Down-regulation of WNK1 protein kinase in neural progenitor cells suppresses cell proliferation and migration.

WNK1, a Ser/Thr protein kinase, is widely expressed in many tissues. Its biological functions are largely unknown. Disruption of the WNK1 gene in mice leads to embryonic lethality at day 13, implicating a critical role of WNK1 in embryonic development. To investigate this potential function, we used antisense strategy to knock down the expression of WNK1 in a mouse neural progenitor cell line, C17.2. Down-regulation of WNK1 in C17.2 cells greatly reduced cell growth. Addition of epidermal growth factor (EGF), a mitogen for C17.2 cells, had no effect on growth. The WNK1-knockdown cells showed a flat and rounded morphology, characteristic of the immature and non-differentiated phenotype of the progenitor cells; this was further demonstrated by immunostaining for the progenitor and neuronal markers. Migration of the WNK1-knockdown C17.2 cells was reduced as tested in culture dishes or Matrigel-covered chambers. Moreover, activation of extracellular signal-regulated kinase (ERK1)/2 and ERK5 by EGF in the WNK1-knockdown cells was suppressed. These results demonstrate a novel function of WNK1 in proliferation, migration, and differentiation of neural progenitor cells, likely by mechanisms involving activation of the mitogen-activated protein (MAP) kinase ERK1/2 and/or ERK5 pathways.

Sp1 and AP2 enhance promoter activity of the mouse GM3-synthase gene.

Promoters of the glycosyltransferase genes for ganglioside synthesis are TATA-less and often have multiple binding sites for transcription factors Sp1 and AP2 in their proximal regions. However, the function of Sp1 and AP2 in the promoters has not yet been defined. Here, we cloned 5'-flanking fragments of the mouse GM3-synthase gene and assessed the promoter activity of these fragments in mouse Neuro-2a cells. This promoter is TATA-less and contains a number of potential transcription factor-binding sites. Multiple putative transcriptional initiation sites for this gene were identified, including several downstream initiation sites. We then set out to dissect the regulatory elements important for GM3-synthase promoter function. We found that a 5'-flanking 254-bp DNA fragment of the gene contained regulatory elements including two Sp1-binding and six AP2-binding sites that were essential for the basal activity of the promoter in mouse Neuro-2a cells. The effects of the individual Sp1- and AP2-binding sites on basal activity of the GM3-synthase gene were investigated. Mutations in the juxtaposed Sp1/AP2-binding site and in an AP2-binding site decreased the activity of the proximal promoter to approximately 50%. In vitro and in vivo interactions between transcription factors Sp1 and AP2 and these regulatory elements were confirmed by EMSA and the chromatin immunoprecipitation approach, respectively. Therefore, our results demonstrate that Sp1 and AP2 enhance the basal activity of the TATA-less mouse GM3-synthase promoter.

Expression of the mouse WNK1 gene in correlation with ganglioside GD3 and functional analysis of the mouse WNK1 promoter.

WNK1 is one of WNK (With No K=Lysine) protein kinases which comprise a newly described subfamily. Our studies showed that expression of the mouse WNK1 gene was dramatically suppressed in a tumor cell line when its phenotype was altered by suppression of the GD3-synthase gene expression. The mouse WNK1 gene was expressed at a high level at early stage of embryonic brain and its expression decreased as brain developed, similar to the expression pattern of the GD3-synthase gene. To study transcriptional regulation, we cloned a 5'-flanking 1239-bp fragment of the mouse WNK1 gene. This fragment contains a number of potential consensus binding sites for transcription factors, including Sp1, AP2, CCAAT, Est-1, Oct-1, CNBP, and NFkB, but lacks a TATA box. Primer extension identified multiple putative transcriptional initiation sites, including several sites downstream of the ATG codon. Activities of the promoter fragments were assessed in mouse breast Sa/R-MT cells by transient transfection and the results showed that the promoter elements between -700 and -977 is required for maintaining a high level of promoter activity of the TATA-less mouse WNK1 gene.

