Patch-clamp measurement of ICRAC and ORAI channel activity

Transient receptor potential melastatin 4 channel contributes to migration of androgen-insensitive prostate cancer cells

Impaired Ca²⁺ signaling in prostate cancer contributes to several cancer hallmarks, such as enhanced proliferation and migration and a decreased ability to induce apoptosis. Na⁺ influx via transient receptor potential melastatin 4 channel (TRPM4) can reduce store-operated Ca²⁺ entry (SOCE) by decreasing the driving force for Ca²⁺. In patients with prostate cancer, gene expression of TRPM4 is elevated. Recently, TRPM4 was identified as a cancer driver gene in androgen-insensitive prostate cancer.

We investigated TRPM4 protein expression in cancer tissue samples from 20 patients with prostate cancer. We found elevated TRPM4 protein levels in prostatic intraepithelial neoplasia (PIN) and prostate cancer tissue compared to healthy tissue. In primary human prostate epithelial cells (hPEC) from healthy tissue and in the androgen-insensitive prostate cancer cell lines DU145 and PC3, TRPM4 mediated large Na⁺ currents. We demonstrated significantly increased SOCE after siRNA targeting of TRPM4 in hPEC and DU145 cells. In addition, knockdown of TRPM4 reduced migration but not proliferation of DU145 and PC3 cells. Taken together, our data identify TRPM4 as a regulator of SOCE in hPEC and DU145 cells, demonstrate a role for TRPM4 in cancer cell migration and suggest that TRPM4 is a promising potential therapeutic target.

Differential Redox Regulation of Ca²⁺ Signaling and Viability in Normal and Malignant Prostate Cells

In prostate cancer, reactive oxygen species (ROS) are elevated and Ca(2+) signaling is impaired. Thus, several novel therapeutic strategies have been developed to target altered ROS and Ca(2+) signaling pathways in prostate cancer. Here, we investigate alterations of intracellular Ca(2+) and inhibition of cell viability caused by ROS in primary human prostate epithelial cells (hPECs) from healthy tissue and prostate cancer cell lines (LNCaP, DU145, and PC3). In hPECs, LNCaP and DU145 H2O2 induces an initial Ca(2+) increase, which in prostate cancer cells is blocked at high concentrations of H2O2. Upon depletion of intracellular Ca(2+) stores, store-operated Ca(2+) entry (SOCE) is activated. SOCE channels can be formed by hexameric Orai1 channels; however, Orai1 can form heteromultimers with its homolog, Orai3. Since the redox sensor of Orai1 (Cys-195) is absent in Orai3, the Orai1/Orai3 ratio in T cells determines the redox sensitivity of SOCE and cell viability. In prostate cancer cells, SOCE is blocked at lower concentrations of H2O2 compared with hPECs. An analysis of data from hPECs, LNCaP, DU145, and PC3, as well as previously published data from naive and effector TH cells, demonstrates a strong correlation between the Orai1/Orai3 ratio and the SOCE redox sensitivity and cell viability. Therefore, our data support the concept that store-operated Ca(2+) channels in hPECs and prostate cancer cells are heteromeric Orai1/Orai3 channels with an increased Orai1/Orai3 ratio in cells derived from prostate cancer tumors. In addition, ROS-induced alterations in Ca(2+) signaling in prostate cancer cells may contribute to the higher sensitivity of these cells to ROS.

Cell type-specific glycosylation of Orai1 modulates store-operated Ca2+ entry

N-glycosylation of cell surface proteins affects protein function, stability, and interaction with other proteins. Orai channels, which mediate store-operated Ca(2+) entry (SOCE), are composed of N-glycosylated subunits. Upon activation by Ca(2+) sensor proteins (stromal interaction molecules STIM1 or STIM2) in the endoplasmic reticulum, Orai Ca(2+) channels in the plasma membrane mediate Ca(2+) influx. Lectins are carbohydrate-binding proteins, and Siglecs are a family of sialic acid-binding lectins with immunoglobulin-like repeats. Using Western blot analysis and lectin-binding assays from various primary human cells and cancer cell lines, we found that glycosylation of Orai1 is cell type-specific. Ca(2+) imaging experiments and patch-clamp experiments revealed that mutation of the only glycosylation site of Orai1 (Orai1N223A) enhanced SOCE in Jurkat T cells. Knockdown of the sialyltransferase ST6GAL1 reduced α-2,6-linked sialic acids in the glycan structure of Orai1 and was associated with increased Ca(2+) entry in Jurkat T cells. In human mast cells, inhibition of sialyl sulfation altered the N-glycan of Orai1 (and other proteins) and increased SOCE. These data suggest that cell type-specific glycosylation influences the interaction of Orai1 with specific lectins, such as Siglecs, which then attenuates SOCE. In summary, the glycosylation state of Orai1 influences SOCE-mediated Ca(2+) signaling and, thus, may contribute to pathophysiological Ca(2+) signaling observed in immune disease and cancer.

Measuring endogenous ICRAC and ORAI currents with the patch-clamp technique

Although ICRAC and other store-operated currents are often analyzed by Ca(2+) imaging, whole-cell patch clamp, described here, is the preferred technique to analyze ICRAC whenever possible. The whole-cell patch-clamp protocol can even be used to record endogenous ICRAC in primary cells. The small endogenous current size of ICRAC requires some precautions: First, it is important to inhibit potential interferences from other channels in the cell by carefully choosing the combination of pipette and bath solutions. Second, the noise should be <150 fA root mean square (RMS) when the pipette holder (with its wire) with or without a patch pipette is adjusted over (but not in!) the solution using a high amplification gain (50 mV/pA or higher) of the patch-clamp amplifier. In addition, this protocol draws attention to measures that should be considered when recording ICRAC currents from an overexpression system. This protocol also suggests sets of solutions that can be used for distinguishing ICRAC from potentially interfering currents. In addition to the solutions, the identity of ICRAC can be confirmed by the characteristic inward rectification, its high Ca(2+) selectivity, and the reversal potential of more than +50 mV. A few (mostly nonspecific) CRAC channel blockers are known, which can also be applied for characterization purposes.