Interleukin-6 repression is associated with a distinctive chromatin structure of the gene

Expression of the interleukin-6 (IL-6) gene is usually tightly controlled a nd may be induced in specific tissues only after treatment with appropriate stimuli. The molecular mechanisms responsible for IL-6 gene repression in specific tissues or cell lines remain poorly defined. In order to address t his question we have studied two human breast carcinoma cell lines, MDA-MB- 231, in which the IL-6 gene is expressed, and MCF-7, in which it is not. Th e promoter region of the IL-6 gene was analysed in both cell lines with ref erence to two different parameters: (i) DNase I hypersensitivity; (ii) the in vivo pattern of DNA-protein interactions. We show herein that the mechan ism responsible for silencing IL-6 gene expression in MCF-7 cells most prob ably involves a modification of chromatin structure, as suggested by a decr eased sensitivity of the IL-6 promoter to DNase I relative to the IL-&6-xpr essing cell line MDA-MB-231. Moreover, we show that a 'closed' nucleosomal structure in MCF-7 cells does not inhibit the binding of nuclear proteins t o IL-6 gene regulatory sequences in vivo, We suggest, therefore, that, in n on-expressing cells, local chromatin remodelling at the proximal promoter i s inhibited by negative regulators, as suggested by two specific hallmarks of nuclear factor binding that are not observed in expressing cells: an add itional in vivo footprint spanning positions -135/-119 and an additional DN ase I hypersensitive site far upstream, around position -1400. Furthermore, a specific factor binding in vitro to the -140/-116 region of the IL-6 pro moter is found in MCF-7 cells.