Forkhead L2 Is Expressed in the Ovary and Represses the Promoter Activity of the Steroidogenic Acute Regulatory Gene

Abstract

Premature ovarian failure in a subgroup of women with blepharophimosis-ptosis-epicanthus inversus type 1 syndrome has been associated with nonsense mutations in the gene encoding a Forkhead transcription factor, Forkhead L2 (FOXL2). However, the exact function of FOXL2 in the ovary is unclear. We investigated the expression of FOXL2 in the mouse ovary during follicular development and maturation by RT-PCR and in situ hybridization. The FOXL2 mRNA is expressed in ovaries throughout development and adulthood and is localized to the undifferentiated granulosa cells in small and medium follicles as well as cumulus cells of preovulatory follicles. FOXL2 belongs to a group of transcription factors capable of interacting with specific DNA sequences in diverse gene promoters. With the presence of multiple putative forkhead DNA consensus sites, the promoter of the human steroidogenic acute regulatory (StAR) gene was used to test for regulation by FOXL2. Cotransfection studies revealed that wild-type FOXL2 represses the activity of the StAR promoter, and the first 95 bp upstream of the transcriptional start site of the StAR gene is sufficient for FOXL2 repression. EMSAs confirmed that FOXL2 interacts directly with this region. Analyses using FOXL2 mutants also demonstrated the importance of the entire alanine/proline-rich carboxyl terminus of FOXL2 for transcriptional repression. Furthermore, these mutations produce a protein with a dominant-negative effect that disables the transcriptional repressor activity of wild-type FOXL2. Dominant-negative mutations of FOXL2 could increase expression of StAR and other follicle differentiation genes in small and medium follicles to accelerate follicle development, resulting in increased initial recruitment of dormant follicles and thus the premature ovarian failure phenotype.

Prognostic significance of FOXL2 mutation and mRNA expression in adult and juvenile granulosa cell tumors of the ovary

“…Knockout mice present a blockage in ovarian development secondary to failure in granulosa cell differentiation. 14,15 It has been recently reported that wild-type FOXL2 and its mutant may have differential regulatory activities of the apoptosis. According to Kim et al,17 FOXL2 mutant induces a lower number of apoptotic cells due to deregulation of the caspase activation.…”

“…12 FOXL2 overexpression induces apoptosis in ovarian granulosa cells, whereas its alteration induces premature ovarian failure due to the abnormal granulosa cell differentiation. [13][14][15] The missense point mutation (C402G) induces an amino-acid change in FOXL2 (C134W) but neither altered function nor mislocalization of the protein have been reported. 10 Previous studies have shown that this mutation is specific for the adult type granulosa cell tumors and is present in up to 95-97% of these tumors.…”

Potential targets of FOXL2, a transcription factor involved in craniofacial and follicular development, identified by transcriptomics

“…We found that PPARGC1A and NR5A2, both involved in cholesterol homoeostasis (35), are stimulated by FOXL2. Previous works (13) have shown that FOXL2 represses expression of Star, a protein that controls cholesterol transport from the outer to the inner mitochondrial membranes. In agreement with this finding, FOXL2 up-regulated the cholesterol 25-hydroxylase whose product is a potent inhibitor of sterol synthesis (42).…”

“…Only three of them, namely the genes encoding the gonadotropin-releasing hormone receptor (GnRHr), the alpha subunit of the gonadotropins (Cga), and the steroidogenic acute regulatory protein (StAR) have been reported so far (11)(12)(13). We have recently suggested that the aromatase gene (CYP19A1) is transcriptionally activated by FOXL2 (14,15).…”

Differential apoptotic activities of wild-type FOXL2 and the adult-type granulosa cell tumor-associated mutant FOXL2 (C134W)

“…Mutations in FOXL2 cause blepharophimosis, ptosis, epicanthus inversus syndrome (OMIM #110100), and blepharophimosis, ptosis, epicanthus inversus syndrome type I is accompanied by premature ovarian failure of the affected female (Crisponi et al, 2001). FOXL2 is mainly expressed in undifferentiated granulosa cells, and blockage of ovarian follicle development because of the failure of granulosa cell differentiation was observed in FoxL2 lacZ homozygous mutant mice (Pisarska et al, 2004;Uda et al, 2004).…”