Cancer-Associated Intermediate Conductance Ca2+-Activated K⁺ Channel KCa3.1

Abstract

Several tumor entities have been reported to overexpress K_Ca3.1 potassium channels due to epigenetic, transcriptional, or post-translational modifications. By modulating membrane potential, cell volume, or Ca²⁺ signaling, K_Ca3.1 has been proposed to exert pivotal oncogenic functions in tumorigenesis, malignant progression, metastasis, and therapy resistance. Moreover, K_Ca3.1 is expressed by tumor-promoting stroma cells such as fibroblasts and the tumor vasculature suggesting a role of K_Ca3.1 in the adaptation of the tumor microenvironment. Combined, this features K_Ca3.1 as a candidate target for innovative anti-cancer therapy. However, immune cells also express K_Ca3.1 thereby contributing to T cell activation. Thus, any strategy targeting K_Ca3.1 in anti-cancer therapy may also modulate anti-tumor immune activity and/or immunosuppression. The present review article highlights the potential of K_Ca3.1 as an anti-tumor target providing an overview of the current knowledge on its function in tumor pathogenesis with emphasis on vasculo- and angiogenesis as well as anti-cancer immune responses.

Keywords: (1-[(2-chlorophenyl) diphenylmethyl]-pyrazole; 1-EBIO; 1-Ethyl-1,3-dihydro-2H-benzimidazol-2-one; 17-estradiol; BK; E2; KCa3.1; TRAM-34; big conductance Ca2+- and voltage-activated K+ channels; intermediate conductance calcium-activated K+ channel.

Figure 1

KCNN4 mRNA expression levels in breast cancer and their association with patient survival. (A) mRNA expression levels of KCNN1-4 coding for SK1-SK3 and K_Ca3.1 were compared between healthy and breast tumor tissues, measured by RNA sequencing as fragments per kilobase of transcript per million mapped reads (FPKM). Data obtained from The Cancer Genome Atlas [30] revealed no significant difference in a Kruskal–Wallis test with Dunn’s test for multiple comparisons (α = 0.05 for n = 113 healthy and n = 1095 breast tumor tissues). (B) In the Kaplan–Meier plotter [31], significantly prolonged overall survival (OS) was associated with low KCNN4 mRNA levels. Groups were statistically compared by log-rank test (hazard ratio = 1.37 (confidence interval 1.08–1.72) for n = 1030 low and n = 372 high KCNN4-expressing tumors).

Figure 2

Tumorigenesis, progression, and CD45 status in MMTV-PyMT WT and K_Ca3.1 KO mice. Tumor-free survival (TFS) and overall survival (OS) were studied in spontaneous breast cancer-prone MMTV-PyMT wildtype (WT) and K_Ca3.1 KO mice. At a diameter of 15 mm, tumors were harvested for investigating immune cell infiltration. (A) As previously reported [71], tumorigenesis and tumor progression measured by TFS and OS, respectively, were not dependent on MMTV-PyMT WT or K_Ca3.1 KO genotypes (n = 6 each). (B) Staining against the CD45 pan leukocyte marker revealed moderate immune cell infiltration in WT tumors (green), which was absent in K_Ca3.1 KO. Immune cells were generally more abundant in the tumor-surrounding stroma of WT mice, but mostly absent in K_Ca3.1 KO tumor samples. DAPI labelling was performed to visualize nuclei. Results are presented as means ± SEM for n = 4 stroma sections and n = 5 tumor sections of MMTV-PyMT WT (black squares) or K_Ca3.1 KO (red squares) genotypes. Unpaired t-tests differentiated between groups with ** p < 0.01 and *** p < 0.001.

Figure 3

Role of the K_Ca3.1 channel in tumor-associated cells. Tumors from different entities and various microenvironmental cell types, i.e., immune cells, vasculature, fibroblasts (not shown) express functional K_Ca3.1 channels. Physiological roles and tumor behaviors of K_Ca3.1 are cell type-dependent, but involve proliferation, migration and cancer progression. K_Ca3.1 channel expression seems to be a crucial determinant of cancer risk and, in established cancers, K_Ca3.1 upregulation at the end of G₁ phase of the cell cycle was seen in various tumor cell types [36]. By its interaction with [Ca²⁺]_i via its constitutively bound calmodulin (CaM), with other ion channels such as TRP or STIM/Orai, with changes in the membrane potential (V_m), and with apoptotic pathways, K_Ca3.1 may further contribute to aberrant tumor cell signaling. Beyond that, tumor-promoting K_Ca3.1 activity in stromal cells has been described. Several studies find evidence for K_Ca3.1 expression in endothelial and in activated smooth muscle cells of the vasculature pointing to its role in tumor angiogenesis and/or vasculogenesis. Moreover, growth factor signaling was linked to K_Ca3.1 in fibroblasts to promote epithelial-mesenchymal transition in breast cancer (not depicted) [84]. Proper activation and function of various immune cell subsets requires K_Ca3.1. Therefore, perturbed K_Ca3.1 signaling may prevent cancer progression and disturb e.g., the immune cell´s pro-angiogenic program, but also its activity to recognize and eliminate tumor cells. Apparently, the impact of a tumor and stromal versus immune cell K_Ca3.1 inhibition on tumor progression and therapy success and thus also interaction between the different cell types, as indicated by dotted lanes, requires further investigations.