Complete structure of the murine C4b-binding protein gene and regulation of its expression by dexamethasone

C4b-binding protein (C4BP) is involved in the fluid-phase regulation of the classical pathway of complement. A murine genomic library was screened, and five clones were selected that covered the remaining four exons in the 5'-region of the C4BP gene. Together with previous work (Barnum, S. R., Kristensen, T., Chaplin, D. D., Seldin, M. F., and Tack, B. F. (1989) Biochemistry 28, 8312-8317), the entire C4BP gene has now been shown to be 23 kilobases (kb) long and comprised of 10 exons ranging in size from 86 to 442 base pairs (bp). Primer extension analysis revealed the major transcription start site to be 46 bp upstream of the published cDNA start site. Northern blot analysis of RNA isolated from several mouse tissues demonstrated that the C4BP gene is expressed in a liver-specific manner. Several regions homologous to known response elements were identified upstream of the C4BP gene including a strong hepatocyte nuclear factor 1 binding site and four putative glucocorticoid response elements. Furthermore, dexamethasone increased C4BP mRNA and protein levels in the mouse liver cell line, NMuLi. The stimulation of C4BP gene expression was rapid and independent of protein synthesis. These results suggest dexamethasone induction of the C4BP gene is a primary response and therefore a transcriptional effect. Inhibition of the dexamethasone effect on C4BP by actinomycin D supports this theory. These studies also provide evidence that, for optimal induction of the C4BP gene, the glucocorticoid receptor complex may cooperatively interact with accessory transcription factors. It is likely that stimulation of C4BP gene expression by dexamethasone may allude to a mechanism by which glucocorticoids exert their anti-inflammatory effects.