Expression of the Interleukin-6 Gene Is Constitutive and Not Regulated by Estrogen in Rat Vascular Smooth Muscle Cells in Culture

Abstract Vascular smooth muscle cells (SMC) are major constituents of the medial layer of blood vessels and are involved in the development of atherosclerotic plaque. SMC secrete copious IL-6 under basal conditions that can be increased by cytokines such as tumor necrosis factor-α and interleukin-1β (IL-1β). The goal of our studies was to define the role of estrogen in IL-6 production by SMC. In a first series of experiments, the expression of specific messenger RNAs as well as the production of IL-6 bioactivity by rat SMC in culture could be demonstrated in basal and IL-1-stimulated conditions, but was unaffected by estrogen treatment. Different constructs containing deleted or mutated fragments of the human IL-6 promoter driving luciferase or chloramphenicol acetyltransferase reporter gene were then transiently transfected in these cells. A significant basal activity that was increased 2- to 4-fold after IL-1β stimulation was observed with the total IL-6 promoter. Deletion analysis indicated that the− 158/+11 region containing activator protein-1 and cAMP response element sites was apparently the minimal region of IL-6 promoter to confer both constitutive and IL-1-inducible activities. Site-directed mutagenesis experiments suggest that basal activity is dependent upon the promoter sequence −158 to −112 containing the nuclear factor (NF)-IL6(−153) and Sp1 sites, whereas IL-1β stimulation would depend on the residual −112 nucleotides containing NF-IL6(−75) and NF-κB sites. In contrast to the down-regulation of IL-6 expression by estrogen described in osteoblasts, ethinyl estradiol as well as 17β-estradiol did not influence stimulated IL-6 activity in our experimental conditions whatever the construct tested, even when either estrogen receptor α or β was overexpressed. Thus, the atheroprotective properties of estrogen are probably not mediated through the regulation of IL-6 production by SMC.