Crosstalk between the Estrogen Receptor and the HER Tyrosine Kinase Receptor Family: Molecular Mechanism and Clinical Implications for Endocrine Therapy Resistance

Breast cancer evolution and tumor progression are governed by the complex interactions between steroid receptor estrogen receptor (ER) and progesterone receptor and growth factor receptor signaling. In recent years, the field of cancer therapy has witnessed the emergence of multiple strategies targeting these specific cancer pathways and key molecules (ER and growth factor receptors) to arrest tumor growth and achieve tumor eradication; treatment success, however, has varied and both de novo (up front) and acquired resistance have proven a challenge. Recent studies of ER biology have revealed new insights into ER action in breast cancer and have highlighted the role of an intimate crosstalk between the ER and HER family signaling pathways as a fundamental contributor to the development of resistance to endocrine therapies against the ER pathway. The aim of this review article is to summarize the current knowledge on mechanisms of resistance of breast cancer cells to endocrine therapies due to the crosstalk between the ER and the HER growth factor receptor signaling pathways and to explore new available therapeutic strategies that could prolong duration of response and circumvent endocrine resistant tumor growth. (Endocrine Reviews 29: 217-233, 2008) I. Introduction II. Biology of the Estrogen Receptor (ER) A. ER and ER subtypes B. Genomic action of ER C. Nongenomic rapid ER activity III. The HER Tyrosine Kinase Receptor Family and Its Role in the Development of Endocrine Resistance: Studies in Preclinical Models A. HER/ER crosstalk as a mechanism for endocrine therapy resistance: molecular determinants B. HER2/ER crosstalk in de novo endocrine resistance models C. HER2/ER crosstalk in acquired endocrine resistance models IV. The HER Tyrosine Kinase Receptor Family and Its Role in the Development of Endocrine Resistance: Clinical Evidence A. De novo endocrine resistance B. Progesterone receptor (PgR) negativity associated with endocrine resistance and HER signaling C. HER2/ER crosstalk in acquired endocrine resistance V. Novel Therapeutic Strategies to Overcome HER/ER Pathway Crosstalk and Endocrine Resistance A. Preclinical studies B. Clinical studies VI. Future Challenges