Rac1 attenuates macrophage MMP‐9 gene expression via inhibition of SP‐1 at S586

Our previous observations suggest that macrophage‐derived matrix metalloproteinase‐9 (MMP‐9) attenuates asbestos‐induced pulmonary fibrosis in Rac1 null mice. We found that Rac1 null macrophages have a high level of MMP‐9 gene expression which was due, in part, to increased SP‐1 transcriptional activity. In addition to S7, that is known to play a role in SP‐1 stability, we have identified a PEST domain (575–595) in SP‐1 with a score of 6.1 containing one threonine (T578) and two serine residues (S586 and S587). As ERK, a serine/threonine kinase, is constitutively active in Rac1 null cells, we hypothesized that SP‐1‐driven MMP‐9 gene expression is regulated by Rac1 at the threonine and/or serine residues. Transfection of macrophages with SP‐1 mutated at these sites revealed that, compared to empty vector, constitutively active Rac1 suppressed MMP‐9‐luciferase in cells overexpressing WT SP‐1 or SP‐1 mutants S7A, T578A and S587A but not in cells expressing the S586A mutant. Taken together these novel observations suggest that SP‐1 is a critical target of ERK in the regulation of MMP‐9 gene expression in the absence of Rac1 activity. This work was supported by NIH grants ES‐015981 and ES014871.