Regulation of synthesis of complement protein C4 in human fibroblasts: cell- and gene-specific effects of cytokines and lipopolysaccharide.

Synthesis and secretion of the class III major histocompatibility complex (MHC) gene product, C4, were detected in human skin fibroblasts by metabolic labelling, immunoprecipitation and SDS-PAGE analysis. Pro-C4 (approximately 185,000 MW) was present in intracellular lysates, and the mature protein was present in extracellular media, with three bands of approximately 93,000, 75,000 and 33,000 MW, corresponding to the alpha, beta and gamma chains, respectively. C4 expression was increased in a dose-dependent manner by interferon-gamma (IFN-gamma), but was unaffected by interleukin-1 beta (IL-1 beta), IL-6 and tumor necrosis factor-alpha (TNF-alpha) alone, each of which augmented the expression of factor B, C3 and other complement proteins synthesized in fibroblasts. Simultaneous incubation of fibroblasts with IFN-gamma and TNF resulted in a synergistic increase in C4 synthesis. RNA blot analyses indicated that regulation of C4 synthesis by IFN-gamma and the combination of IFN-gamma and TNF was mediated primarily at a pretranslational level. Lipopolysaccharide (LPS) had no effect on C4 or HLA-DR synthesis in fibroblasts, either constitutive or IFN-gamma-regulated. These results are in contrast to the effects of LPS in monocytes, where LPS decreased constitutive synthesis and counter-regulated the IFN-gamma-enhanced expression of both C4 and HLA-DR. C2 expression in fibroblasts was also increased primarily by IFN-gamma. However, C2 synthesis was increased by LPS, 1L-1 and TNF, although to a lesser extent than the increase in synthesis of factor B stimulated by these mediators. These results show that up-regulation by IFN-gamma is a common feature of C2 and C4 expression in human cells that constitutively synthesize these proteins. In contrast, regulation of MHC class III and class II genes by LPS, TNF, IL-1, and IL-6 is cell- and gene-specific.