ICAM-1 expression by vascular smooth muscle cells is phenotype-dependent

Atherosclerosis is an inflammatory disease characterised by increased expre ssion of adhesion molecules for leukocytes on both the surface of dysfuncti onal endothelium and on smooth muscle cells (SMC) within the lesion. It is also characterised by altered SMC phenotypic expression, indicated by a dec reased Volume fraction of myofilaments (V(v)myo) [1,2] and changes in gene expression [3]. The present study used an in vitro model to investigate, by immunofluorescence staining and flow cytometry, the influence of phenotype on vascular SMC expression of the adhesion molecule for leukocytes, intrac ellular adhesion molecule-1 (ICAM-1), and the regulatory mechanisms involve d in this process. Smooth muscle cells with a high V(v)myo, freshly isolate d from rat aortic media, expressed little or no ICAM-1 and this could not b e induced by interleukin-1 beta (IL-1 beta). As SMC modulated phenotype, in dicated by decreasing V(v)myo over the first 5 days of culture, there was a concomitant increase in ICAM-1 expression. At day 9 of primary culture, wh en SMC cultures had returned to the high V(v)myo phenotype, ICAM-1 expressi on was markedly lower. However, these cells retained the capacity to expres s ICAM-1 in response to IL-1 beta. After several passages in culture, cells (with a low V(v)myo) constitutively expressed ICAM-1, with levels further up-regulated in response to IL-1 beta. These changes in ICAM-1 expression w ere not related to proliferative state, since similar results were obtained with growth arrested SMC. Investigation of signalling pathways involved in regulating ICAM-1 expression by primary vascular SMC suggested a complex r egulatory mechanism. Activation of adenyl cyclase (with forskolin) caused a significant increase in cells expressing ICAM-1. Treatment with inhibitors of protein kinase C (chelerythrine chloride), protein tyrosine kinase (gen istein), or the transcription factor NF-kappa B (PDTC) had no significant e ffect on IL-1-induced ICAM-1 expression. However, in the presence of serum, both genistein and PDTC caused a significant increase in basal expression. The results indicate that ICAM-1 expression by SMC is phenotype-dependent, with expression evident only after cells have modulated to a low V(v)myo p henotype. They also indicate the existence of complex regulatory mechanisms , possibly involving the SMC cytoskeleton. (C) 2000 Elsevier Science Irelan d Ltd. All rights reserved.