Department of Reproductive Medicine (S.B.R., P.L.M.) and Neurosciences (P.L.M.), University of California, San Diego, La Jolla, California 92093-0674
Address all correspondence and requests for reprints to: Pamela L. Mellon, Ph.D., Department of Reproductive Medicine 0674, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0674. E-mail: pmellon{at}ucsd.edu.
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ABSTRACT |
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INTRODUCTION |
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The gonadotropins, LH and FSH, are essential reproductive hormones that are specifically produced in the gonadotrope population of the anterior pituitary. LH and FSH are both members of the glycoprotein hormone family that also includes TSH, made in pituitary thyrotropes, and CG, made in human and primate placenta (1). Each of these hormones shares a common -subunit and contains a unique ß-subunit that confers the physiological specificity of the hormone. The ß-subunit genes of LH and FSH are expressed exclusively in the pituitary gonadotrope, thus limiting the synthesis and secretion of LH and FSH to these cells.
The molecular mechanism controlling the gonadotrope-specific expression of the LH and FSH ß-subunit genes has been difficult to study due to the heterogeneous nature of the anterior pituitary in vivo and the lack of a differentiated gonadotrope cell line that produces endogenous LHß and FSHß in which to study their expression. Our laboratory has used targeted oncogenesis in transgenic mice to develop several pituitary-derived cell lines of the gonadotrope and thyrotrope lineages (2). The T1-1 cell line expresses the common
-glycoprotein hormone subunit (
-GSU) of the glycoprotein hormones, but is not committed to either the gonadotrope or thyrotrope lineage. The
T3-1 cell line corresponds to a determined, but immature gonadotrope, expressing not only
-GSU, but also specific markers of the gonadotrope lineage, such as steroidogenic factor-1 (SF-1) and GnRH receptor (GnRH-R). In addition to these early markers of gonadotrope differentiation, the LßT2 cell line expresses the ß-subunits of LH and FSH (3, 4) and therefore corresponds to a more mature gonadotrope.
LH ß-subunit gene expression has been studied extensively using transgenic mice, heterologous cell lines (GH3, CV1) (5, 6), and more recently the T3-1 and LßT2 cell lines (7, 8, 9). Several cis-regulatory elements important for both basal and GnRH regulated transcription of the LHß gene have been identified. Binding sites for the orphan nuclear receptor SF-1 are located at -127 bp and -59 bp relative to the transcriptional start site of the rat LHß promoter (10). Within the pituitary, SF-1 expression is gonadotrope specific, making it a key identifier of the gonadotrope cell population (11). In transfected cells, SF-1 has been shown to activate transcription of numerous gonadotrope markers, including GnRH-R (12), the glycoprotein
-subunit (13), and LHß. Supporting the importance of SF-1 to gonadotrope function in vivo, SF-1 null mice are infertile and have markedly reduced levels of
-GSU, LHß, FSHß, and GnRH-R (11). Pituitary-specific SF-1 knockout mice indicate that the impaired gonadotropin expression is a primary defect due to the lack of SF-1 in the pituitary and not a secondary defect due to SF-1 deficiency in other tissues (14). Although SF-1 is an important basal regulator of LHß gene expression, it is not sufficient to confer gonadotrope specificity of the gonadotropin ß-subunit genes because it is expressed earlier in development and in steroidogenic tissues outside of the pituitary (15) as well as in
T3-1 cells, which do not produce the LH or FSH ß-subunits.
Binding sites for early growth response protein 1 (Egr-1) are adjacent to the SF-1 binding sites, located at -112 bp and-50 bp relative to the transcriptional start site (5). Egr-1 (NGFI-A, Krox-24, zif/268) is an early-response gene that is important for GnRH regulation of the LHß gene. Egr-1 null animals are infertile and fail to produce LHß, basally or in response to GnRH (16, 17). In LßT2 and T3-1 cells, basal expression of Egr-1 is low; however, expression is dramatically increased upon treatment with GnRH (6, 7, 18). Egr-1 can activate transcription of the LHß gene by direct physical interactions and synergies with SF-1 and the pituitary homeobox 1 transcription factor (Ptx1, also termed Pitx1 or P-OTX) (5, 6).
Ptx1 is a member of the Ptx family of Paired-like homeobox transcription factors that also includes Ptx2 and Ptx3. Whereas both Ptx1 and Ptx2 are expressed in gonadotrope cells and are important for pituitary development (19, 20, 21), Ptx3 is absent from the pituitary (22). Ptx1 expression is first detected in the presumptive pituitary around embryonic d 9.5, after invagination of Rathkes Pouch (23), and continues throughout development of the anterior pituitary. In the adult pituitary, Ptx1 expression is maintained in all pituitary cells, albeit at varying levels (24, 25). Mice null for Ptx1 die at birth and exhibit developmental defects in the anterior pituitary gland, including decreases in the number of gonadotropes and thyrotropes and corresponding decreases in TSHß, LHß, and FSHß gene transcripts (19). Indeed, using transient transfections of heterologous cells (CV1), it has been demonstrated that Ptx1 is capable of transactivating numerous pituitary-specific promoters, including -GSU, LHß, FSHß, GnRH-R, TSHß, POMC, PRL, and GH (22, 26). In CV1 cells, Ptx1 can activate LHß gene expression through the homeodomain (HD) binding element located at -100 bp of the rat LHß promoter (27). Although this HD element is essential for overall LHß promoter activity in transgenic mice (9), it is not required for Ptx1 regulation in CV1 cells. Even in the presence of a mutated HD-binding site, Ptx1 can activate transcription of the LHß gene through synergy with SF1 and Egr-1 (6).
