The pituitary gland is essential for maintaining homeostasis in vertebrates, and controls a variety of processes including growth, metabolism, reproduction, and stress. The human growth hormone (hGH-N) gene, expressed in pituitary somatotropes, is an excellent model system in which to study the molecular mechanisms that underlie the control of tissue-specific and developmental stage-specific gene expression. The hGH-N gene is a member of a five-gene cluster of paralogs expressed differentially in pituitary and placenta, under the dominant control of a complex distal locus control region (LCR), located 15 to 40 kb 5' to thehGH gene cluster. The LCR is necessary for both thetranscriptional activation of thehGH-N gene and the associated covalent modification of chromatin at the human growth hormone (hGH) locus during pituitary development in transgenic mouse models. The primary determinants of LCR activity are multiple binding sites for the homeodomain transcription factor Pit-1, which recruits histone acetyltransferase (HAT) activity to the hGH locus. This results in the establishment of a 32 kb domain of hyperacetylation of histones H3 and H4, characteristic of transcriptionally active chromatin, at the hGH locus. A major focus of my lab is to identify the components of these Pit-1-dependent complexes, specifically chromatin modifying activities, employing in vitro biochemical approaches and in vivointeraction screens, and to establish the role of these proteins in the molecular mechanisms of hGH gene activation. Of particular interest is the mechanistic basis for long-range gene activation in higher eukaryotes as exemplified by the effect of the hGH LCR on the expression of the hGH-N gene over 15 kb away. The broad goal of this research is to explore the various mechanisms by which genes are selectively activated and silenced, with a focus on epigenetic mechanisms.
In addition to answering fundamental questions, these studies will elucidate general mechanisms and themes involved in complex eukaryotic gene regulation pathways that may further our understanding of human diseases caused by epigenetic effects that result in defective gene regulation. Of particular interest is the mechanistic basis for the aberrant activation of the growth hormone gene that has been documented in certain cancers, including those of the breast, prostate and myeloid lineages. These tissue types express the GH receptor, suggesting the possibility of an autocrine regulatory loop that can augment the tumorigenic potential of these cells, given the major mitogenic and anti-apoptotic pathways stimulated by growth hormone. We are currently investigating the underlying cause of ectopic GH gene expression, and the effects of this expression on tumor cell proliferation and survival in breast and prostate cancer cell models.
Shewchuk, B., S. Liebhaber and N. Cooke. 2006. A single base difference between the Pit-1 binding sites at the hGH promoter and locus control region specifies distinct Pit-1 conformations and functions. Mol. Cell. Biol. 26:6535-6546.
Shewchuk, B., S. Liebhaber and N. Cooke. 2002. Specification of unique Pit-1 activity in the hGH locus control region. Proc. Natl. Acad. Sci. USA 99:11784-11789.
Shewchuk, B., N. Cooke and S. Liebhaber. 2001. The human growth hormone locus control region mediates long-distance transcriptional activation independent of nuclear matrix attachment regions. Nucleic Acids Research 16:3356-3361.
Shewchuk, B., S. Asa, N. Cooke and S. Liebhaber. 1999. Pit-1 binding sites in the somatotrope-specific HS I,II region of the hGH LCR are essential for in vivo hGH-Ngene activation. The Journal of Biological Chemistry 274:35725-33.
Shewchuk, B. and R. Hardison. 1997. CpG islands from the a-globin gene cluster increase gene expression in an integration-dependent manner. Molecular and Cellular Biology 17: 5856-5866.
James-Pederson, M., S. Yost, B. Shewchuk, T. Zeigler, R. Miller and R. Hardison. 1995. Flanking and intragenic sequences regulating the expression of the rabbit a-globin gene. The Journal of Biological Chemistry 270:3965-3973.
Yost, S., B. Shewchuk and R. Hardison. 1993. Nuclear protein-binding sites in a transcriptional control region of the rabbit a-globin gene. Molecular and Cellular Biology 13:5439-5449.
Assistant Professor of Biochemistry & Molecular Biology
The Brody School of Medicine at East Carolina University
Greenville, NC 27834