Browsing Theses and Dissertations by Subject "Transcriptional Regulation"
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Distinct Mechanisms Regulate Induction of Stress Effector, gadd45bThe GADD45 family of proteins consists of three small nuclear proteins, GADD45A, GADD45B, and GADD45G, which are implicated in modulating the cellular response to various types of genotoxic/physiological stress. This family of proteins has been shown to interact with and modulate the function of cell-cycle control proteins, such as p21 and cdc2/cyclin B1, the DNA repair protein, PCNA, key stress response MAP kinases, including MEKK4 (an upstream regulator of JNK kinase), and p38 kinase. Despite similarities in amino acid sequence, structure and function, each gadd45 gene is induced differentially, depending on the type of stress stimuli. For example, the alkylating agent, methylmethane sulfonate (MMS), rapidly induces all three genes, whereas hydrogen peroxide and sorbitol preferentially induce gadd45a and gadd45b, respectively. Studies of the mechanisms of the stress-mediated induction of the gadd45 genes have predominantly focused on gadd45a, with knowledge of gadd45b and gadd45g regulation lacking. Thus, in order to generate a more complete understanding of the collective regulation of the gadd45 genes, a comprehensive analysis of the stress-mediated induction of gadd45b has been carried out. Towards this end, a gadd45b promoter-reporter construct was generated, consisting of 3897bp sequence upstream of the transcription start site of gadd45b, fused to a luciferase reporter. In a human colorectal carcinoma cell line (RKO), in which gadd45b mRNA levels profoundly increase by various stress stimuli, we observe similar, high levels of induction of the gadd45b-luciferase construct with MMS or UVC treatments, but surprisingly not with sorbitol or anisomycin. Linker-scanning mutagenesis of the gadd45b promoter reveals several important MMS and UVC cis-acting responsive elements contained within the proximal promoter, including a GC-rich region and the CCAAT box. Furthermore, we have identified three constitutively bound transcription factors, Sp1, MZF1, and NFY, and one inducible factor, Egr1, which bind to these regions and which contribute to MMS-responsiveness. In contrast, a post-transcriptional mechanism appears to regulate gadd45b induction upon sorbitol treatment, as this treatment increases the gadd45b mRNA half-life, compared to MMS treatment. Interestingly, with the exception of a common cis-element, the stress-mediated induction of gadd45b appears to be mechanistically distinct from gadd45a. In conclusion, this study provides novel evidence that gadd45b induction by distinct stress agents, MMS and sorbitol, is regulated differentially at the level of mRNA transcription or mRNA stability, respectively.
TRANSCRIPTIONAL REGULATION OF OSTEOACTIVIN EXPRESSION BY BMP-2 IN OSTEOBLASTSOsteoactivin (OA) is a glycoprotein required for the differentiation of osteoblasts. In osteoblasts, Bone Morphogenetic Protein-2 (BMP-2) activated Smad1 signaling enhances OA expression. However, the transcriptional regulation of OA gene expression by BMP-2 is still unknown. The aim of this study was to characterize BMP-2-induced transcription factors that regulate OA gene expression during osteoblast differentiation. The stimulatory effects of BMP-2 on OA transcription were established by cloning the proximal 0.96kb of rat OA promoter region in a luciferase reporter vector in various osteogenic cell types. Further, by deletion and mutagenesis analyses of the cloned OA promoter, key binding sites for osteogenic transcription factors namely, Runx2, Smad1, Smad4 and homeodomain proteins (Dlx3, Dlx5 and Msx2) were identified and characterized. Utilizing specific siRNAs to knock down Runx2, Smad1, Smad4, Dlx3, Dlx5 or Msx2 proteins in osteoblasts, we found that Runx2, Smad1, Smad4, Dlx3 and Dlx5 proteins up-regulate OA transcription, whereas, Msx2 down-regulated OA gene expression. These specific effects of transcription factors on OA promoter regulation were confirmed by forced expression of transcription factors. Most notably, BMP-2-stimulated cooperative and synergistic interactions between Runx2-Smad1 proteins and Dlx3-Dlx5 proteins that up-regulate OA promoter activity. Electrophoretic mobility shift and supershift assays demonstrated that BMP-2 stimulates interactions between Runx2, Smad1 and Smad4 and homeodomain transcription factors with the OA promoter regions flanking the -585 Runx2 binding site, the -248 Smad binding site and the region between the -852 and the -843 homeodomain binding sites relative to transcription start site. The OA promoter region was occupied by Runx2 and also Dlx3 transcription factors during proliferation stages of osteoblast differentiation. As the osteoblasts progress from proliferation to matrix maturation stages of differentiation, the OA promoter was predominantly occupied by Runx2 and to a lesser extent Dlx5 in response to BMP-2. Finally, during matrix mineralization stages of osteoblast differentiation, BMP-2-induced a robust recruitment of Dlx5, Smad1, Dlx3 and Msx2 proteins with simultaneous dissociation of Runx2 from the OA promoter region. In conclusion, the BMP-2-induced osteogenic transcription factors Runx2, Smad1, Smad4, Dlx3, Dlx5 and Msx2 provide key molecular switches that regulate OA transcription during osteoblast differentiation.