Despite the importance of SF-1, Egr-1, and Ptx1 for expression of the LHß gene, they are not sufficient to confer LHß gene expression because all of these factors are present in T3-1 cells (which do not express endogenous LHß), as well as being present early in gonadotrope development in vivo before the onset of LHß gene expression (11, 17, 19). The specific expression of endogenous LHß in LßT2 cells indicates that there are additional factors, not present in the early gonadotrope precursor
T3-1 cells, which are involved in the maturation of the gonadotrope and the onset of gonadotropin ß-subunit gene expression and the subsequent production and secretion of LH. These cell lines present an opportunity to identify lineage specific and developmentally regulated transcription factors involved in gonadotrope differentiation and the cell-specific transcriptional regulation of LHß gene expression in the more differentiated LßT2 cells when compared with the precursor
T3-1 cells.
Using a transient transfection paradigm to directly compare reporter gene expression in a variety of pituitary- and nonpituitary-derived cell lines, we show that an LHß-luciferase (Luc) reporter plasmid is specifically expressed in the mature LßT2 gonadotrope cell line. Truncation and mutagenesis analyses indicate that the HD element is necessary for the specific expression in LßT2 cells when compared with precursor T3-1 cells. Using EMSA, we show that this HD element interacts with a protein in LßT2 cell nuclear extract that does not appear in
T3-1 or
T1-1 cell nuclear extracts. Further analyses indicate that this LßT2 nuclear protein complex does not contain Ptx1 or Ptx2 HD transcription factors but does contain a protein related to Otx HD family members.
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RESULTS |
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Results from these experiments show that the transfected LHß reporter gene is specifically expressed in the more differentiated LßT2 pituitary gonadotrope cell line (Fig. 1). LHß promoter activity is approximately 4.5- to 5-fold higher in LßT2 cells than in the gonadotrope precursor
T3-1 cells and up to 20-fold higher than in nongonadotrope-derived cell lines such as AtT20 (corticotrope-derived), GT1-7 (hypothalamic neuroendocrine), JEG-3 (placental), HeLa (cervical fibroblast), CV1 (kidney-derived), and NIH3T3 (fibroblast-derived). These data show that the 1.8-kb rat LHß promoter is sufficient to direct LßT2 cell-specific expression.
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To establish that complexes 2 and 3 are actually binding to the HD element, oligonucleotides with single base-pair point mutations spanning the HD binding site and surrounding sequences were used as competitors and probes in EMSA (Fig. 4, C and D, respectively, sequences shown in Table 1
). Point mutations within the HD-binding element (mutants D, E, F, G, and H) almost completely block binding and competition of both complexes 2 and 3 to the LH 121/87 probe. In contrast, oligonucleotides containing point mutations outside the core HD-binding element (mutants A, B, and I) bind equally as well as wild-type. Slight differences in binding ability between complexes 2 and 3 were observed using these mutants as probes (5' end of HD site, mutants C and D, Fig. 4D
). These gelshift data indicate that the core bases of the HD-binding element, but not additional bases outside this core, are necessary for binding of complexes 2 and 3 to the LHß promoter.
The LHß Promoter HD Element Is Necessary for LßT2 Cell-Specific Expression
Having examined the proteins binding to the HD and Egr/SP1 elements of the LHß promoter, mutations were introduced into -1800LHßLuc and -122LHßLuc and transiently transfected into LßT2, T3-1, and NIH3T3 cells to ascertain whether either of these elements is involved in LßT2 cell-specific expression (Fig. 5
). Site-directed mutagenesis was performed to mutate the 5' Egr-1 binding site, the HD element, and the 3' SF-1 binding site. The 3' SF-1 site was mutated because of the importance of SF-1 for LHß gene expression and the ability of SF-1 to physically interact and synergize with both Ptx1 and Egr-1. Mutation of the element that binds Egr-1 has little or no effect on LßT2 cell-specific expression of the LHß gene under our basal (non-GnRH stimulated) conditions. Similarly, mutation of the 3' SF-1 binding site has no effect on LßT2 cell specificity (4-fold for -1800LHßLuc and 2-fold for -122LHßLuc), although it does reduce basal levels in both LßT2 and
T3-1 cells within -122LHßLuc.
