FHL-1/reconectin and factor H: two human complement regulators which are encoded by the same gene are differently expressed and regulated.
FHL-1/reconectin and factor H are two human complement regulators which are encoded by a single gene. FHL-1/reconectin contains the first 7 of 20 SCR protein domains of factor H and has four unique residues attached to its C-terminal end. The overlapping region of 445 amino acids explains the related complement regulatory functions of the two proteins. However, unique biological functions have also been reported for FHL-1/reconectin, such as cell adhesion and binding to microbial surfaces. Both proteins are synthesised and secreted by the liver. Extrahepatic synthesis occurs in a wide variety of cells, e.g. in monocytes, fibroblasts or neuronal cells. Unexpectedly, FHL-1/reconectin and factor H exhibit distinct expression patterns. This is also observed in disease situations such as in rheumatoid arthritis or malignancies. In rheumatoid arthritis a potentially protective role is suggested by the local synthesis of both FHL-1/reconectin and factor H in synovial fibroblasts and their induction by the anti-inflammatory agent dexamethasone and the cytokine IFN-gamma, but not by TNF-alpha. FHL-1/reconectin is overexpressed in certain tumor cells such as glioblastoma, conferring an exceptional resistance to such cells against complement mediated lysis. Although FHL-1/reconectin and factor H are encoded by a single gene, regulated by the same gene promoter and initiate transcription at the same start site, their transcripts are differently regulated. The putative control levels, which are responsible for this complex regulation, include transcript elongation, RNA processing, alternative splicing and differential poly(A) site selection.