Autophagy and Tamoxifen Resistance in Breast Cancer.
Authors: Gonzalez-Malerva L. , Park J. , Zou L. , Hu Y. , Moradpour Z. , Pearlberg J. , Harlow E. , LaBaer J. 
Published: December 2010

Resistance totamoxifen in breast cancer patients is a serious therapeutic problem and majorefforts are underway to understand underlying mechanisms. Resistance can be either intrinsic or acquired. We derived matched tamoxifen sensitive and resistant MCF7 sub-clones that allow us to ask what changes occur in a breast cancer cell when it becomes drug resistant. We consider that this approach mimics de novo tamoxifen resistance and we used these sub-clones to study the factors that lead to drug resistance by two different approaches: expression profiling and an ectopic kinase expression screen. These sub-clones were challenged with steroid depleted media, estrogen, and estrogen plus tamoxifen in order to determine the differential gene response in the sensitive vs. resistant cells. We identified 227 genes that were estrogen responsive in both sub-clones but were not blunted in their response to tamoxifen in the resistant cells. We examined a subset of 67 of these genes that showed differences in baseline expression between sensitive and resistant tumors. Using publicly available data from two different clinical studies of women taking tamoxifen, we found that women classified as matching the sensitive pattern had statistically significant longer disease free survival. The tamoxifen response signature was an independent predictor of outcome compared with other clinical variables by multivariate analysis (p=0.05). The hazard ratio (1.9) was among the strongest observed for the study. Signatures like this one may help personalize therapy by planning regimens of adjuvant therapy best suited to each particular patient. In parallel, high-throughput cell-based screens, in which more than 500 human kinases were independently ectopically expressed, identified 31 kinases that conferred drug resistance on sensitive cells. Both expression profiling and an ectopic kinase expression screen led us to HSPB8, a less well known atypical kinase that has been linked to breast cancer. The ectopic expression of HSPB8 in tamoxifen sensitive cells enabled them to grow in the presence of the drug. Moreover, silencing HSPB8 by shRNA in tamoxifen resistant cells led to cell death. Loss of HSPB8 did not cause cell death by apoptosis, but rather by autophagy.Our findings indicate that HSPB8 is a negative regulator of autophagy and may play an important role in tamoxifen resistance. Interestingly, high expression level of HSPB8 predicted an earlier relapse on tamoxifen in breast cancer patients. Tamoxifen itself induced autophagy in sensitive cells but not in resistant ones and tamoxifen resistant cells were sensitive to the induction of autophagy by other drugs.These results may point to an important role for autophagy in the sensitivity to tamoxifen. We anticipate that this kind of analysis will provide insight into the signaling mechanisms related to the action of estrogen and also may facilitate efforts to identify novel targets for therapeutic intervention for ER positive breast tumors.

Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5135.