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Complex 2 Contains an Otx-Related Factor that Is Not a Known Member of the Ptx or Otx HD Families
Because of their ability to regulate the LHß promoter in CV1 cells (26) and their role in pituitary development (19, 21), it was of interest to ascertain whether complex 2 contains Ptx1 or Ptx2 HD transcription factors. Additionally, Otx HD transcription factors were examined because they are related to the Ptx family and Otx1 can also activate transcription of the LHß promoter in cotransfections (27, 29). Expression of Ptx and Otx family members in gonadotrope-derived cell lines was determined to identify whether any of these factors is restricted to LßT2 cells. Ptx1 RNA expression has previously been shown to be widespread throughout the anterior pituitary (24, 25). Western analysis reveals that Ptx1 protein is present in LßT2, T3-1, and
T1-1 cells, consistent with its expression throughout pituitary development (Fig. 6A
). In contrast, all three isoforms of Ptx2 are expressed in committed gonadotropes (LßT2,
T3-1), but not uncommitted pituitary progenitor cells (
T1-1). Both Ptx1 and Ptx2 levels are somewhat higher in LßT2 cells than
T3-1 cells. Northern analysis of Otx1 and Otx2 expression reveals the presence of Otx1 mRNA in all cell lines tested except NIH3T3 (Fig. 6B
). As with the Ptx family members, Otx1 is expressed at higher levels in LßT2 cells than
T3-1 cells. Otx2 is not expressed in any of the gonadotrope-derived cell lines. These expression studies demonstrate the presence of Ptx1, Ptx2, and Otx1 in LßT2 cells but also show that none of these factors are restricted to these mature gonadotrope cells; in particular, they are all expressed in the
T3-1 cells.
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Complex 2 Contains a K50 HD Transcription Factor
The Ptx and Otx HD proteins are Paired-like HD transcription factors that contain a lysine in the 50th position of the HD and are therefore referred to as K50 HD proteins (30). The 50th amino acid of the HD interacts with the nucleotide bases 5' to the ATTA HD recognition core and is important in determining DNA-binding specificity of HD transcription factors (31). Whereas K50 HDs exhibit a preference for binding GATTA target sequences, as is found in the LHß promoter, Q50 HDs (containing a glutamine at the 50th position) show a preferred binding sequence of either AATTA or CATTA (31, 32). To examine the importance of a K50 amino acid for HD protein interactions with the LHß promoter HD element, we used site-directed mutagenesis to mutate the Ptx1 and Otx1 HDs from K50 to Q50 (AAG CAG). EMSA was performed using nuclear extracts from NIH3T3 cells transiently transfected with expression vectors coding for either wild-type (NIH3T3 + Otx1 or NIH3T3 + Ptx1) or mutant (NIH3T3 + Otx1 K50Q or NIH3T3 + Ptx1 K50Q) proteins (Fig. 10A
). Expression of both wild-type and mutant proteins in the cells was confirmed by Western blotting (data not shown). This single amino acid change from a K50 to Q50 blocks the ability of both Otx1 and Ptx1 to bind to the LHß promoter, indicating that a lysine at the 50th position of the HD is important for interaction with this HD element.
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DISCUSSION |
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In the present study, the specificity of expression of LHß to the more mature gonadotrope was analyzed using transient transfection assays in gonadotrope and nongonadotrope-derived cell lines. Activity of the 1800 bp rat LHß promoter is 4.5- to 5-fold higher in LßT2 cells as compared with T3-1 cells. The activity in
T3-1 cells when compared with nongonadotrope-derived cell lines is not significantly different, demonstrating that even these committed gonadotrope precursors lack conditions necessary for gonadotrope-specific LHß gene expression. This is especially notable because
T3-1 cells do express several factors thought to be important for LHß gene expression, including SF-1, Egr-1, Ptx1, and Ptx2. Most of the studies concerning LHß gene regulation have been performed using heterologous cell lines, such as CV1, GH3, and JEG-3 (5, 6, 7); however, the use of these cells does not accurately reflect cell-specific responses since distinct sets of transcription factors, kinases, G proteins, receptors, and other classes of molecules are expressed in different cell types. Furthermore, cotransfections with expression vectors for transcription factors not normally expressed in a cell can show activation regardless of the actual role of that factor in regulating LHß in the gonadotrope. In the current work, the selectivity of LHß promoter activity to LßT2 cells underscores the importance of studying LHß gene regulation in a homologous cell system.
As development relies on both the activation and inactivation of lineage specific genes, several models could account for the differential expression of the LHß gene between LßT2 and T3-1 cells. One possibility is the presence of inhibitory factors in early gonadotrope cells (
T3-1) preventing LHß gene expression. Alternatively, there may be activating factors present in mature gonadotrope cells (LßT2) conferring LHß gene expression. The promoter truncation analysis described herein suggests that both mechanisms of gene regulation are at work within the LHß promoter. The approximately 4.5-fold difference in LHß promoter activity between LßT2 and
T3-1 cells is reduced by truncation of two regions of the LHß promoter. Deletion from -179 bp to -146 bp reduces the difference to 2-fold. This is due to a relative increase in expression of LHßLuc in
T3-1 cells (though notably not in NIH3T3 cells), consistent with the possibility of an
T3-1 specific inhibitor acting on this region of the promoter. Interestingly, down-regulation of the LHß promoter after 24 h of tonic GnRH treatment also maps to this region (Vasilyev, V. V., M. A. Lawson, and P. L. Mellon, manuscript submitted). Further characterization of this region to identify potential repressors might provide useful insights into both hormonal and basal input into gonadotrope development.
