Isolation and characterization of a growth factor (embryonin) from bovine fetuin which resembles alpha 2-macroglobulin.

A serum-free hormone-supplemented medium which was previously formulated for the growth of mouse embryonal carcinoma (EC) cells and rat mammary epithelial (REM) cells required the presence of crude bovine fetuin as a medium supplement for maintaining cell growth. This requirement could not be replaced by purified fetuin preparations. The major growth-promoting activity (embryonin) in the crude fetuin preparation has been purified to near homogeneity by carboxymethylcellulose chromatography and high performance gel filtration chromatography. Purified embryonin, 2 to 4 micrograms/ml, is able to stimulate the growth of mouse EC cells in a serum-free hormone-supplemented medium to a level that is achieved with 0.5 to 1 mg/ml of the crude fetuin preparation. The biological activity resides in a high molecular weight glycoprotein (Mr = 270,000). Three polypeptide chains are observed following reduction, a major polypeptide (Mr = 185,000) and two minor chains (Mr = 116,000 to 120,000 and 68,000). Embryonin differs from pure fetuin in molecular weight, isoelectric point, amino acid composition, and immunological reactivity. However, embryonin is similar to human alpha 2-macroglobulin (alpha 2M) in these physicochemical and immunological properties. Resemblance to alpha 2M is also indicated by the observation that alpha 2M used over the same protein concentration range as embryonin produces a comparable stimulation of EC cell growth in the absence of serum. In addition, embryonin and alpha 2M produce a 2- to 10-fold differential stimulation of collagen production in cultures of normal rat kidney and RME cells, a response which may be linked to the growth-promoting activity of these two proteins.