Constitutive expression of interleukin-1α precursor promotes human vascular smooth muscle cell proliferation

Abstract

Vascular smooth muscle cell (VSMC) proliferation plays a critical role in the failure of vascular surgeries and contributes to the development of atherosclerotic lesions. Evidence that interleukin-1 (IL-1) is a mitogen for cultured VSMC has implicated its release by activated macrophages in the development of atherosclerosis. VSMC also produce IL-1, including the precursor form of IL-1α. However, it is not known whether IL-1α precursor is processed to mature IL-1α or released from VSMC, nor is it known whether either precursor or mature IL-1α functions as an autocrine growth factor. The goals of the present study were to establish whether proliferation is enhanced in human VSMC transfectants producing IL-1α constitutively at levels comparable to those produced after activation, and to determine which domains of IL-1α are important for its activity. Human VSMC were stably transfected with expression vectors directing constitutive expression of either full-length IL-1α precursor [IL-1α-(1—271)], its NH2-terminal domain [IL-1α-(1—112)], or mature IL-1α [IL-1α-(113—271)]. Both IL-1α-(1—271) and IL-1α-(113—271) stable transfectants produced moderate levels of IL-1α (0.2–1.0 ng/106cells) and released low levels of IL-1α into the supernatant (<20 pg/ml). VSMC stably transfected with either IL-1α-(1—271) or IL-1α-(113—271) expression plasmids proliferated rapidly compared with nontransfected or vector-transfected VSMC and displayed a distinct morphology characterized by elongated, spindle-shaped cells. Stable transfection with IL-1α-(1—271) was somewhat more effective than transfection with IL-1α-(113—271). Interestingly, VSMC transfected with IL-1α-(113—271) expression plasmids also expressed IL-1α-(1—271) mRNA, suggesting that IL-1α-(113—271) activates an IL-1-induced IL-1 autocrine loop. In contrast, neither proliferation rates nor morphology was affected by stable transfection with IL-1α-(1—112) expression plasmids. Exogenous IL-1 receptor antagonist partially reversed the enhanced DNA synthesis in VSMC transfected with either IL-1α-(1—271) or IL-1α-(113—271) expression plasmids, suggesting that the pro-proliferative effect of VSMC-derived IL-1α is at least partially mediated by signaling via the type I IL-1 receptor. These results demonstrate that IL-1α precursor is an autocrine growth factor for human VSMC and further indicate that amino acids 113–271 play a crucial role in its actions.