The remaining selectivity in LHßLuc expression between the gonadotrope-derived cell lines is lost upon deletion of the region between -122 bp and -87 bp. This is due to the loss of activity specifically in LßT2 cells, indicating the removal of a promoter element specifically active in the more mature gonadotrope cell line. This region of the promoter contains the previously characterized 5' Egr-1 binding site and the HD element. Consistent with the negligible levels of Egr-1 in uninduced LßT2 and T3-1 cells (6, 7, 18), Egr-1 is not detected binding to its promoter elements using nuclear extracts from non GnRH-treated cells; rather, the ubiquitous SP1 transcription factor binds to the GC-rich Egr-1 elements [(28), and data not shown]. Mutation of the 5' Egr-1 element has little or no effect on either basal (18) or cell-specific activity of the LHß promoter. This contrasts with the importance of Egr-1 in GnRH regulation of the LHß gene; responsiveness of the LHß promoter to GnRH in both
T3-1 (6, 18) and LßT2 (7, 8) cells is attenuated by mutation of the Egr-1 binding sites.
Whereas Egr-1 elements are not involved in basal activity of the LHß promoter, the HD element is essential for both basal activity as well as cell-specific differences in promoter activity in LßT2 vs. T3-1 cells. We used two 3-bp HD mutations that exhibited different protein interactions in EMSA to examine the role of the HD element in LHß gene regulation. The partial decrease in LßT2 cell-specificity observed with HDm5, which reduces basal activity in both LßT2 and
T3-1 cells, may indicate a contribution of complex 3, Otx1, or Ptx1 to LHß promoter activity; alternatively, the decrease in activity may be due to the reduced binding of complex 2. Unlike HDm5, HDm4 completely blocks binding of complex 2, in addition to the other proteins, and only reduces basal activity in LßT2 cells. The greater effect of the HDm4 mutation (vs. HDm5) on LßT2 cell specificity of -1800LHßLuc correlates with the loss of complex 2 binding, indicating a role for this protein in LßT2 cell-specific expression of LHß. Whereas HDm4 in the context of the 1800-bp promoter only decreases LßT2 cell-specificity to 1.5-fold, the same mutation in the context of the 122-bp promoter abolishes the difference. One element present in the longer promoter but absent from the shorter version is the 5' SF-1 binding site. When this element is mutated along with the 3' SF-1 binding site and the HD element (m4) (a triple mutant), the 1800-bp promoter loses its specific activity in LßT2 cells. The necessity for mutating both SF-1 sites as well as the HD element to eliminate the differences between LßT2 and
T3-1 cells in -1800LHßLuc despite the presence of SF-1 in both cell lines is probably due to interactions of SF-1 with the protein(s) acting through the HD element. SF-1 is capable of interacting with Ptx1, physically and synergistically (26, 27), and may be capable of interacting with additional HD proteins as well. Data from transgenic mice confirms the importance of this HD site for LHß promoter activity in vivo. A -776/+12 bovine LHß promoter confers gonadotrope-specific expression and regulation by both GnRH and gonadal steroids to a CAT reporter gene in transgenic mice (36). Recently, it was shown that mutation of the HD element completely abrogates basal activity as well as GnRH responsiveness of this bovine LHß promoter in transgenic mice (9).
Despite the clear importance of the HD element for LHß promoter activity, the proteins acting through this element are not well characterized, and no comparative studies have been performed. When we compared the proteins binding to the LHß promoter HD element in LßT2 and various other cell types, the major difference observed between cell lines of the gonadotrope lineage is the presence of complex 2 in LßT2 but not T3-1 or
T1-1 cells. This nuclear protein complex has the binding characteristics of a K50 HD protein and is recognized by an Otx1 antibody. Of interest, complex 3, which appears at equal levels in LßT2 and
T3-1 nuclear extracts but to a lesser extent in
T1-1 nuclear extract, has binding specificity almost identical to complex 2 and is also recognized by the Otx1 antibody. This suggests that the proteins comprising complexes 2 and 3 are closely related. These complexes may represent alternatively spliced variants of the same protein or individual members of the same protein family. Although complexes with migration similar to complex 2 appear in some nongonadotrope endocrine-derived cell types, this complex may account for the difference in activity of the LHß promoter between the mature gonadotrope LßT2 and precursor gonadotrope
T3-1 cells. Tissue-specific gene expression is often conferred by complex control regions in which several cell-restricted, but not necessarily cell-specific, factors interact (37, 38). The spatial and temporal pattern in which regulatory factors are expressed is determining when cell-fate decisions are being made. With regards to the LHß gene, a factor expressed in mature gonadotropes that is also present elsewhere in the pituitary could still contribute to gonadotrope-specific expression and activation during development by interaction with SF-1, which, within the anterior pituitary, is uniquely expressed in the gonadotrope population.
