MinireviewMultiple mechanisms of growth hormone-regulated gene transcription
Introduction
Pituitary growth hormone (GH) has long been known as a major regulator of normal growth and metabolism [1], [2], [3]. Among its diverse actions, GH promotes statural growth in conjunction with insulin-like growth factor 1 (IGF-1) by stimulating chondrocytes in long bones [4], [5], [6], [7]. GH promotes a relative increase in lean body mass and decrease in body lipid, reflecting changes that include the ability of GH to increase cellular protein synthesis, stimulate lipolysis and impair lipogenesis under physiological conditions [8], [9]. GH excess can result in acromegaly and insulin resistance [8], [10], [11].
GH-regulated gene transcription underlies many of the diverse responses to GH (Fig. 1). These responses are initiated by the interaction of GH with the GH receptor, a member of the cytokine receptor superfamily [12], [13]. Janus kinase 2 (JAK2), a non-receptor tyrosine kinase, associates with dimerized GH receptors [14]. The activated JAK2 phosphorylates itself and the cytoplasmic domain of the GH receptor to initiate downstream signaling. Cytoplasmic signaling molecules, including signal transducers and activators of transcription (Stats), pathways mediated by mitogen activated protein kinases (MAPKs), and phosphatidyl inositol 3′ kinase (PI3K), relay GH signals to the nuclei of target cells to modulate gene transcription [12], [13], [15], [16].
Changes in gene transcription in response to GH occur at multiple levels, including post-translational modifications of nuclear proteins, formation of nucleoprotein complexes and cellular re-localization of factors that regulate transcription. While some aspects of these events have been analyzed for the signal transducers and activators of transcription (Stats), this review focuses on regulation of CCAAT/enhancer binding protein β (C/EBPβ), a transcription factor essential for GH-stimulated c-fos expression and serves to illustrate the complexity in the diverse mechanisms of gene regulation by GH.
Section snippets
GH regulates transcription by modifying the activity of multiple transcription factors
As part of the mechanism of gene regulation by GH, it has been amply demonstrated that Stat family members, especially Stat 5a and 5b, mediate the GH-dependent activation of a number of genes [15]. Interaction of GH with its receptors activates the tyrosine kinase Jak2, which phosphorylates and activates cytoplasmic Stats 1, 3, and 5. Phosphorylated Stats translocate to the nucleus and bind to DNA elements within GH target genes [17]. Stat 5b has been implicated in the transcription of multiple
GH regulates transcription factors through a variety of post-translational modifications
The demonstration that GH activates Stats by stimulating their tyrosine phosphorylation was a landmark in understanding GH signaling [35], [36], [38]. The importance of other signaling events in GH-stimulated transcription is demonstrated by observations of GH-regulated changes in the phosphorylation state of C/EBPβ (Fig. 2A). Isoelectric focusing identified at least six different phosphorylated forms of C/EBPβ that are regulated by GH in 3T3-F442A cells [57]. A MAPK substrate site at T235 in
GH regulates the composition of nucleoprotein complexes that mediate gene transcription
Our current understanding of transcription supports a mechanism in which binding of regulated sequence-specific transcription factors, such as the GH-regulated Stats and C/EBPβ, to their cognate response elements, provides a platform for formation of multiprotein complexes. The complexes often include factors such as co-activators, co-repressors, HATS, HDACs, chromatin remodeling factors and other proteins, which communicate signals from regulated factors to the general transcription machinery
GH regulates the cellular localization of transcription factors
The cellular localization of gene regulatory proteins is an important determinant of transcription. It is well established that activation of Stats 1, 3, and 5 by GH is accompanied by their translocation from the cytoplasm to nucleus, where they activate target genes [15]. In the case of C/EBPβ, its subnuclear localization appears to be a regulated event [81]. Immunofluorescence analysis of the nuclear localization of C/EBPβ revealed that GH dramatically shifts the distribution of C/EBPβ within
GH regulates transcription by multiple mechanisms
GH regulates transcription by multiple mechanisms involving a variety of post-translational modifications of transcription factors, dynamic assembly of nucleoprotein complexes and re-localization of transcription regulatory proteins in target cells. These mechanisms apply not only to Stat-dependent effects of GH, but are also involved in the similarly complex C/EBPβ-dependent transcription of GH-stimulated target genes. The following model is suggested for C/EBPβ-mediated transcription of
Acknowledgments
These studies were supported by NIH Grant DK46072 and NSF Grant 00-80193 to J. Schwartz, by NIH Grant 5P60 DK20572 to R. Kwok, NIH Grant DK061656 to J. Iñiguez-Lluhí. This work used multiple core facilities of the Michigan Diabetes Research and Training Center funded by NIH (5P60DK20572). T. Cesena was supported by an individual NIH predoctoral fellowship (DK074377), a predoctoral traineeship from the Center for Organogenesis (NIH T32-HD07505), the Training Program in Cellular and Molecular
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These authors contributed equally to this work.
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Present address: Neuroscience Program, University of Michigan, Ann Arbor, MI 48109, USA.