The Partial Isolation and Characterization of Ilotropin, a Novel Islet-specific Growth Factor

In this series of studies, we have presented evidence for a novel, pancreatic islet-specific growth factor, which we call ilotropin. Ilotropin is acid stable, heat stable, ethanol-precipitable, and sensitive to trypsin digestion. It appears to have a molecular weight between 29 - 44,000, and preliminary data not presented here suggests that it has a relatively basic pI. Unlike many other growth factors, ilotropin does not bind to heparin. Ilotropin is distinguishable from most of the known growth factors on the basis of at least one of the characteristics established in these studies. The apparent molecular weight of 29 - 44,000 eliminates all but the larger growth factors such as PDGF and hepatic growth factor. The fact that ilotropin is acid stable rules out identity with hepatic growth factor, and its lack of binding to heparin and apparent basic pI rules out identity with PDGF. Thus, the combination of characteristics described in these studies eliminates most of the known growth factors as candidates for the role of ilotropin. Certain growth factor precursor molecules (e.g. TGF-a) and several interleukins and cytokines (e.g. pro-IL-1 and melanocyte growth factor) also fall into this molecular weight range. Whether these proteins might be related to ilotropin or play a role in its biological activity remains to be determined. Current studies of ilotropin include further purification to homogeneity, determination of the peptide sequence of ilotropin, and development of an in vitro bioassay using trophic responses of primary cultures of pancreatic duct cells as an indicator of ilotropin activity. With purified material we ought to be able to identify the cells of origin and the target cells for the action of ilotropin, and establish assays to determine the relationship to failure of beta-cell regeneration that accompanies diabetes. Ultimately we hope that ilotropin may lead to new ways of approaching aspects of the problems presented in pancreatic beta-cell failure.