Because of their known role in early pituitary development and their presence in gonadotrope cells, Ptx1 and Ptx2 were considered as possible candidates for the complex 2 protein. Although both LßT2 and T3-1 cells express Ptx1 and Ptx2 mRNA and protein (Ref. 22 ; Fig. 6
), previous studies have not demonstrated binding of these proteins to the LHß promoter HD element. Tremblay et al. (22, 27) established the presence of Ptx1 DNA-binding activity in
T3-1 cells using the POMC genes Ptx1 promoter element, yet Ptx1 was not shown binding to the LHß promoter element. In their work demonstrating the necessity of the HD element for LHß promoter activity in vivo, Quirk et al. (9) observe several proteins binding to the HD element using LßT2 cell nuclear proteins. They presume one of these bands to be Ptx1 but were unable to detect it using a Ptx1 antibody. Here we demonstrate for the first time that Ptx1 does indeed bind the LHß promoter HD element, albeit at quite low levels in LßT2 cells. These same experiments show that Ptx1 protein is not contained within complex 2. As with Ptx1, we demonstrate the ability of Ptx2 from transfected NIH3T3 cells to bind the LHß HD element. Because Ptx2 protein is present in LßT2 cells, our inability to detect Ptx2 binding the LHß promoter using LßT2 nuclear extracts may indicate that Ptx2 is not interacting with the HD element in these gonadotrope cells. Ptx1 and Ptx2 are capable of transactivating the LHß promoter in heterologous cell systems (CV1, HeLa) (23, 26) and have been suggested to be involved in cell-specific transcriptional regulation but neither has been proven to be functional in gonadotrope-derived cell lines (LßT2,
T3-1). Ptx1 and Ptx2 HD proteins are necessary for pituitary development and therefore indirectly necessary for gonadotrope function; whether or not they directly regulate LHß gene expression through the HD element in mature gonadotropes needs further clarification.
Interestingly, an Otx1 antibody that can recognize both Ptx and Otx HD proteins also recognizes complex 2, which is our best candidate for an LßT2 HD protein required for LHß promoter specificity to mature gonadotropes. The Ptx and Otx families are Paired-like HD proteins, a subclass of Paired-class proteins; they share homology in the HD to Paired-type proteins, but lack a Paired domain (30). The fact that Ptx and Otx proteins lack homology outside the HD (39) suggests that the Otx1 antibody recognizes the Paired-like HD. This is further supported by data demonstrating that the Otx1 antibody does not recognize HD proteins outside the Paired-class, including the TALE-HD proteins, Pbx1/2/3 and PREP-1, the POU domain factor, Oct-1, and the Antennapedia Class proteins, Msx1 and Dlx2 (Givens, M. L., and P. L. Mellon, personal communication, January 2002). Unlike the Antennapedia class, which includes only Q50 HD proteins, the Paired-like subclass includes both K50 and Q50 HD proteins. As we have not tested Q50 Paired-like proteins, we cannot rule out the possibility that the Otx1 antibody also recognizes these HD proteins. However, the DNA-binding data suggest that complex 2 contains a K50 HD protein related to Otx and not a Q50 HD protein.
First of all, the LHß promoter HD element resembles the preferred target sequence for K50 HD proteins, GATTA, and binds Ptx and Otx family members. Furthermore, the presence of this lysine is essential to the ability of Ptx and Otx to interact with this sequence. When Ptx1 and Otx1 are mutated to convert them from K50 to Q50, they are no longer capable of binding the LHß promoter HD element. Finally, experiments with consensus oligonucleotides indicate that complex 2 only binds a HD recognition sequence of GATTA. In addition to supporting the presence of a K50 HD protein in complex 2, experiments using these HD recognition sequences reveal differences in binding specificity between the Otx and Ptx families. Whereas Otx proteins bind the GATTA consensus sequence, the Ptx proteins do not bind any of the consensus sequences. Although seemingly unexpected at first, the inability of Ptx to bind the GATTA HD consensus element is not entirely surprising. Ptx1 interacts with the LHß promoter HD element with binding specificity distinct from both Otx1 and complex 2 (Figs. 4 and 8
and Table 1
). Whereas Otx1 and complex 2 binding relies mainly on the core HD recognition sequence in the LHß probe, Ptx1 binding also requires several bases outside this core. These additional outside bases are not the same in the GATTA HD consensus oligonucleotide used in Fig. 10B
, consistent with the inability of Ptx to interact with this site. Ptx and Otx families are grouped together in the K50 Paired-like subclass based on the presence of a lysine at position 50 of the HD. In fact, phylogenetic analysis of Paired-like HD proteins indicates that the Ptx and Otx families evolved from separate ancestral origins (30). Their separate origins are probably the reason for observed differences in binding specificity between the families. This distinction between Ptx and Otx factors leads us to conclude that complex 2 includes a protein more closely related to Otx than Ptx.
