissue (Fig. 5A). Interestingly, FNIP1 mRNA was significantly higher in clear cell RCC compared to normal kidney (clear cell RCC vs normal kidney: median=2.197 vs 1.624, p<0.001;Fig. 5B). Conversely FNIP2 mRNA was significantly lower in clear cell RCC compared to normal kidney (clear cell RCC vs normal kidney: median=0.197 vs 1.517, p<0.001; Fig. 5C). Furthermore, FNIP1 mRNA was significantly higher in chromophobe RCC (chromophobe RCC vs normal kidney: median=2.965 vs 1.624, p=0.003) and FNIP2 mRNA was significantly higher in oncocytoma compared to normal kidney (oncocytoma vs normal kidney: median=2.261 vs 1.517, p=0.019). The physiological significance of elevated FNIP1 and FNIP2 expression in sporadic chromophobe RCC and oncocytoma is unclear. However it is remarkable that FNIP1 and FNIP2 expression levels were higher in histologic types of renal tumors that are frequently seen in BHD patients, suggesting the existence of a common pathway for the development of sporadic chromophobe RCC/oncocytoma and BHD-related kidney tumors. It is also interesting that FNIP1 expression levels in sporadic clear cell RCC were higher than in normal kidney, whereas FNIP2 expression levels in sporadic clear cell RCC were lower than in normal kidney. Although the physiological significance of higher FNIP1 and lower FNIP2 mRNA expression levels in sporadic clear cell RCC is unknown, these data may suggest a possible involvement of FLCN/FNIP1/FNIP2 signaling pathways in sporadic clear cell carcinogenesis. The opposing expression levels of FNIP1 and FNIP2 specific to sporadic clear cell RCC may suggest unique molecular functions for these homologous proteins. Alternatively the relative ratio of FNIP1 to FNIP2 may affect the formation of homo- or hetero-multimers, and consequently, their physiologic role in normal and neoplastic kidney cells. It will be very important to elucidate the physiologic function of FNIP1 and FNIP2 as well as FLCN in normal cells in order to understand the role of these proteins in BHD syndrome and sporadic renal carcinogenesis.

Figure 5
FLCN, FNIP1 and FNIP2 mRNA expression analysis in sporadic renal cell carcinoma and normal kidney. FLCN(A), FNIP1(B) and FNIP2(C) mRNA levels in human clinical samples were evaluated by qRT-PCR. The gene expression levels are shown by dot and box-whisker ...

4. Conclusions

In summary, we have identified a novel FLCN-interacting protein, FNIP2, with homology to FNIP1 that is conserved across species. FNIP2 interacts with the C-terminal half of the folliculin tumor suppressor protein and with an important nutrient- and energy-sensing molecule, AMPK, a negative regulator of mTOR. Importantly we demonstrate that FNIP1 and FNIP2 can form multimers independent of FLCN expression, suggesting that they may function independently or cooperatively with FLCN. Their differential expression in human tissues may indicate tissue-specific roles for FNIP1 and FNIP2 in normal cell signaling. Moreover, the elevated expression levels of FNIP1 and FNIP2 in the histologic variants of renal tumors found most often in BHD patients may suggest a common pathway for the development of BHD-associated renal tumors, sporadic chromophobe RCC and sporadic oncocytoma.

Acknowledgements

We acknowledge Kazusa DNA Research Institute as the source of the KIAA1450 clone used in this work. This research was supported in part by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research. This project has been funded in part with federal funds from the National Cancer Institute, National Institutes of Health, under contract N01-CO-12400. The content of this publication does not necessarily reflect the views or policies of the Department of Heath and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.

Abbreviations

BHD
Birt-Hogg-Dube’syndrome
FLCN
folliculin
FNIP1
folliculin interacting protein1
FNIP2
folliculin interacting protein 2
AMPK
5′AMP-activated protein kinase
mTOR
mammalian target of rapamycin
RCC
renal cell carcinoma
GST
glutathione-S-transferase
HEK
human embryonic kidney
qRT
quantitative real time

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