Transcription of major histocompatibility complex (MHC) class I genes is regulated by both tissue-specific (basal) and hormone/cytokine (activated) mechanisms. Although promoter-proximal regulatory elements have been characterized extensively, the roles of the core promoter and downstream elements in mediating regulation have been largely undefined. Basal and activated transcriptions of an MHC class I gene target distinct core promoter domains, nucleate distinct transcription initiation complexes and initiate at distinct sites within the promoter. Basal and activated transcription pathways recruit distinct transcription factor complexes to the core promoter elements and target distinct transcription initiation sites. Basal transcription is completely dependent upon the general transcription factor TAF1 whereas activated transcription is TAF1 independent. To further characterize regulation of class I gene expression, we have undertaken to characterize core promoter elements in vivo and to identify novel downstream promoter elements. To understand these mechanisms in more detail, we have characterized elements within the core promoter of the MHC class I gene. The minimal class I core promoter has been localized to a segment between -50 bp and +14 bp. Within this segment are sequences similar to canonical TATA and Inr promoter elements and an Sp1 binding site; however, no single element is necessary for transcription, although each provides some level of regulation in different cell types. For instance, the Inr element is a negative regulator in fibroblasts, but a positive regulator in T and B cell lines. Conversely, the TATA-like element is a positive regulator in T cell lines, but a negative regulator in B cell and fibroblast lines. Importantly, the roles of these core promoter elements has been examined and verified in vivo in transgenic mice bearing MHC class I genes mutated in the relevant promoter elements. Of particular interest, the downstream region of the MHC class I promoter region, between +1 and +32 bp, contains two novel regulatory elements. Under constitutive conditions, the two elements act in concert to down-regulate promoter activity, preferentially suppressing the use of TSS located at the 5 end of the cluster of multiple start sites. These elements only repress constitutive, TAF1-dependent transcription. Under activated TAF1-independent conditions, one element functions independently as an enhancer. Thus, the downstream regulatory elements associated with the class I promoter function to fine-tune and integrate both intracellular and extracellular signaling pathways to ensure the appropriate level of MHC class I transcription to maintain immune homeostasis. These novel downstream elements are functionally and mechanistically distinct from previously described downstream elements in mammalian promoters. Finally, novel regulatory elements that serve to establish tissue specific patterns of regulation have been identified by in vivo analysis and localized to intronic regions.

Agency
National Institute of Health (NIH)
Institute
National Cancer Institute (NCI)
Type
Intramural Research (Z01)
Project #
1Z01BC009279-23
Application #
7732943
Study Section
Project Start
Project End
Budget Start
Budget End
Support Year
23
Fiscal Year
2008
Total Cost
$362,193
Indirect Cost
Name
National Cancer Institute Division of Basic Sciences
Department
Type
DUNS #
City
State
Country
United States
Zip Code
Howcroft, T Kevin; Weissman, Jocelyn D; Gegonne, Anne et al. (2005) A T lymphocyte-specific transcription complex containing RUNX1 activates MHC class I expression. J Immunol 174:2106-15
Lee, Maxwell P; Howcroft, Kevin; Kotekar, Aparna et al. (2005) ATG deserts define a novel core promoter subclass. Genome Res 15:1189-97
Mozes, E; Lovchik, J; Zinger, H et al. (2005) MHC class I expression regulates susceptibility to spontaneous autoimmune disease in (NZBxNZW)F1 mice. Lupus 14:308-14
Grassadonia, Antonino; Tinari, Nicola; Fiorentino, Bruno et al. (2004) The 90K protein increases major histocompatibility complex class I expression and is regulated by hormones, gamma-interferon, and double-strand polynucleotides. Endocrinology 145:4728-36
Howcroft, T Kevin; Raval, Aparna; Weissman, Jocelyn D et al. (2003) Distinct transcriptional pathways regulate basal and activated major histocompatibility complex class I expression. Mol Cell Biol 23:3377-91
Howcroft, T Kevin; Singer, Dinah S (2003) Expression of nonclassical MHC class Ib genes: comparison of regulatory elements. Immunol Res 27:1-30