Evidence for epigenetic mechanisms that silence both basal and Immune-Stimulated transcription of the IL-8 gene

It is becoming increasingly clear that epigenetic silencing of gene transcr iption plays a critical role in the regulation of gene expression in many b iological processes. Tight regulation of immunomodulatory substances that a re important for the initiation of the inflammatory cascade, such as chemoa ttractive cytokines, is essential to prevent initiation of unrestrained imm une activation. Using the Caco-2 intestinal cell line as a model, we reveal two distinctly different mechanisms by which the gene for the neutrophil c hemoattractive cytokine IL-8 is silenced. Nuclear run-on studies, as well a s stably transfected reporter and marked minigene constructs, demonstrate t hat cellular differentiation inhibits immune-activated transcription of the IL-8 gene, a mechanism that is dependent on histone deacetylase activity. Unexpectedly, this silencing mechanism does not involve previously describe d regulatory elements in the IL-8 promoter but rather cis-acting regions lo cated at a distance from the IL-8 gene locus. Genomic elements distant to t he immediate IL-8 locus are also required to silence aberrant basal transcr iptional activity of the IL-8 promoter in the absence of immune activation. However, in this case, silencing occurs in a histone deacetylase-independe nt fashion. These findings were confirmed in transgenic mice in which, in t he absence of these elements, aberrant IL-8 gene activity was present prima rily in the intestinal tract. Epigenetic silencing of cytokine gene transcr iption through distant genomic elements is an important level of gene regul ation that may be relevant to the pathogenesis of immunologic disease state s.