Background: Salt-inducible kinases (SIKs) belonging to an AMP-activated kinase (AMPK) family, have functions in tumourgeniesis and metastasis in many solid tumours. However, the role of SIKs in breast cancer is not clear. In this study we aimed to determine the function of SIKs (SIK1, SIK2 and SIK3) in human breast cancer. Method: A cohort of breast cancer tissues (n=102) were analysed using qPCR to determine changes in SIK expression (SIK1, SIK2, SIK3 and housekeeping GAPDH ) between cancer and normal tissues and different aetiologies of cancer. In addition, a breast disease spectrum array (TMA) was probed for SIK expression at the protein level. SIK1, 2 and 3 were knocked down using shRNA in the human breast cancer cell lines MCF-7 and MDA-MB-231 and changes in cell behaviour were assessed using a number of cell function assays.Results: At the messenger level, expression of SIK2 was lower in breast cancer than normal tissue (p=0.0179), while expression of SIK1 and SIK3 had no significant difference between breast cancer and normal tissue. SIK3 expression was associated with ER status in the cohort (p=0.02). The level of SIK2 expression was significantly different in Stage I and Stage III. While SIK3 expression was significantly higher in ER positive breast cancer than ER negative breast cancer. Further analysis using Kaplan-Meier survival was performed for SIKs expression in an online database (http://kmplot.com/analysis/index.php?p=background) which showed that that an increased expression of SIKs was associated with good prognosis in the breast cancer cohort (p<0.01). Moreover, IHC staining correlation of SIKs in tumour with different clinicopathological characteristics showed a SIK1 Klein score significantly higher in ER positive breast cancer than ER negative breast cancer; in SIK3, a higher Klein score was observed in breast cancer with higher Grade, negative lymph node and higher TNM stage (p <0.05). In vitro, reduced expression of SIK2 and SIK3 increases the proliferation of breast cancer cells. However, SIK2 and SIK3 had differential effects on breast cancer cell adhesion. Knockdown of SIK2 only enhanced the adhesion of triple negative breast cancer cells (MDA-MB-231), while knockdown of SIK3 decreased the adhesion of both MDA-MB-231 and MCF 7 cells. Knockdown of SIK1 and SIK3 increased the invasion of MDA-MB-231 cells (p<0.05). Knockdown SIK1 increased the barrier but knockdown of SIK2 and SIK3 decreased the barrier of the breast cancer cells (p<0.05). We also determined the effect of possible chemotherapeutic resistance in these cell lines: the cells were treated with various concentration of chemotherapy drugs and placebo and found that reduced SIKs increased the resistance of breast cancer cell to paclitaxel and cisplatin.Conclusion: In conclusion, we show that low expression of SIK1,2 and 3 in breast cancer was correlated with poor prognosis and chemotherapy resistance of breast cancer. Reduced SIKs promote tumour metastasis by enhancing tumour function and tight junction. SIKs could be potential biomarkers for prognosis and chemotherapy sensitivity in breast cancer.

Citation Format: Ling Xin, Chang Liu, Yinhua Liu, Robert E. Mansel, Eleri Davies, Wen G. Jiang, Tracey A. Martin. Salt-inducible kinases suppress tumour function and regulate drug resistance in breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS4-41.