Proper regulation of the CC chemokine MCP-1 (monocyte chemoattractant protein-1) is important for normal inflammatory responses. MCP-1 is regulated by a wide variety of agents, including platelet-derived growth factor-BB (PDGF-BB) and tumor necrosis factor-␣ (TNF).Using both in vivo and in vitro assays, the elements required for expression between these two cytokines were compared. In vivo genomic footprinting showed that PDGF-BB induction occurred through the occupancy of the proximal regulatory region, and unlike TNF induction, no changes in the NF-B binding, distal regulatory region occurred. Treatment of cells with trans-retinoic acid, an inhibitor of PDGF-BB activity, resulted in a 50% reduction in PDGF-BB-mediated induction and a concomitant block in the assembly of the proximal regulatory region. trans-Retinoic acid had minimal effect on TNF induction or promoter occupancy. An inhibitor of histone deacetylation was found to stimulate expression of MCP-1 in a manner that correlated with increased accessibility to the proximal regulatory region. These results show that the mechanisms of PDGF-BB and TNF activation of MCP-1 are distinct, although they both require the proximal regulatory region Sp1 binding site. The results also suggest that part of the mechanism used by both of these cytokines involves a process that regulates transcription factor access to the regulatory regions.MCP-1 (monocyte chemoattractant protein-1), a member of the CC chemokine superfamily, functions in attracting monocytes, T lymphocytes, and basophils to sites of inflammation (1-3). The expression of MCP-1 is important for antibacterial responses, antitumor immunity, and other normal immune reactions. Aberrant expression of MCP-1 is associated with glomerular disease (4), allergic and chronic inflammatory diseases (5-7), HIV replication (8, 9), and the pathogenesis of atherosclerosis (10). MCP-1 expression is transcriptionally regulated. A variety of factors, which include tumor necrosis factor-␣ (TNF) 1 (11), platelet-derived growth factor (PDGF-BB) (12), interferon-␥ (13, 14), stress factors (15-17), and viral infection (18), induce MCP-1 transcription. In contrast, retinoic acid, glucocorticoids, and estrogen inhibit induced MCP-1 expression in certain cell lines (19 -22). Elucidating the mechanisms regulating MCP-1 expression is important for understanding the interplay between this chemokine and immune responses and for providing possible approaches to block inflammation. Toward this goal, we and others have identified several regulatory regions that are important for MCP-1 expression (11,(23)(24)(25)(26). Using in vivo genomic footprinting (IVGF) protocols to map regions of factor occupancy and changes in factor occupancy that result from TNF induction in vivo, a distal and a proximal regulatory region separated by 2.1 kilobase pairs of DNA were described (11). The distal regulatory region contains four elements: site A, B1, B2, and HS. Site A, the most 5Ј of these sites, was constitutively occupied in vivo and required for ma...