KPNA1 mediates the nuclear import of BQ323636.1 and confer tamoxifen resistance in breast cancer cells

Abstract

Breast cancer is one of the prevalent causes of cancer in women. Two thirds of breast cancer patients are ER-positive and can be benefited from tamoxifen treatment. However, 50% of the patients will eventually develop the resistance. Identifying molecular targets associated with tamoxifen resistance would help in designing better therapeutic strategies. Previous study from our group identify a novel factor BQ323636.1 (BQ), a spliced variant of NCOR2, could confer tamoxifen resistance. From our in vivo study, nuclear enrichment of BQ in primary breast tumor was found to be associated with tamoxifen resistance. However, the molecular mechanism governed the nuclear import of BQ and was not clear. From bioinformatics prediction, we identified a functional nuclear localization signal (NLS) on BQ. We therefore performed a protein screening to search for the import receptor which could interact with BQ. KPNA1 was found to interact with BQ. To further validate such an interaction, we fused the NLS to GPF reporter protein (GFP-NLS) and performed a co-immunoprecipitation. The result showed that the GFP-NLS was able to interact with KPNA1. Knockdown of KPNA1 by RNAi could prevent the nuclear import of BQ. This confirmed the functional role of KPNA1 in the nuclear import of BQ. In addition, knockdown of KPNA1 in BQ overexpressing breast cancer cells could reverse the tamoxifen resistance. The results highlighted the functional significance of KPNA1 in the tamoxifen resistance. Subsequently, we investigated the functional role of BQ in the nucleus. We found BQ could deplete the gene repressive function of NCOR2. We found that NCOR2 could interact with HSF4 and should repress the expression of HIF1α. In the presence of BQ, the repressive function of NCOR2 on HIF1α was compromised, leading to up-regulation of HIF1α. To further validate the effect of HIF1α in the tamoxifen resistance, HIF1α inhibitor was employed and found that the treatment of the inhibitor could reduce the resistance in BQ overexpressing breast cancer cells. Therefore, our study uncovered the role of KPNA1 in the development of tamoxifen resistance through which KPNA1 could promote the nuclear import of BQ. Subsequently, BQ could lead to the up-regulation of HIF1α which also play significant role in the development of tamoxifen resistance.