Otx HD proteins are related to the orthodenticle (otd) gene of Drosophila and are involved in brain morphogenesis (40). Whereas Otx2 is not expressed in any of the gonadotrope-derived cell lines and is therefore not a candidate for the complex 2 protein, both Otx1 and the Otx-related cone-rod homeobox (Crx) mRNAs are expressed in LßT2 cells (Fig. 6 and data not shown) and were considered as possible candidates. Consistent with a role in the postnatal pituitary, Otx1 knockout mice exhibit transient dwarfism and hypogonadism and a corresponding reduction of LH, FSH, and GH levels; however, these defects are temporary and the mice appear indistinguishable from wild-type by 4 months of age (29). Though Otx1 does not comigrate with complex 2, since the Otx1 antibody cross-reacts with related HD proteins, we cannot completely rule out the possibility that complex 2 contains Otx1 along with an additional protein. Paired-like HD proteins are capable of binding DNA as cooperative heterodimers, but this requires a palindromic repeat of the TAAT recognition sequence (33). The LHß promoter HD element does not contain this palindromic target element necessary for recognition by such dimers, suggesting that the proteins interacting with this element are monomers and that Otx1 is not interacting with an additional protein to form complex 2. In addition, subtle, but important, differences exist in the binding specificities of Otx1 and complex 2. Although complex 2 could be an unidentified isoform of Otx1, to date no additional isoforms of Otx1 have been reported and only a single band is observed by Northern analysis (Ref. 41 and Fig. 6
). Furthermore, we have not observed a protein in LßT2 nuclear extract that comigrates with Otx1 by Western analysis (data not shown) or EMSA.
Crx is an Otx-related HD protein that is expressed in mature and developing photoreceptor cells and the pineal gland (42, 43). As with Otx1 and Otx2, Crx is capable of binding the LHß promoter HD element, but does not comigrate with complex 2 (data not shown). As an antibody directed against Crx (kindly provided by Dr. Shiming Chen) does not supershift complex 2 (data not shown), it appears that Crx is not the complex 2 protein. Although the binding specificity of complex 2 and its recognition by the Otx1 antibody suggest that it is an Otx-related HD protein, the lack of comigration with Otx1, Otx2, or Crx, the absence of Otx2 from LßT2 cells, and the failure of a Crx antibody to interact with complex 2, indicate that this may be a novel protein. The complete annotation of the mouse and human genomes should assist in the identification of new members of the Otx HD family.
In summary, the studies presented here emphasize the importance of using a homologous cell system to study tissue-specific gene regulation. We have identified the HD binding site as a key element involved in the activation of the LHß gene in mature LßT2 gonadotrope cells as compared with precursor T3-1 cells. Complex 2 is an attractive candidate for involvement in this LßT2 cell-specific expression of the LHß gene: it is present in LßT2 gonadotrope cells but not
T3-1 or
T1-1 precursor gonadotrope cells and binds the HD element of the LHß promoter that is essential for both basal activity and LßT2 cell-specific expression. Although our data suggest that Ptx1, Ptx2, and Otx1 are not members of complex 2, this same evidence suggests that complex 2 contains a K50 HD protein related to the Otx family. Future work will aim at identifying this protein and examining its role in LHß gene expression, gonadotrope development, and normal reproductive function. Successful identification of this protein may also lead to a greater understanding of cell-type specification and maturation in the anterior pituitary.
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MATERIALS AND METHODS |
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The mouse Ptx1 and human Otx1 cDNAs were kindly provided by Dr. Jacques Drouin and Dr. Antonio Simeone, respectively. The Ptx1 and Otx1 cDNAs were amplified using Pfu DNA polymerase (Stratagene, La Jolla, CA) and PCR primers designed to the translational start and stop sites of each cDNA. The cDNAs were ligated into the TopoII vector using the TOPO TA cloning kit (Invitrogen, Carlsbad, CA) according to the manufacturers protocol. The sequences of the entire cDNAs were confirmed by automated sequencing performed by the Center for AIDS Research Molecular Biology Core at University of California, San Diego. The cDNAs were then recloned into the pcDNA3 expression vector using the EcoRI restriction sites of both plasmids. The RSV-Ptx2a expression vector was a gift from Dr. Jacques Drouin; the mouse Otx2 expression vector (pSG-mOtx2) has been previously described (44).
Plasmid DNA was prepared from overnight bacterial cultures using a cesium chloride protocol adapted from Sambrook et al. (45).
