The acquisition of hormone independence is a crucial event in breast cancer since it is involved in the progression into a hormone resistant and more aggressive phenotype. We have developed an experimental model in which the continuous administration of medroxyprogesterone acetate (MPA) induces mammary carcinomas in female BALB/c mice. As most breast cancers, these ductal mammary carcinomas express high levels of estrogen (ER) and progesterone receptors (PR) and transit through different stages of hormone dependence. Hormone dependent (HD) tumor variants need exogenous administration of progestin to grow while hormone-independent (HI) tumor variants grow without exogenous hormone supply, even though they maintain high levels of ER and PR. We have previously provided evidence that these receptors are still playing a pivotal role regulating HI growth. The aim of the current study was to investigate which transduction pathways were involved in the steroid receptor expression and activation that could be leading to hormone independence. Two different approaches were used: a) a tissue array involving five different HD tumors and several HI variants to evaluate key proteins from the Akt pathway, and b) in vitro studies in primary cultures derived from HI and HD tumors using specific inhibitors of PI3K/Akt (LY294002) and Erk1/2 (PD98059) to evaluate the role of these pathways on steroid receptor expression and cell proliferation. Analysis of the arrays showed high levels of phospho Akt (pAkt, Ser473 and Thr 308) in the nuclei of most HI and HD tumors and, in agreement, low levels of GSK3 α/β. The in vitro studies involving the inhibitors LY294002 and PD98059 indicated that both kinases are responsible for maintaining cell proliferation and high levels of ERα, PRA and PRB in primary cultures derived from HD and HI variants. No differences between the HD and the HI tumors were observed suggesting that in this tumor model the acquisition of constitutive activation of those pathways is not restricted to hormone-independent variants. Finally, we present evidence showing that inhibiting the PI3K/Akt pathway causes an up-regulation of Erk1/2 levels, indicating that they regulate each other and suggesting that the overall control of steroid receptor expression and function might depend on a delicate balance between the relative activity of these two kinase pathways. In summary, in this study we demonstrate that deregulated Akt and Erk1/2 pathways are involved in maintainace of high levels of ER and PR in MPA-induced tumors. Our results show the involvement of alternative pathways on the regulation of steroid receptor expression and could contribute to the understanding of the acquisition of different hormone responsiveness in breast cancer progression.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA