K(Ca)1.1 K(+) Channel Inhibition Overcomes Resistance to Antiandrogens and Doxorubicin in a Human Prostate Cancer LNCaP Spheroid Model

Several types of K(+) channels play crucial roles in tumorigenicity, stemness, invasiveness, and drug resistance in cancer. Spheroid formation of human prostate cancer (PC) LNCaP cells with ultra-low attachment surface cultureware induced the up-regulation of cancer stem cell markers, such as NANOG, and decreased the protein degradation of the Ca(2+)-activated K(+) channel K(Ca)1.1 by down-regulating the E3 ubiquitin ligase, FBXW7, compared with LNCaP monolayers. Accordingly, K(Ca)1.1 activator-induced hyperpolarizing responses were larger in isolated cells from LNCaP spheroids. The pharmacological inhibition of K(Ca)1.1 overcame the resistance of LNCaP spheroids to antiandrogens and doxorubicin (DOX). The protein expression of androgen receptors (AR) was significantly decreased by LNCaP spheroid formation and reversed by K(Ca)1.1 inhibition. The pharmacological and genetic inhibition of MDM2, which may be related to AR protein degradation in PC stem cells, revealed that MDM2 was responsible for the acquisition of antiandrogen resistance in LNCaP spheroids, which was overcome by K(Ca)1.1 inhibition. Furthermore, a member of the multidrug resistance-associated protein subfamily of ABC transporters, MRP5 was responsible for the acquisition of DOX resistance in LNCaP spheroids, which was also overcome by K(Ca)1.1 inhibition. Collectively, the present results suggest the potential of K(Ca)1.1 in LNCaP spheroids, which mimic PC stem cells, as a therapeutic target for overcoming antiandrogen- and DOX-resistance in PC cells.