Over expression of estrogen receptor (ER) has been implicated in over 70% of breast cancers. Thus therapy targeting ER directly or indirectly is the most important modality in the two-thirds of patients with an ER-positive early breast cancer. The mainstay of endocrine therapy targeting ER in postmenopausal women that are currently available includes selective ER modulators such as tamoxifen and raloxifene, and the ‘third-generation’ aromatase inhibitors (AIs), anastrozole, exemestane and letrozole (click here for more information: http://www.cancer.gov/cancertopics/understandingcancer/targetedtherapies/breastcancer_htmlcourse/page2).
Even though endocrine therapy is the most effective treatment for ER-positive metastatic breast cancer, its effectiveness is limited by high rates of innate (intrinsic) and acquired resistance during treatment. Only about 30% of patients with metastatic disease have objective regression of tumor with initial endocrine treatment, while another 20% have prolonged stable disease.
Even though mutations of ER are rarely reported, other mechanisms such as ER-phosphorylation has been implicated in resistance to tamoxifen. In addition, several clinical studies suggested potential mechanisms of resistance to endocrine therapy. Some of the mechanisms implicated include loss of ER, loss of progesterone receptor (PR), upregulation of HER-2, and response to sequential endocrine therapy.
Using a high throughput screening, a recent study by Stebbing et al. identified a regulator of ER-α, Lemur tyrosine kinase 3 (LMTK3), and noted that LMTK3 gene amplification in both circulating free DNA and primary tumors are predictive of resistance to tamoxifen. Using an orthotopic breast cancer model with tamoxifen-resistant breast cancer cells BT474 that overexpress LMTK3, Stebbing and his colleagues noted that tamoxifen treatment along with LMTK3 knock-down resulted in significant inhibition of tumor growth compared to untreated control mice. To evaluate the clinical relevance of this observation, levels of LMTK3 were determined by immunohistochemistry in tumor samples from ER-positive breast cancer patients treated with endocrine therapy. High levels of LMTK3 were observed in non-responders compared to responders suggesting the association of LMTK3 in limiting efficacy of endocrine therapy. To identify genes and signaling pathways affected by LMTK3, a genome-wide gene expression analysis was performed using BT747 cells. One gene whose expression was found to be significantly regulated by LMTK3 was HSPB8 (heat shock 22kD protein 8). Both overexpression of HSPB8 in breast cancer and potential involvement in tamoxifen resistance have been reported by other studies. Taken together, these results suggests that LMTK3 can contribute to tamoxifen resistance.
ER targeted therapy has improved the quality of life and survival of millions of women around the world, however, resistance to therapy continues to be a major problem. Identification of the role LMTK3 in resistance would facilitate to formulate strategies to overcome this problem.
Ali S, Coombes RC. Endocrine-responsive breast cancer and strategies for combating resistance. Nat Rev Cancer. 2002;2(2):101-112.
Osborne CK, Schiff R. Mechanisms of endocrine resistance in breast cancer. Annu Rev Med. 2011;62:233-247.
Stebbing J, Filipovic A, Lit LC, et al. LMTK3 is implicated in endocrine resistance via multiple signaling pathways. Oncogene. 2013;32(28):3371-3380.