Inducible expression of manganese superoxide dismutase by phorbol 12- myristate 13-acetate is mediated by SP1 in endothelial cells

Abstract

The expression of manganese superoxide dismutase (Mn-SOD), an important component of the cellular defense system against oxidative stress, is induced in response to a variety of stimuli, including cytokines and phorbol esters, in endothelial cells. To define the molecular mechanisms regulating the expression of Mn-SOD, we have characterized the promoter of the human Mn-SOD gene. In calf pulmonary artery endothelial cells, phorbol 12-myristate 13- acetate (PMA) gradually increased Mn-SOD mRNA levels, with a peak at 6 to 12 hours after stimulation. The increase in Mn-SOD mRNA was significantly inhibited by a protein kinase C (PKC) inhibitor (calphostin C) but not by a mitogen-activated protein kinase kinase-1 inhibitor (PD98059) or a p38 mitogen-activated protein kinase inhibitor (SB203580). By reporter gene transfection experiments of a series of promoter deletions and site-directed mutation constructs, we found 2 consensus Spl binding sequences located at - 97 and at -77 to play an important role in PMA-induced Mn-SOD transcription. Electrophoretic gel mobility shift assays have indicated that this sequence serves as an Spl binding site. Northern and Western blot analysis has revealed that PMA-induced promoter activity of Mn-SOD correlates with an increased expression of Sp1. Nuclear proteins from PMA-treated calf pulmonary artery endothelial cells displayed an increased DNA binding to the Sp1 site. Furthermore, the Mn-SOD promoter was activated either by overexpression of Sp1 or the constitutively activated form of PKCβ in an Sp1 site-dependent manner. These results suggest that PMA stimulates transcription of the Mn-SOD gene through an increase in Spl expression and thus implicate Sp1 as an effector mediating the PKC-signaling pathway elicited by extracellular signals.