Orai1 Boosts SK3 Channel Activation.

Herwig Grabmayr,Adéla Tiffner,Rainer Schindl,Romana Schober,Irene Frischauf,Carmen Höglinger,Matthias A Hediger,Lena Maltan,Isabella Derler,Marc Fahrner,Denis Krivic,Matthias Sallinger,Valentina Hopl,Victoria Lunz,Rajesh Bhardwaj

doi:10.3390/cancers13246357

Abstract

Simple SummaryBreast, colon, and prostate cancer account for about a third of cancer cases and a fifth of cancer deaths. At the molecular level, one reason for the development of cancer is the dysfunction or altered co-regulation of cellular proteins. In this study, we focused on the co-regulation of ion channels, specifically the prominent Ca2+ ion channel Orai1 and the Ca2+ activated K+ ion channel SK3. It has recently been reported that their interplay promotes the growth of breast and colon cancer cells, but the molecular determinants for their co-regulation have remained elusive. In this study, we set out to characterize their interplay and the crucial regions therefore required. Moreover, we found that the function of prostate cancer cells is also controlled by the interplay of Ca2+ and the Ca2+ sensitive K+ channels. Our findings provide a better understanding of the co-regulation of these ion channels, which could be used in the future for the development of novel therapeutics.The interplay of SK3, a Ca2+ sensitive K+ ion channel, with Orai1, a Ca2+ ion channel, has been reported to increase cytosolic Ca2+ levels, thereby triggering proliferation of breast and colon cancer cells, although a molecular mechanism has remained elusive to date. We show in the current study, via heterologous protein expression, that Orai1 can enhance SK3 K+ currents, in addition to constitutively bound calmodulin (CaM). At low cytosolic Ca2+ levels that decrease SK3 K+ permeation, co-expressed Orai1 potentiates SK3 currents. This positive feedback mechanism of SK3 and Orai1 is enabled by their close co-localization. Remarkably, we discovered that loss of SK3 channel activity due to overexpressed CaM mutants could be restored by Orai1, likely via its interplay with the SK3–CaM binding site. Mapping for interaction sites within Orai1, we identified that the cytosolic strands and pore residues are critical for a functional communication with SK3. Moreover, STIM1 has a bimodal role in SK3–Orai1 regulation. Under physiological ionic conditions, STIM1 is able to impede SK3–Orai1 interplay by significantly decreasing their co-localization. Forced STIM1–Orai1 activity and associated Ca2+ influx promote SK3 K+ currents. The dynamic regulation of Orai1 to boost endogenous SK3 channels was also determined in the human prostate cancer cell line LNCaP.

Highlights

Calcium (Ca2+ ) ionsplay a variety of critical roles in many vital aspects of cellular life.Ca2+ signaling triggers short- and long-term cellular processes responsible for fundamental physiological functions, including secretion, gene regulation, muscle contraction, activation of the immune system, cell proliferation, cell motility, and apoptosis [1,2,3,4,5,6,7]
We show that Orai1 promotes SK3 K+ currents and restores SK3 channel activity, which has been abolished by CaM mutants
Prior to our investigations of a potential co-regulation of SK3 and Orai1 channels in HEK 293 cells, we systematically examined individual SK3 and STIM1-Orai1 current activation upon their heterologous expression, using distinct solution conditions, differing mainly in intracellular

Summary

Introduction

Calcium (Ca2+ ) ionsplay a variety of critical roles in many vital aspects of cellular life. Ca2+ signaling triggers short- and long-term cellular processes responsible for fundamental physiological functions, including secretion, gene regulation, muscle contraction, activation of the immune system, cell proliferation, cell motility, and apoptosis [1,2,3,4,5,6,7]. Defects in the cellular Ca2+ homeostasis due to dysfunction or changes in Ca2+ signal transduction can lead to severe immune deficiencies, neurological diseases, cardiovascular problems, and various types of cancers [8]. Ca2+ enters the cell via diverse Ca2+ ion channels. The resulting elevations of intracellular Ca2+ levels can regulate the activity of Ca2+ sensing ion channels. Several reports have already provided evidence of an interplay of Ca2+ and Ca2+ regulated potassium (K+ ) ion channels such as CaV 2.3 with BK channels [9], Orai with the ether a-gogo K+ channel hEag1 [10] or BK channels [11,12], Orai1 [13], or TRPV6 [14] with the small conductance Ca2+ -activated activated K+ channels SK4 and Orai with its homolog

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