Fetuin: its enigmatic property of growth promotion.

A variety of cell types in culture respond to fetuin, a glycoprotein from fetal bovine serum, which is often an important supplement to many serum-free media. Bovine fetuin preparation has been shown to inhibit trypsin activity and promote cellular attachment, growth, and differentiation in many different culture systems. In addition, fetuin associates with various growth factors or growth-promoting substances. However, whether the growth-promoting activity of fetuin preparation is due to fetuin per se or to its minor contaminant(s) has been a long-standing puzzle. The present review surveys the literature concerning this enigmatic property of fetuin and summarizes three possibilities: 1) fetuin itself is active, although the majority of studies do not support this; 2) various contaminants of fetuin preparations, including potentially unidentified ones, are responsible for the activity, a possibility supported by numerous reports; and 3) one of the fetuin subspecies, one of its contaminants, or a combination of both of these is responsible for growth of a specific cell type. In addition, the basic physicochemical properties and other biological functions of fetuin have also been presented.