Molecular Characterization andin VitroBiological Activity of Placentin, a New Member of the Insulin Gene Family
Journal of Biological Chemistry
Insulin and insulin-like growth factors belong to a family of polypeptides involved in essential physiological processes. Placentin, a new member of the insulin family, was recently identified as a 139-amino acid open reading frame from a cDNA clone isolated from a subtracted library of first trimester human placenta. Tris/ Tricine/SDS-polyacrylamide gel electrophoresis and immunoblot analyses of histidine-tagged recombinant placentin indicate that it is composed of two peptide chains of
... t molecular masses of 4 and 13 kDa. Conditioned media produced by recombinant expression of placentin cDNA in the placental 3AsubE cell line were assayed for biological activity and found to stimulate tyrosine phosphorylation and DNA synthesis. While these effects closely mimicked those of insulin, they were not mediated by the insulin receptor as shown by the lack of tyrosine phosphorylation of this receptor upon placentin treatment. Moreover, in cytotrophoblast primary culture, production of chorionic gonadotropin, a marker of trophoblast differentiation, was increased upon treatment with placentin-conditioned media, while unaffected by insulin. These results suggest that placentin might participate in the cellular proliferation and/or differentiation processes during placental development. Insulin and insulin-like growth factors belong to a family of polypeptides essential for proper regulation of physiological processes such as energy metabolism, cell proliferation, development, and differentiation (1, 2). These polypeptides exert their effects through membrane receptors belonging to the superfamily of polypeptide growth factor receptor tyrosine kinases and their oncogenic analogs (3-5). The intracellular signal transduction pathway for these receptors is characterized by a tyrosine kinase activity that initiates a chain of phospho-rylation-related events. The insulin family consists of insulin, IGF 1 I and II, relaxin (6), and LEY I-L (7). The consensus elements defining their relationship are the positions of six half-cystine residues essential for proper tertiary structure formation (8). While receptors mediating the effects of relaxin and LEY I-L have not yet been described, insulin and IGF I act through distinct tyrosine kinase subclass II receptors (3), and the actions of IGF II appear to be mediated through several receptors, including those for insulin and IGF I (1).