The Role of Sp Family Members, Basic Krupple-Like Factor, and E Box Factors in the Basal and IFN-γ Regulated Expression of the Human Complement C4 Promoter

Abstract

The fourth component of human complement (C4) is a serum protein that is expressed in the liver and other organs. The promoter region of the C4 gene has been analyzed in reporter gene assays in two cell lines that represent hepatic (HepG2) and monocytic (U937) lineages. Analysis indicated that regions important for basal transcription in HepG2 cells included Sp1 and E box sites within the first 100 bp upstream of the transcription initiation site but not the nuclear factor-1 site important in the control of the mouse C4 gene. Also, a region encompassing −468 to −310 was able to repress activity 2-fold. However, when a CACCC or GT box sequence at −140 was mutated the repressive activity of the upstream region resulted in almost no activity. The −140 region consists of a series of four closely positioned GT boxes that were shown to bind Sp1, Sp3, and basic Krupple-like factor in EMSA. This novel two-part regulatory element may be involved in the regulated expression of C4. However, IFN-γ a major activator of C4 expression did not signal through this two-part regulatory element. We were able to map the position of an IFN-γ responsive element in U937. IFN-γ was able to increase transcription by up to 20-fold with mutations in the E box sequence at −78 to −73, thus completely abolishing induction. We conclude that the E box binding factors, which appear to be distinct from upstream stimulatory factors 1 and 2, are totally responsible for IFN-γ induction of C4.