Angela Costantino

Insulin receptor activation by IGF-II in breast cancers: evidence for a new autocrine/paracrine mechanism

IGF-II, produced by breast cancer epithelial and stromal cells, enhances tumor growth by activating the IGF-I receptor (IGF-I-R) via autocrine and paracrine mechanisms. Previously we found that the insulin receptor (IR), which is related to the IGF-I-R, is overexpressed in breast cancer cells. Herein, we find that, in breast cancer the IR is activated by IGF-II. In eight human breast cancer cell lines studied there was high affinity IGF-II binding to the IR, with subsequent IR activation. In these lines, IGF-II had a potency up to 63% that of insulin. In contrast, in non malignant human breast cells, IGF-II was less than 1% potent as insulin. Via activation of the IR tyrosine kinase IGF-II stimulated breast cancer cell growth. Moreover, IGF-II also activated the IR in breast cancer tissue specimens; IGF-II was 10 – 100% as potent as insulin. The IR occurs in two isoforms generated by alternative splicing of exon 11; these isoforms are IR-A (Ex11−) and IR-B (Ex11+). IR-A was predominantly expressed in breast cancer cells and specimens and the potency of IGF-II was correlated to the expression of this isoform (P<0.0001). These data indicate, therefore, that the IR-A, which binds IGF-II with high affinity, is predominantly expressed in breast cancer cells and represents a new autocrine/paracrine loop involved in tumor biology.

Overexpression of the RON gene in human breast carcinoma

Constitutive activation of the RON gene, known to code for the tyrosine-kinase receptor for Macrophage Stimulating Protein (also known as Scatter Factor 2), has been shown to induce invasive-metastatic phenotype in vitro. As yet, nothing is known about the expression of this novel member of the MET-oncogene family in spontaneously occurring human cancers. Here we report that Ron is expressed at abnormally high levels in about 50% primary breast carcinomas (35/74 patients). Among these, the expression is increased more than 20-fold in 12 cases and the overexpressed protein is constitutively phosphorylated on tyrosine residues. Notably, Ron is only barely detectable in epithelial cells of the mammary gland, and its expression remains unchanged in benign breast lesions (including adenomas and papillomas). Overexpression was observed in different histotypic variants of carcinomas; it is associated with the disease at any stage and correlates with the post-menopausal status. In breast carcinoma cells grown in vitro, activation of the Ron receptor resulted in proliferation, migration and invasion through reconstituted basement membranes. Altogether, these data suggest a role for the RON gene in progression of human breast carcinomas to the invasive-metastatic phenotype.

Insulin Receptor: What Role in Breast Cancer?

It is commonly believed that the insulin receptor mainly mediates the metabolic effects of insulin, whereas the closely related IGF-I receptor is considered a major factor for the regulation of cell proliferation. Experimental and epidemiological evidence indicates, however, that insulin and insulin receptors may play an important role in breast cancer. This article reviews evidence indicating that (a) insulin receptors are overexpressed in human breast cancer, (b) insulin stimulates growth in breast cancer cells, (c) cells transfected with human insulin receptor may acquire a ligand-dependent transformed phenotype, and (d) breast cancer is associated with insulin resistance and hyperinsulinemia. These findings may open new possibilities in breast cancer prevention, prognosis assessment, and therapy. (Trends Endocrinol Metab 1997; 8:306-312). (c) 1997, Elsevier Science Inc.

Insulin receptor content in tissues of normal and diabetic rats measured by radioimmunoassay

Insulin receptor (IR) content in different tissues has been quantitatively evaluated by means of steady state binding studies with radiolabeled insulin. The information provided by this approach, however, does not give a direct measurement of the receptor protein. Rather, it depends on the binding function of the IR, evaluated on the basis of curvilinear plots derived by Scatchard analysis of the experimental data. In the present report we employed a sensitive and specific radioimmunoassay (RIA) that allows a direct measurement of IR in solubilized cells or tissues. By this method we studied: a) IR distribution in several tissues of the rat, the animal model most frequently used in studies of insulin action; b) IR regulation in streptozotocin-treated, diabetic insulin deficient rats. Tissues from male Wistar rats (11 controls and 6 streptozotocin-treated diabetic animals) were homogenized, solubilized with Triton X-100 in the presence of protease inhibitors and stored at −80 C. IR content in the solubilized material was then measured by RIA. IR were detectable in all 11 tissues tested. Liver, kidney and brain neocortex had the highest IR content. (24.7±1.0, 20.5±1.1, 25.9±1.6 ng/mg protein, m ± SE, respectively). As expected, circulating insulin levels were lower in diabetic rats than in control rats. In diabetic, insulin deficient rats, liver, kidney and skeletal muscle contained more IR than in control rats (p = 0.001; p = 0.018; p = 0.003, respectively), whereas IR content in neocortex was similar in the two groups. The IR RIA may represent a useful tool for the study of IR regulation and pathophysiology. Our data provide a comparative direct measurement of IR distribution in a variety of rat tissues. IR content in diabetic rats is increased in typical target organs for insulin action, as a consequence of up-regulation due to the reduced insulin levels. This is not the case for metabolically insulin-dependent tissues, like brain.