Cell Culture and Transient Transfections
Cells were grown in 60-mm diameter dishes to 5060% confluency in DMEM (Cellgro, Mediatech, Inc., Herndon, VA) supplemented with 10% FBS (Omega Scientific Inc., Tarzana, CA) and 1% penicillin-streptomycin (Life Technologies, Inc., Grand Island, NY) at 37 C with 5% CO2. Transient transfections were performed using FuGENE 6 transfection reagent (Roche Molecular Biochemicals, Indianapolis, IN), after the manufacturers protocol. The cells were harvested 48 h after transfection and Luc and ß-gal assays performed. Experiments were performed using 3 µg (Figs. 2 and 3
) or 2.5 µg (Fig. 4
) of the reporter plasmids. One-half microgram of RSV-ßgal was used as an internal control. All transfection experiments were performed at least three times.
Normalizing Transfection Data and Statistics
To control for differences in expression between the different cell types, each experiment was normalized. The RSV enhancer fused to the RSV promoter driving Luc (RSV-Luc) was transfected in triplicate in each experiment. The internal control, RSV enhancer and promoter fused to ß-galactosidase (RSV-ß-gal), was used as an internal control for each transfected plate of cells. The RSV-Luc values were divided by the RSV-ß-gal ß-gal values, averaged, and set equal to 100. The values for the other plates were normalized to this value in individual cell types; thus, the values from the individual cell types can be directly compared. The mean of at least three experiments is depicted. The error bars represent SEM. Normal or Box Cox Transformed ratios for each promoter construct in each cell type were compared by the ANOVA Factorial test, followed by the Tukey-Kramer HSD post hoc test. In all analyses, P 0.05 was considered significant.
Luc and ß-gal Assays
At harvest, cells were washed twice in 1x PBS and then 1 ml of harvesting buffer (0.15 M NaCl; 1 mM EDTA; 40 mM Tris-HCl pH 7.4) was added to each dish. Cells were scraped from the plate, transferred to 1.5-ml microcentrifuge tubes, and collected by centrifugation at 3000 rpm for 5 min. The harvesting buffer was discarded and the cells were resuspended in 50 µl of lysis solution (100 mM potassium phosphate (pH 7.8), 0.2% Triton X-100). Cells were spun at 14,000 rpm for 5 min and the supernatant was transferred to a new tube and assayed for Luc and ß-gal activity. Fifteen microliters of each sample was used per assay.
Luc activity was measured using an EG\|[amp ]\|G Berthold (Wildbad, Germany) Microplate Luminometer by injecting 100 µl per well of a buffer containing 100 mM Tris-HCl (pH 7.8), 15 mM MgSO4, 10 mM ATP and 65 µM of luciferin. ß-gal assays were performed using the Galacto-Light Plus Kit (Tropix, Bedford, MA) after the manufacturers protocol. Before each ß-gal assay, cell extracts were heat-inactivated at 48 C for 50 min. The Luc and ß-gal values for a mock-transfected plate of cells were subtracted from each transfected plate value. Luc values were divided by the ß-gal values to control for transfection efficiency and normalized to the ratio of RSV-Luc divided by RSV-ß-gal for the appropriate cell type as described earlier.
Mutagenesis
Mutagenesis of LHßLuc reporter genes was performed using the Transformer Site-Directed Mutagenesis Kit (CLONTECH Laboratories, Inc., Palo Alto, CA) according to the manufacturers protocol. Oligonucleotides used for mutagenesis were the following: 3' SF-1 site (5'-GCCTCTGCTTAGTGGAATTCCCACCCCCACAACCCGC-3'), 5' SF-1 site (5'-GCTGGTCCCTGGCTTTTCTGAAATTCTCTGTCTCGCCCCCAAAG-3'), 5' EGR-1 site (5'-CTGACCTTGTCTGTCTAGTACTCAAAGAGATTAGTGTCTAGG-3'), HD element mutant 4 (HDm4) (5'-CGCCCCCAAAGATCGTAGTGTCTAGGTTACCCAAGCC-3'), HD element mutant 5 (HDm5) (5'-CGCCCCCAAAGAGATGCTTGTCTAGGTTACCCAAGCC-3'). Mutagenesis of Otx1 and Ptx1 expression vectors was performed using the QuikChange Site-Directed Mutagenesis Kit (Stratagene) according to the manufacturers protocol. Oligonucleotides used for mutagenesis were the following: Otx1 K50Q (5'-GAGTCCAGGTCTGGTTCCAGAACCGCCGCGCCAAATGCC-3' (top strand)], Ptx1 K50Q (5'-CGAGTGCGGGTCTGGTTCCAGAACCGGCGAGCCAAATGG-3' (top strand). Mutated sequences (underlined) were confirmed by dideoxynucleotide sequencing (46).
