Bioassays for growth factors.

Growth factors have a wide variety of actions in living systems, providing a range of potentially quantifiable responses for measurement of their biological activity. The biological activity has to be assessed by bioassay as it cannot be predicted from physicochemical data alone. Bioassay systems range from in vivo responses to changes in receptor binding and in early components of signal transduction pathways. The most commonly used systems are based on the measurement of responses of immortalized cell lines, which although not as functionally relevant as in vivo assays, are easier to use. Most growth factors have multiple actions on multiple targets, and can show differential changes in their different activities, so use of the biological activity measured in one bioassay system to predict biological activity in another system must be rigorously validated. Since the bioassay systems are themselves inherently variable, measurement of the growth factor's activity must be made relative to a common, stable, reference preparation to permit valid inter-assay and inter-laboratory comparisons.