The ultimate function of the mammary gland is to nourish newborns to sustain mammalian life post-partum. Development and differentiation of the mammary gland occurs postnatally under the control of a complex system of hormonal and environmental cues making the mammary gland an important model of study for both developmental biology and endocrinology. Furthermore, breast cancer is the “number one cause of cancer death in Hispanic women” and the “second most common cause of cancer death in white, black, Asian/Pacific Islander and American Indian/Alaska Native women” according to the Centers for Disease Control and Prevention. These statistics make it clear that ongoing research in both breast cancer and normal mammary gland development is imperative to women’s health.

The primary cause of breast cancer is transformation of mammary epithelial cells (MECs) that result in unregulated cell proliferation and subsequent mammary carcinomas. Several components of the insulin-like growth factor (IGF) system, the IGF ligands and the IGF-type I receptor (IGF-1R) in particular, regulate both normal and transformed MEC growth.

Previous studies from our laboratory examined expression of the IGF ligands and the IGF-1R in normal mammary gland development and demonstrated that the IGF ligands and the IGF-1R are expressed in distinct patterns during normal development. Subsequent analysis of receptor affinity for IGF ligands demonstrated that the insulin receptor isoform A (IR-A) is an IGF-II-sensitive signaling receptor and furthermore, that hybrid receptors consisting of equal parts IGF-1R and insulin receptor (IR), preferentially bind IGF ligands.

Herein, we have developed and tested a quantitative PCR assay to accurately measure IGF-1R, IR-A and IR isoform-B (IR-B) expression on the same scale. We have used this assay to quantify mRNA expression of IGF sensitive receptors in primary MECs during mammary gland development. These data demonstrate that IR isoforms are expressed at much higher levels than the IGF-1R at all times. Subsequent protein analysis demonstrated that due to this disproportion, at least 49% of IGF-1R is present as part of a hybrid receptor during pregnancy stages.

We next provide preliminary data that demonstrate preferential activation of the IGF-1R rather than hybrid receptors by IGF-I in vivo. Furthermore, we demonstrate that insulin stimulates IGF-1R activation in vivo, either by stimulating classic IGF-1R or IGF-1R present in a hybrid receptor. The importance of IGF-1R signaling in MEC development is further supported by microarray analysis of a MEC-specific conditional deletion of the IGF-1R during post-pubertal development. These data suggest a functional role for IGF-1R signaling via the PI3K/Akt pathway in regulating expression of cell cycle regulatory molecules. In the final section of this thesis, we present in vitro data that support the findings of the microarray analysis and demonstrate that activation of the PI3K/Akt/mammalian target of rapamycin pathway stimulates cell cycle progression downstream of IGF-I in normal MECs.