Tumor suppressor FLCN inhibits tumorigenesis of a FLCN-null renal cancer cell line and regulates expression of key molecules in TGF-beta signaling.
BACKGROUND
Germline mutations in the FLCN gene are responsible for the development of fibrofolliculomas, lung cysts and renal neoplasia in Birt-Hogg-Dube' (BHD) syndrome. The encoded protein folliculin (FLCN) is conserved across species but contains no classic motifs or domains and its function remains unknown. Somatic mutations or loss of heterozygosity in the remaining wild type copy of the FLCN gene have been found in renal tumors from BHD patients suggesting that FLCN is a classic tumor suppressor gene.


RESULTS
To examine the tumor suppressor function of FLCN, wild-type or mutant FLCN (H255R) was stably expressed in a FLCN-null renal tumor cell line, UOK257, derived from a BHD patient. When these cells were injected into nude mice, tumor development was inversely dependent upon the level of wild-type FLCN expression. We identified genes that were differentially expressed in the cell lines with or without wild-type FLCN, many of which are involved in TGF-beta signaling, including TGF-beta2 (TGFB2), inhibin beta A chain (INHBA), thrombospondin 1 (THBS1), gremlin (GREM1), and SMAD3. In support of the in vitro data, TGFB2, INHBA, THBS1 and SMAD3 expression levels were significantly lower in BHD-associated renal tumors compared with normal kidney tissue. Although receptor mediated SMAD phosphorylation was not affected, basal and maximal TGF-beta-induced levels of TGFB2, INHBA and SMAD7 were dramatically reduced in FLCN-null cells compared with FLCN-restored cells. Secreted TGF-beta2 and activin A (homo-dimer of INHBA) protein levels were also lower in FLCN-null cells compared with FLCN-restored cells. Consistent with a growth suppressive function, activin A (but not TGF-beta2) completely suppressed anchorage-independent growth of FLCN-null UOK257 cells.


CONCLUSIONS
Our data demonstrate a role for FLCN in the regulation of key molecules in TGF-beta signaling and confirm deregulation of their expression in BHD-associated renal tumors. Thus, deregulation of genes involved in TGF-beta signaling by FLCN inactivation is likely to be an important step for tumorigenesis in BHD syndrome.
Hong
Seung-Beom
SB
Urologic Oncology Branch, Center for Cancer Research, National Cancer Institute, 10 Center Drive MSC1107, 10/CRC/1W-5940, Bethesda, MD 20892, USA.
Oh
HyoungBin
H
Valera
Vladimir A
VA
Stull
Jaime
J
Ngo
Duy-Tan
DT
Baba
Masaya
M
Merino
Maria J
MJ
Linehan
W Marston
WM
Schmidt
Laura S
LS
D000818:Animals; D045744:Cell Line, Tumor; D005786:Gene Expression Regulation / Q000502:physiology*; D016147:Genes, Tumor Suppressor; D005822:Genetic Vectors; D006801:Humans; D007680:Kidney Neoplasms / Q000235:genetics / Q000473:pathology*; D016086:Lentivirus / Q000235:genetics; D051379:Mice; D020411:Oligonucleotide Array Sequence Analysis; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000502:physiology*
C476078:FLCN protein, human; D011518:Proto-Oncogene Proteins; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins