Identification and characterization of genes whose expressions are altered in rat 6 fibroblasts transformed by mutant p53(val135)

Genomic structure, alternative splicing and tissue expression of rFrp/sFRP-4, the rat frizzled related protein gene

Secreted frizzled related proteins (sFRP) are regulators of Wnt signaling pathways that play central roles in developmental processes and oncogenesis. Various sFRP genes have been cloned from different tissues and implicated in diverse biological activities. rFrp, the rat homologue of sFRP-4, was initially identified as being upregulated in mutant p53-induced cellular transformation. Here, we report on the isolation of five novel splice variants, rFrp/sFRP-4 II, II, III, IVa and IVb. The complete rFrp/sFRP-4 genomic structure spans over 31 kb covering 9 exons. Except for the variant IVb, which was derived from IVa by alternative polyadenylation signal, variants I to IVa were alternatively spliced to different exons in the 3'end of mRNA and resulted in transcripts with truncated open reading frame. The deduced proteins of the variants had truncated C-termini, however, the two key functional protein domains, the cysteine-rich domain and the netrin-like domain of the isoforms, were not altered. In addition, different transcriptional initiation sites were found with variants II and IV, implying that these variants may be regulated differently from the rFrp/sFRP-4. RT-PCR analysis showed that these splice variants displayed different patterns of tissue-specific expression. Northern blot analysis revealed that the rFrp/sFRP-4 is most abundant in the ovary. Taken together, our findings suggest that alternative splicing of rFrp/sFRP-4 plays a role in regulating tissue-specific expression. The truncated C terminals of rFrp/sFRP-4 variants may confer structural specificity and hence exert different biological functions in different tissues. Characterization of these novel splice variants should help to elucidate the function of the sFRP family gene.

Cloning and characterization of the promoter region of the mouse frizzled-related protein 4 gene

Frizzled-related protein (Frp) is a newly identified family of secreted proteins involved in the Wnt signaling pathway. To date, little is known about the underlying mechanisms regulating Frp expression. In this study the promoter region of mouse frizzled related protein 4 (sFrp4) gene was cloned, sequenced, and analyzed using transient reporter assays along with site-directed mutagenesis. Two clusters of cis-acting elements, STAT3/Lyf-1/MZF1 (site 1) and C/EBP-beta/ GATA-1/CREB (site 2) located in the promoter region from -238 to -144 were found to be essential for the promoter activity of sFrp4. In addition to sites 1 and 2, putative transcriptional factor binding sites for TFIID, SP1/GC and ATF/CREB exhibited positive, while the site for NRSE exhibited negative regulatory functions, as determined by the alkaline phosphatase activities of the reporter assay. We also demonstrate that the ATF/CREB site may cooperatively interact with the NRSF-like element in regulating sFrp4 promoter activity. The data of our study, which is the first promoter analysis of mouse Frp genes, provide the basis for understanding the functions and the regulation of Frp and its role in regulating Wnt signals.

Transcriptional regulation of the promoter of the rat frizzled related protein gene by CREB

Frizzled related proteins (Frps) are secreted proteins structurally similar to frizzled receptors; they bind Wnt via the cysteine-rich domain and antagonize the Wnt signaling pathway. In this study, we have investigated the mechanisms regulating the transcriptional regulation of rat Frp (rFrp) promoter. From previous findings, we know that the transcriptional activation domain of rFrp resides in the region -202 to -144 relative to the transcription start site, and that it is essential for efficient promoter activity. The study presented here was designed to identify trans-acting factors that bind to this critical domain of the rFrp promoter and to elucidate the pathway involved in the regulation of rFrp expression. Electrophoretic mobility shift assay (EMSA) demonstrated that specific DNA-protein binding activities fall into two adjacent core sequences with (CTTTGGGGG) at -197 to -189 and (AGATGATGTAA) at -151 to -141 of the rFrp promoter. Reporter assay showed that these core sequences are both required for the activation of rFrp promoter. Mutation within either one or both core sequence drastically reduced the promoter activity. Southwestern blotting showed that the estimated molecular mass of the distinct binding protein to the (AGATGATGTAA) domain is about 43 kDa. Further EMSA suggested CREB as the trans-acting factor in the DNA-protein complex, which was out competed by CREB consensus oligonucleotides and supershifted by anti-CREB antibody. Overexpression of PKA and CREB also transactivated rFrp promoter, and dominant-negative CREB inhibited the promoter activity in transient reporter assays. More importantly, CREB, phosphorylated CREB and the adaptor protein CBP were found binding to the endogenous rFrp promoter using chromatin immunoprecipitation assay. Collectively, our results demonstrate the induction of rFrp promoter activity by PKA and CREB in vitro, and the binding of CREB and CBP to the rFrp promoter core motif in vivo.

Induction of nonmuscle myosin heavy chain II-C by butyrate in RAW 264.7 mouse macrophages

RAW 264.7 macrophages express nonmuscle myosin heavy chain II-A as the only significant nonmuscle myosin heavy chain isoform, with expression of nonmuscle myosin heavy chain II-B and II-C low or absent. Treatment of the cells with sodium butyrate, an inhibitor of histone deacetylase, led to the dose-dependent induction of nonmuscle myosin heavy chain II-C. Trichostatin A, another inhibitor of histone deacetylase, also induced nonmuscle myosin heavy chain II-C. Induction of nonmuscle myosin heavy chain II-C in response to these histone deacetylase inhibitors was attenuated by mithramycin, an inhibitor of Sp1 binding to GC-rich DNA sequences. Bacterial lipopolysaccharide alone had no effect on basal nonmuscle myosin heavy chain II-C expression, but attenuated butyrate-mediated induction of nonmuscle myosin heavy chain II-C. The effects of lipopolysaccharide were mimicked by the nitric oxide donors sodium nitroprusside and spermine NONOate, suggesting a role for nitric oxide in the lipopolysaccharide-mediated down-regulation of nonmuscle myosin heavy chain II-C induction. This was supported by experiments with the inducible nitric-oxide synthase inhibitor 1400W, which partially blocked the lipopolysaccharide-mediated attenuation of nonmuscle myosin heavy chain induction. 8-Bromo-cGMP had no effect on nonmuscle myosin heavy chain induction, consistent with a cGMP-independent mechanism for nitric oxide-mediated inhibition of nonmuscle myosin heavy chain II-C induction.

P53-responsive genes and the potential for cancer diagnostics and therapeutics development

P53 protein regulates cell responses to DNA damage to keep genomic stability by transactivation and trans-repression of its downstream target genes. P53 protein also has activators, inactivators, or co-factors via interaction with other proteins. Both the p53-regulated genes and interacted proteins form a huge network. As tumors usually escape from proliferating controls by means of accumulation of genetic alterations, p53 is one of the most important tumor suppressor genes that can be targeted for diagnosis, prognosis, and therapeutic intervention. Reviewing the p53-network is of great importance. In this review, we are focusing on cancer-related p53 downstream-regulated genes. Various methods dealing with the discovery of p53-regulated genes by the detection of gene expression have been applied. Recently high throughput functional genomics methods, such as DNA microarray, serial analysis of gene expression (SAGE), differential display, and protein two-dimensional gel electrophoresis, have provided a wealth of information on the dynamics of cell context responses. Hundreds of genes have been discovered whose transcriptions are regulated by p53 protein. They were grouped, based on their functions, into sub-classes including cell-cycle regulation, DNA repair, angiogenesis, metastasis, and multidrug resistance. P53 plays a pivotal role in keeping genomic stability and tumor suppression. The deeper we investigate the cell responses as mediated by p53, the more complex p53-network becomes. However, understanding p53-network, offers great opportunities to develop more sensitive and accurate diagnostic/prognostic tools, as well as more efficient therapies for cancer.

Expression of frizzled-related protein and Wnt-signalling molecules in invasive human breast tumours

Frizzled-related protein (Frp) is a new family of secreted proteins that contain a region homologous to the extracellular cysteine-rich domain (CRD) of the frizzled family proteins. The role of Frp protein is far from clear. To explore the role of Frp and its relationship to the Wnt-signalling pathway in breast cancer, in situ hybridization and immunohistochemical analyses of Frp, Wnt-1, APC, beta-catenin, and its target genes c-myc and cyclin D1 were conducted in 70 specimens of invasive ductal carcinomas of the human breast. Frp mRNA was down-regulated in 62 and elevated in eight tumour specimens, compared with adjacent normal tissues. In the course of tumour progression, however, Frp mRNA steadily increased in both tumour and the adjacent tissues. Interestingly, the number of cases with axillary lymph node metastasis was significantly lower in the group with elevated Frp than in the group with decreased Frp, suggesting that Frp may contribute as a prognostic factor in invasive breast cancer. Wnt-1, a gene implicated in human breast cancer, was markedly elevated in grade 1 tumours, but declined as tumour grade declined. The level of Wnt-1 was linearly correlated with its downstream target beta-catenin (p<0.05), but was inversely correlated with Frp (p<0.05), suggesting a possible negative regulatory role of Frp with regard to Wnt-1. APC was inversely correlated with beta-catenin (p<0.05). Beta-catenin, a key transcriptional activator responsible for the activation of both c-myc and cyclin D1 in colorectal tumours, was detected at high levels in the plasma membranes of cells in normal tissue. In tumour masses, however, beta-catenin lost its tight association with the membrane and diffused into the cytoplasm. Surprisingly, it clearly did not penetrate the nuclei, despite the fact that both c-myc and cyclin D1 were markedly elevated in all tumour tissues. As revealed in this study, Wnt-1/beta-catenin plays very different roles in the oncogenesis of breast and colon cancers. This first systemic analysis of the Frp and the Wnt-signalling pathway in human breast cancer provides a springboard for further work on the role of Frp in the development of breast cancer.

Transcriptional activity of the promoter region of rat frizzled-related protein gene

Frizzled-related protein (Frp) is a new family of secreted proteins involved in tumorigenesis and Wnt-signaling pathway. Previous study has shown that rat Frp (rFrp) gene was found to be differentially expressed in Rat 6 fibroblast cell line overexpressing p53(val135) (R6#13-8). The rFrp gene was otherwise silent in normal parental Rat 6 cells. To elucidate the molecular basis of the transcriptional activation of rFrp, we have isolated and analyzed a 2-kilobase pair promoter region of the rFrp gene. Mapping of transcription initiation sites of rFrp showed the existence of multiple initiation sites. Transfection studies of serial deletion constructs in both Rat 6 and CHOK1 cell lines revealed that the region from -202 to -144 contains cis-acting elements essential for the efficient transcription of rFrp. This work provides a transcriptional regulation basis for Frp and gives insight into its implication in tumorigenesis.

Incorporating the ABI GeneScan analysis to a RACE-based technique for mapping multiple transcription initiation sites

Determination of transcription initiation sites has commonly been performed by primer extension and RNase protection assay using radioactively labeled oligonucleotides. Recently, a protocol based on modified 5' rapid amplification of cDNA ends (RACE) with the use of fluorescently labeled primer was developed. Here, we describe the use of RACE-based technique in conjunction with the GeneScan analysis for the determination of transcription initiation sites of genes of interest. The RACE technique is based on the ligation of an adapter to both ends of the cDNAs. The gene of interest was first amplified by PCR using a gene-specific and a 5' adapter primer. Subsequently, nested PCR was performed using an internal gene-specific primer paired with a fluorescently end-labeled adapter primer. The size of the fluorescently labeled PCR products was directly determined by the ABI PRISM 377 GeneScan Analyzer. This novel approach provides an accurate, sensitive, and convenient method for mapping transcription initiation sites, especially for genes with multiple transcription initiation sites, for genes expressed at low levels, and for splice variants that display alternative splicing farther than a few hundred nucleotides downstream from the transcription initiation site. This article describes the application of this new method in the mapping of transcription initiation sites of two splice variants of rat frizzled related protein transcripts.

Suppression of the tumorigenicity of mutant p53-transformed rat embryo fibroblasts through expression of a newly cloned rat nonmuscle myosin heavy chain-B

In our previous study, a rat homolog of human nonmuscle myosin heavy chain-B (nmMHC-B) was identified by mRNA differential display comparing of transformed against nontransformed Rat 6 cells overexpressing mutant p53val135 gene. The nmMHC-B was found to be expressed in normal Rat 6 embryo fibroblast cell line, but markedly suppressed in the mutant p53val135-transformed Rat 6 cells. To examine the possible involvement of nmMHC-B in cell transformation, we first cloned and sequenced the full length cDNA of rat nmMHC-B, which was then cloned into an ecdysone-expression vector. The resulting construct was introduced into the T2 cell line, a mutant p53val135-transformed Rat 6 cells lacking the expression of the endogenous nmMHC-B. The clonal transfectants, expressing muristerone A-induced nmMHC-B, displayed a slightly flatter morphology and reached to a lower saturation density compared to the parental transformed cells. Reconstitution of actin filamental bundles was also clearly seen in cells overexpressing the nmMHC-B. In soft agar assays, nmMHC-B transfectants formed fewer and substantially smaller colonies than the parental cells in response to muristerone A induction. Moreover, it was strikingly effective in suppressing the tumorigenicity of the T2 cells when tested in nude mice. Thus, the nmMHC-B, known as a component of the cytoskeletal network, may act as a tumor suppressor gene. Our current finding may reveal a novel role of nmMHC-B in regulating cell growth and cell signaling in nonmuscle cells. Oncogene (2001) 20, 58 - 68.

Molecular cloning and functional analysis of the promoter region of rat nonmuscle myosin heavy chain-B gene

Rat nonmuscle myosin heavy chain-B (r-nmMHC-B) mRNA was previously found downregulated in Rat 6 fibroblasts transformed by mutant p53(val135) [J. W. P. Yam, J. Y. Zheng, and W. L. W. Hsiao (1987) Biochem. Biophys. Res. Commun. 266, 472-480]. Overexpression of exogenous r-nmMHC-B could partially reverse the transforming phenotypes both in vitro and in vivo. The downregulation of r-nmMHC-B was also observed in Rat 6 transformed by c-H-ras and v-myc oncogenes. We cloned a 5.2-kb r-nmMHC-B promoter region. Sequence analysis of -1248 to +1 revealed no TATA box, but did show that it contained CAAT boxes, E12/E47, MyoD, MEF, E2F, CREB, and SP1 binding sites. Based on transient reporter assays, the promoter/enhancer activities were unusually extended to the entire 5.2 kb region in normal Rat 6 cultures, but markedly suppressed in p53(val135)-, and c-H-ras-transformed cells. The activity detected by the reporter assay corresponded to levels of mRNA as analyzed previously by Northern blots in each respective cell line. Thus, the switch-off of the r-nmMHC-B in the transformed cells is very likely controlled by upstream transcriptional factors, which might have been altered in the course of neoplastic transformation.