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Insulin-Like Growth Factor Type 1 Receptor and Insulin Receptor Isoform Expression and Signaling in Mammary Epithelial Cells

Department of Neurology and Neuroscience (A.M.R., T.L.W.), University Hospital Cancer Center, New Jersey Medical School/University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07101; and ImClone Systems (D.L.L.), a wholly owned subsidiary of Eli Lilly and Co., New York, New York 10014

Address all correspondence and requests for reprints to: Dr. Teresa L. Wood, University of Medicine and Dentistry of New Jersey, New Jersey Medical School Cancer Center, 205 South Orange Avenue, H1200, Newark, New Jersey 07101. E-mail: woodte{at}umdnj.edu.

The insulin receptor (IR) isoforms and the IGF type 1 receptor (IGF-1R) share a high degree of structural homology but differ in ligand binding kinetics and functions. We developed a highly specific quantitative PCR assay to quantify and compare IR-A, IR-B, and IGF-1R expression within an RNA population. We determined receptor expression in primary murine mammary epithelial cells (MECs) during postnatal development. Both IR isoform mRNAs were 3- to 16-fold higher than IGF-1R expression at all developmental times. IR protein was also 3- to 10-fold higher than IGF-1R protein; however, significantly less IGF-1R was found in hybrid receptors at early (49%) vs. late (79%) pregnancy, indicating that the amount of hybrid receptor is developmentally regulated. Despite high IR expression, IGF ligands were more effective than insulin in stimulating the insulin receptor substrate-1/phosphatidylinositol 3-kinase/Akt pathway in acutely isolated MECs from virgin glands. Although approximately 40% of IR transcripts were the IGF-II-sensitive IR-A isoform, IGF-II failed to stimulate IR phosphorylation, and an IGF-1R-specific blocking antibody completely abrogated IGF-II-mediated Akt phosphorylation in the virgin MECs. Taken together, these data suggest that the IGF-1R is more active in signaling than the IR and is the predominant mediator of IGF actions in virgin MECs.