부산대학교 도서관

Forkhead Box O3 (FOXO3) is a tumour suppressor whose functional activity is predominantly regulated by post-translational modifications (PTMs). Of all PTMs, the best-studied PTM of FOXO proteins is phosphorylation, however, acetylation by EP300 or the CREB-binding protein (CBP) has been increasingly recognised as a novel axis regulating FOXO proteins. Herein, using the BT474 breast cancer cells and its lapatinib resistant derivative BT474 LapR cells, I observed that higher FOXO3 and acetylated Ac-FOXO3 levels correlate with lapatinib sensitivity. Subsequent ectopic expression of EP300, as well as CBP, led to an increase in acetylated-FOXO3 in sensitive, but not in resistant BT474 cells. Moreover, lapatinib sensitive BT474 cells exhibited enhanced lapatinib cytotoxicity upon over-expression of wild-type but not acetylation-deficient EP300 or CBP as revealed by drug sensitivity assays. Besides, using SIRT1/6 specific siRNAs and chemical inhibitor, I also found that SIRT1 and SIRT6 play a part in fine-tuning FOXO3 acetylation and lapatinib sensitivity. Notably, we next explored that not only SIRT1/6 but also SIRT2 plays a significant role in regulating FOXO3 acetylation and lapatinib sensitivity of BT474 and nasopharyngeal cancers. Taken together, these results suggest the involvement of acetyltransferases EP300/CBP and nuclear sirtuins in regulating FOXO3 acetylation and lapatinib sensitivity of breast cancers. Despite having a tumour-suppressive role, some reports suggested the oncogenic role of EP300 in the development and progression of cancers. To uncover this, I used Hs578T cells which have E-cadherin promoter hypermethylated. Interestingly, overexpression of EP300 led to the up-regulation of mesenchymal and stemness markers, increases in cell migration, invasion, anchorage-independent growth, and drug resistance. Moreover, genome-wide expression profiling identified Aldo-keto reductases AKR1C1-3 as positive modulators of cancer stemness and drug resistance, since their pharmacological inhibition with flufenamic acid restored both doxorubicin and paclitaxel sensitivity and reduced mammosphere formation. Thus, EP300 functions as an oncogene via the up-regulation of AKR1Cs in cells where the E-cadherin promoter is hypermethylated. This important finding offers the rationale of using current AKR1Cs inhibitors in the treatment of triple-negative breast cancers.