SOMATOMEDIN &INSULIN RECEPTORS: A COMPARATIVE ANALYSIS

The somatomedins comprise a group of growth hormone-dependent, insulin-like
peptides with anabolic and mitogenic activity for a wide variety of cell
lines. The two sequenced human somatomedins, insulin-like growth factors I
and II (IGF-I and II) are structurally homologous to human proinsulin,
suggesting concservation of a core peptide skeleton which has evolved into
a family of hormones capable of responding to both metabolic and mitogenic
cellular requirements. The actions of insulin and the somatomedins are
presumably mediated through the binding of these peptides to specific
membrane receptors on target tissues, but attempts to define the mechanism
of action of each peptide have been handicapped by the significant
structural and functional homology of their receptors. This project will
attempt to characterize these receptors by employing a panel of polyclonal
and monoclonal antibodies generated against receptor preparations at
various levels of purification. Antibodies will be produced which are
specific for the insulin and the IGF-I receptor, and which are directed
against various protions of each receptor, including 1) the ligand binding
site, 2) antigenic sties removed from the binding site, and 3) the
"effector" protion of the receptor. The techniques of immunoprecipitation
of solubilized receptors, competitive radioligand binding studies, and
affinity cross-linking and electrophoresis will be employed to study 1)
tissue and species specificity of each receptor, 2) the ontogeny of each
receptor, 3) alterations in receptors associated with cell differentiation,
and 4) mechanism of modulation of each receptor by homologous and
heterologous ligands. Antibodies which are specifically directed against
each receptor will be employed to define the respective role of the insulin
receptor, IGF-I receptor and IGF-II receptor in cell metabolism and
replication. Finally, anti-receptor antibodies will be administered to
both normal and hypophysectomized rats, and growth parameters will be
measured after treatment with growth hormone, insulin and IGF-I. These
studies will help define the specific roles of insulin and the somatomedins
in cell growth, and will provide the first direct assessment of the
fundamental somatomedin hypothesis, that peptides mediate the anabolic
actions of growth hormone.