Urologic Oncology Branch, Center for Cancer Research, National Cancer Institute, 10 Center Drive MSC1107, 10/CRC/1W-5940, Bethesda, MD 20892, USA.
Mol Cancer. 2010 Jun 23;9:160. doi: 10.1186/1476-4598-9-160.
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.
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.
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.
FLCN 基因的种系突变导致 Birt-Hogg-Dube'（BHD）综合征中纤维滤泡瘤、肺囊肿和肾肿瘤的发生。编码的滤泡素（FLCN）蛋白在物种间是保守的，但不含经典的基序或结构域，其功能仍不清楚。在 BHD 患者的肾肿瘤中发现了 FLCN 基因的剩余野生型拷贝的体细胞突变或杂合性丢失，这表明 FLCN 是一种经典的肿瘤抑制基因。
为了研究 FLCN 的肿瘤抑制功能，在源自 BHD 患者的 FLCN 缺失肾肿瘤细胞系 UOK257 中稳定表达野生型或突变型 FLCN（H255R）。当这些细胞被注射到裸鼠中时，肿瘤的发生与野生型 FLCN 表达水平呈负相关。我们鉴定了在有或没有野生型 FLCN 的细胞系中差异表达的基因，其中许多基因参与 TGF-β 信号通路，包括 TGF-β2（TGFB2）、抑制素βA 链（INHBA）、血小板反应蛋白 1（THBS1）、gremlin（GREM1）和 SMAD3。支持体外数据，与正常肾组织相比，BHD 相关肾肿瘤中的 TGFB2、INHBA、THBS1 和 SMAD3 表达水平显著降低。尽管受体介导的 SMAD 磷酸化不受影响，但与 FLCN 恢复细胞相比，FLCN 缺失细胞中 TGF-β2、INHBA 和 SMAD7 的基础和最大诱导水平显著降低。与 FLCN 缺失细胞相比，FLCN 恢复细胞中分泌的 TGF-β2 和激活素 A（INHBA 的同源二聚体）蛋白水平也较低。与生长抑制功能一致，激活素 A（而不是 TGF-β2）完全抑制了 FLCN 缺失的 UOK257 细胞的锚定非依赖性生长。
我们的数据表明 FLCN 在 TGF-β 信号通路中关键分子的调节中发挥作用，并证实了它们在 BHD 相关肾肿瘤中的表达失调。因此，FLCN 失活引起的 TGF-β 信号通路相关基因的失调可能是 BHD 综合征肿瘤发生的重要步骤。
