Regulatory Nodes That Integrate and Coordinate Signaling As Potential Targets for Breast Cancer Therapy

Blockade of the estrogen receptor (ER) with antiestrogens and aromatase inhibitors is effective in the treatment of breast cancer. Why ER plays such a dominant role in breast cancer and represents such an excellent target remains to be defined. The ability of ER to respond to multiple inputs and to control expression of multiple downstream genes may be one of the reasons why ER is such a powerful target for breast cancer treatment. The recent modest performance of a number of targeted therapies in breast cancer has raised the question whether we will ever develop therapies that have such success as antiestrogens. Targeted therapies tend to inhibit a single pathway that is probably altered in only a subset of patients. Even within this subset, only a limited number of patients respond. The evidence that virtually all pathways can cross-talk and that they exhibit several layers of redundancy reveals a complexity of signaling networks that may defy the generation of targeted therapies with efficacy similar to antiestrogens. However, there are clearly regulatory nodes that can integrate multiple upstream inputs and elicit diverse downstream outputs. We provide evidence and rationales for integrins, insulin receptor substrates (IRSs), and cyclin D1 as potential therapeutic targets. These proteins, similar to ER, can integrate and coordinate multiple signals in breast cancer cells and thus mediate diverse aspects of breast cancer progression. New treatment targets will emerge in light of more global models of signal transduction that fully integrate all aspects of cell biology such as the role of the extracellular matrix and will hopefully result in the development of targeted therapies that show efficacy similar to antiestrogens.