Recombinant expression and properties of the human calcium-binding extracellular matrix protein BM-40

A cDNA construct (approximately 1 kb) of human BM-40 in a plasmid with the cytomegalovirus promoter and enhancer was used to produce several stable clones by transfecting two human cell lines (293, HT 1080). These clones showed a high expression of exogenous 1-kb BM-40 mRNA and no or only little endogenous 2.2-kb mRNA. These clones also secreted BM-40 at high rates (5-50 micrograms ml-1 day-1) into serum-free culture medium as shown by electrophoresis, radioimmunoassay and metabolic labelling. Transfection with the plasmid and overexpression of BM-40 had no effect on cell spreading, proliferation rate and adhesion patterns to extracellular matrix substrates. Recombinant human BM-40 was purified by anion-exchange chromatography and showed the expected N-terminal sequence and amino acid composition. The protein was also identical or similar to authentic BM-40 purified from the mouse Engelbreth-Holm-Swarm tumor in hexosamine content, electrophoretic mobility, circular dichroism and binding activity for calcium and collagen IV. Reduction of both authentic and recombinant BM-40 decreased binding activity which indicates correct formation of disulfide bonds in the recombinant protein. A specific and sensitive radioimmunoassay for human BM-40 was shown to be useful for detecting small quantities of the protein in human cell culture medium and blood. No significant cross-reaction was, however, detected between human and mouse BM-40.