Effect of DMBA on the expression of prolactin receptors and IGF1 genes in rat mammary gland

Abstract Prolactin receptor and IGF1 gene expression were measured in mammary glands from Sprague-Dawley rats at different times (10,30; and 58 d) after administration of a single dose of 15 mg dimethylbenz(a)-anthracene (DMBA) per os at 55 d of age, and in DMBA-induced mammary tumors appearing in these rats at ≈ 2 months after DMBA administration: The relative gene expression of prolactin receptor and insulin-like growth factor (IGF1) mRNAs was measured by hybridization to Northern blots prepared from pools of tissue. The probes used were 32 P-labelled cDNAs specific to the extracellular domain of the receptor (E probe), common to all forms, and a probe specific to the intracellular position of the long form of the receptor (I probe), a human IGF1 probe, and chicken β-actin probe, to correct for loss of tissue and different metabolic activity of the tissues. Hybridization with the prolactin receptor probes revealed bands at 2.5, 3; and 5.5 kb hybridizing with the long form of the receptor and a more intense band at 1.8 kb that corresponded to the short form of the receptor. There were no changes in the relative expression of prolactin receptor mRNAs in the mammary gland of control (oil-treated) or DMBA-treated rats, although there was a gradual diminution of expression with increasing age of the animals. In contrast, in DMBA-induced mammary tumers, there was a marked increase in the relative expression of prolactin receptor mRNAs with, however, no modification in the relative proportion of short and long forms. Measurement of membrane prolactin receptors by binding to a 125 I-labelled specific monoclonal antibody (U5) also revealed a much greater concentration of prolactin receptor in membranes from tumors in comparison to normal mammary tissue. Hybridization with the IGF1 probe revealed bands at 1, 1.8 and 7.5 kb that did not differ between control or DMBA-treated rats, or with time. In contrast, in tumors, an additional band appeared at ≈ 0.6 kb. These results indicate that modifications in prolactin receptor or IGF1 gene expression do not participate in the early stages of DMBA-induced tumor transformation of mammary tissue. However, the altered expression of these proteins in tumor tissue may participate in the increase in cell proliferation characteristic of the transformed slate.