Nuclear Extracts and EMSAs
Crude nuclear extracts were prepared by the method of Lee et al. (47). Glycerol was added to a final concentration of 20%. Nuclear extracts from NIH3T3 cells overexpressing Ptx or Otx family members were prepared 2 d after transient transfection of NIH3T3 cells with expression vectors coding for either Ptx1, Ptx2, Otx1, Otx2, Ptx1 K50Q, or Otx1 K50Q. NIH3T3 cells were grown on 100-mm dishes and transiently transfected with 5 µg of the appropriate expression vector using the FuGENE 6 transfection reagent (Roche Molecular Biochemicals, Indianapolis, IN), after the manufacturers protocol. Extracts were assayed for protein concentration using the Bio-Rad Laboratories, Inc. (Hercules, CA) protein assay reagent, aliquoted, and frozen at -80 C. For the EMSA, annealed oligonucleotides (1 pmol, Operon Technologies, Alameda, CA) were phosphorylated with [-32P]ATP (7000 Ci/mmol; ICN Biomedicals, Inc., Aurora, IL) and T4 Polynucleotide Kinase (New England Biolabs, Inc., Beverly, MA). The probes were phenol/chloroform extracted and passed over two Microspin G-25 columns (Amersham Pharmacia Biotech, Piscataway, NJ). Each binding reaction (20 µl) contained 2 fmol probe and 2 µg nuclear extract in 10 mM HEPES (pH 7.9), 50 mM KCl, 5 mM MgCl2, 0.25 mg BSA, 0.1% Nonidet P-40, 0.1 µg poly(deoxyinosine-deoxycytidine), and 5 mM DTT. Reactions were incubated for 5 min at room temperature after addition of probe, loaded onto a 17 cm x 30 cm 7% nondenaturing polyacrylamide gel acrylamide:bis (30:1); 0.25 x 130 mM Tris, 45 mM boric acid, 2.5 mM EDTA; and electrophoresed for 4 h at 250 V. After electrophoresis, gels were dried and subjected to autoradiography. In Figs. 4
and 710
, the probe was cut off for better viewing of the relevant protein-DNA complexes. For competition and antibody experiments, 100-fold excess unlabeled competitor oligonucleotide (200 fmol) or 0.5 µl antibody (Otx1) or 1 µl antibody (Ptx1, Ptx2, Oct-1, Pbx1/2/3, PREP-1) was added 20 min before addition of probe. Oligonucleotides used as probes and in competitions (Operon Technologies) are described in Table 1
(for Figs. 4
and 8
) or legend (Figs. 9
and 10
). The Ptx1 (48), Ptx2 (49), and Otx1 (29) antibodies were kindly provided by Dr. Jacques Drouin, Dr. Tord Hjalt, and Dr. Antonio Simeone, respectively. The Oct-1, Pbx 1/2/3, and PREP-1 antibodies were obtained from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA).
Western Blotting
Preparation of nuclear extracts has been described above. Fifteen micrograms of each nuclear extract were electrophoresed in a 10% SDS-polyacrylamide gel and transferred to Immobilon-P membrane (Millipore Corp., Bedford, MA). Ponceau S staining before immunoblotting was used to ascertain the efficiency of transfer. Membrane was blocked in PBS-T (PBS with 0.2% Tween-20) with 5% nonfat dry milk for 1 h and then incubated with the primary antibody for 1 h. The Ptx1 antibody (48) was diluted 1:2000 and the Ptx2 antibody (49) was diluted 1:100 in blocking buffer (PBS-T/5% milk). The membrane was washed three times for 5 min each with blocking buffer and then incubated with a horseradish peroxidase-labeled anti-rabbit secondary antibody (diluted in blocking buffer 1:2000 for Ptx1 and 1:1000 for Ptx2). The membrane was then washed 3 times for 5 min each before detection using enhanced chemiluminescence (Amersham Pharmacia Biotech). All washes and incubations were done at room temperature.
Northern Analysis
Total RNA was prepared using TRI REAGENT (Sigma, St. Louis, MO) and polyA+ RNA was extracted using PolyATract mRNA Isolation System IV (Promega Corp., Madison, WI) according to the manufacturers protocols. Two micrograms of each polyA+ RNA sample were electrophoresed in a denaturing 1% formaldehyde-agarose gel and transferred to Hybond N+ nylon membrane (Amersham Pharmacia Biotech) by capillary blotting. The RNA was fixed by UV cross-linking and then the membrane was hybridized overnight with 32P-labeled cDNA probe at 55 C in a 25% formamide solution. DNA probes were labeled by random priming (50). Excess probe was removed by washing at 65 C (Otx2) or 50 C (Otx1) with 0.2x SSPE and 0.1% SDS. The Otx1 probe consisted of a 396 nucleotide SacI-PstI fragment from the mouse Otx1 cDNA (41) and the Otx2 probe consisted of a 900 nucleotide EcoRI fragment of pSG-mOTX2 (44).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Abbreviations: Egr-1, Early growth response protein 1; ß-gal, ß-galactosidase; -GSU,
-glycoprotein hormone subunit; GnRH-R, GnRH receptor; HD, homeodomain; Luc, luciferase; Ptx1, pituitary homeobox 1 transcription factor; RSV, Rous sarcoma virus; SF-1, steroidogenic factor 1.
Received for publication November 2, 2001. Accepted for publication February 4, 2002.
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REFERENCES |
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