Schultz K, Murthy V, Tatro JB, Beasley D. Endogenous interleukin-1␣ promotes a proliferative and proinflammatory phenotype in human vascular smooth muscle cells. Am J Physiol Heart Circ Physiol 292: H2927-H2934, 2007. First published February 9, 2007 doi:10.1152/ajpheart.00700.2006.-During vascular disease and following injury, vascular smooth muscle cells (VSMC) proliferate and produce inflammation-promoting cytokines and chemokines. Similar phenotypic changes can be elicited in vitro by activation of Toll-like receptors (TLR) within VSMC. TLR-activated VSMC also produce IL-1␣, but it is unknown whether endogenous IL-1␣ stimulates VSMC in an autocrine manner. Here we tested the hypothesis that endogenous IL-1␣ contributes to TLR-induced proliferation and chemokine release in human VSMC by using RNA interference to knock down IL-1␣ expression. Knockdown of IL-1␣ abolished TLR-induced proliferation and suppressed TLR4-induced release of monocyte chemoattractant protein-1 (MCP-1) by VSMC, indicating that endogenous IL-1␣ plays a crucial role in both responses. Serum, PDGF, FGF-2, and EGF each increased cellular IL-1␣ concentrations, and IL-1␣ knockdown inhibited serum-and PDGF-induced DNA synthesis, further indicating that endogenous IL-1␣ also contributed to VSMC responses to growth factors. IL-1 receptor antagonist, a competitive inhibitor of IL-1 receptor I (IL-1RI), also attenuated TLRinduced proliferation and both basal and TLR-induced MCP-1 expression, indicating at least a partial role of the IL-1RI in mediating these responses. The results support the hypothesis that autocrine actions of endogenous IL-1␣, mediated at least in part via IL-1RI signaling, contribute to a proproliferative and proinflammatory phenotypic shift in TLR-activated human VSMC, which might play a pathogenic role in vascular disorders. monocyte chemoattractant protein-1; cell proliferation; type I interleukin-1 receptor EXCESSIVE PROLIFERATION OF vascular smooth muscle cells (VSMC) is a prototypical response to vascular injury and is also a feature of early-stage atherogenesis. Such proliferative responses of VSMC may contribute to vascular repair in some cases, but they can also lead to restenosis when excessive (1). Another fundamental and dramatic change displayed by VSMC following either vascular injury or chronic exposure to intravascular lipids is their shift to a proinflammatory phenotype, characterized by the synthesis of proinflammatory cytokines and chemokines, including interleukin-1␣ (IL-1␣) and monocyte chemoattractant protein-1 (MCP-1) (7,9,22,29). The functions of endogenous VSMC-derived IL-1␣ in this context are unknown, but we have hypothesized that it may play an autocrine role in promoting VSMC proliferation, based on in vitro and in vivo lines of evidence. For example, in coronary bypass patients, IL-1␣ was found in spindle-shaped cells within saphenous vein grafts that had become stenotic but not in those that remained patent (5). Similarly, IL-1␣ mRNA was found in medial smooth muscle cells (SMC) within arterial ...