Characterization of the promoter and the transcription factors for the mouse UDP-Gal:betaGlcNAc beta1,3-galactosyltransferase gene.

Galbeta1-3Gal-NAcbeta1-4Gal(3-2alphaNeuAc)beta1-4Glcbeta1-1Cer (GM1) is one of the most extensively investigated gangliosides that plays critical roles in the development and functions of the nervous system. UDP-Gal:betaGlcNAc beta1,3-galactosyltransferase (Gal-T-II) is responsible for synthesis of ganglioside GM1 in the ganglioside biosynthetic pathway. To understand the transcriptional regulation of Gal-T-II gene expression, we cloned a 1448 bp 5'-flanking fragment from the mouse Gal-T-II gene. The transcriptional activity of the fragment was demonstrated in mouse Neuro-2a cells by a luciferase assay. The proximal 550 bp fragment showed the highest transcriptional activity as determined by a series of 5'-truncated constructs of the promoter. One negative regulatory region was also identified. Primer extension assay revealed a transcription initiation site approximately 242 bp upstream from the ATG translation start codon. Analysis of the promoter sequence revealed a number of potential binding sites for known transcription factors. To determine which transcription factors bind to the promoter, we carried out a systematic search for the binding proteins using the 1142 bp Gal-T-II promoter fragment containing both positive and negative regulatory regions in a combination of DNA pull-down assay and transcription factor array analysis. Twenty-seven transcription factors bound to consensus sites in the promoter region. In addition, four other factors without consensus binding sites in this region were also recruited, possibly as components of transcription factor complexes. These data indicate that the basal regulation of Gal-T-II gene transcription involves multiple transcription factors, some of which may be present in complexes.

Characterization of the 5'-flanking fragment of the human GM3-synthase gene.

To investigate the transcriptional regulation of human GM3-synthase, a 5'-flanking fragment of 1379 bp was cloned by a PCR-based procedure. Analysis of the human genomic sequence showed that the gene consists of seven exons, locates at chromosome 2, and spans over 62 kb. There are a number of potential consensus binding sites in the cloned promoter region, but TATA and CCAAT boxes were not found in the promoter. Primer extension analysis identified two transcription start sites approximately 11 and 57 bp upstream of the exon 1. The transcription activity of the promoter was assessed in human HeLa cells by transient transfection. Of the fragments assayed, the proximal 409 bp fragment exhibits the highest transcription activity. Transcription factors that bound to the 409 bp fragment were pulled down by DNA-coupled magnetic beads. Identities of the pull-down proteins were determined by array analysis. Eight transcription factors were identified, which might either bind to the proximal region or be recruited as co-activators of the transcription factor complexes.

Variations in gene expression patterns correlated with phenotype of F-11 tumor cells whose expression of GD3-synthase is suppressed.

Alteration in ganglioside composition in F-11 cells by suppression of GD3-synthase gene expression resulted in greatly reduced tumor growth and metastasis when the cells were injected into nude mice. To identify genes whose expression is correlated with the decreased level of ganglioside GD3, we analyzed gene expression profiles of the GD3-suppressed F-11 cells and the control F-11 cells using DNA microarrays. We identified a set of GD3-related genes, most of which are involved in tumor growth and development. The genes that define the proliferation-transformation signature are down-regulated, such as creatine kinase-B (CKB), upstream stimulation factor 1 (USF-1), type II cAMP-dependent protein kinase regulatory subunit (RII PKA), and tyrosine hydroxylase (TH). On the other hand, the genes that define the differentiation-reverse transformation signature are up-regulated, including p160 myb-binding protein (P160), brain factor-2, insulin-like growth factor-binding protein (IGFBP), and growth/differentiation factor 11. Transcriptional levels of the genes that showed the most distinct GD3-related expression change were validated by reverse transcription-polymerase chain reaction (RT-PCR). Defining GD3-related genes may lead to identification of clinically relevant therapeutics and to understanding of the mechanism(s) by which ganglioside GD3 affects tumor growth and metastasis.