The role of Orai–STIM calcium channels in melanocytes and melanoma

Abstract

Calcium signalling within normal and cancer cells regulates many important cellular functions such as migration, proliferation, differentiation and cytokine secretion. Store operated Ca²⁺ entry (SOCE) via the Ca²⁺ release activated Ca²⁺ (CRAC) channels, which are composed of the plasma membrane based Orai channels and the endoplasmic reticulum stromal interaction molecules (STIMs), is a major Ca²⁺ entry route in many cell types. Orai and STIM have been implicated in the growth and metastasis of multiple cancers; however, while their involvement in cancer is presently indisputable, how Orai–STIM‐controlled Ca²⁺ signals affect malignant transformation, tumour growth and invasion is not fully understood. Here, we review recent studies linking Orai–STIM Ca²⁺ channels with cancer, with a particular focus on melanoma. We highlight and examine key molecular players and the signalling pathways regulated by Orai and STIM in normal and malignant cells, we expose discrepancies, and we reflect on the potential of Orai–STIMs as anticancer drug targets. Finally, we discuss the functional implications of future discoveries in the field of Ca²⁺ signalling.

Abbreviations

Brn2

POU domain

class 3

transcription factor 2

CRAC

calcium release activated calcium

ER

endoplasmic reticulum

ERK

extracellular‐signal‐regulated kinases

JARID1B

histone demethylase Jumonji AT‐rich interactive domain 1B

MITF

microphthalmia‐associated transcription factor

PKB

protein kinase B

PKC

protein kinase C

SOCE

store operated calcium entry

STIM

stromal interaction molecule

Tg

thapsigargin

TRP

transient receptor potential

Wnt5a

wingless‐type MMTV integration site family, member 5A

Store operated Ca²⁺ entry and Ca²⁺ release activated Ca²⁺ channels

Store operated Ca²⁺ entry (SOCE), also referred to as capacitive Ca²⁺ entry, is a mechanism by which Ca²⁺ enters cells from the extracellular space (Putney, 1986, 1990; Parekh & Putney, 2005). SOCE is initiated by the engagement of surface receptors which cause depletion of the endoplasmic reticulum (ER) Ca²⁺ stores and subsequent opening of Ca²⁺ selective ion channels at the plasma membrane. While the phenomenon of SOCE was discovered almost 30 years ago, the molecular identity of the plasma membrane channels defining this process took longer to reveal. The first major breakthrough in identifying and characterizing these channels arose in 1992, when Hoth et al. recorded a small Ca²⁺ current in mast cells that was induced by the depletion of ER stores (Hoth & Penner, 1992, 1993). The current was named Ca²⁺ release activated Ca²⁺ (CRAC) current (I _CRAC). Studies that followed contributed in further characterizing the biophysical properties of the channel responsible for I _CRAC (Fasolato et al. 1993; Hoth, 1995; Zweifach & Lewis, 1995; Lepple‐Wienhues & Cahalan, 1996; Parekh et al. 1997; Gilabert & Parekh, 2000; Hoth et al. 2000; Mignen & Shuttleworth, 2000; Prakriya & Lewis, 2001; Voets et al. 2001; Ashot Kozak et al. 2002; Glitsch et al. 2002; Bautista & Lewis, 2004; Zitt et al. 2004). However, the molecular identity of CRAC channels remained elusive until 2005 and 2006 when an ER based Ca²⁺ binding protein named stromal interaction molecule (STIM) and plasma membrane proteins termed Orai were identified as major components of the channel (Liou et al. 2005; Roos et al. 2005; Zhang et al. 2005; Feske et al. 2006; Vig et al. 2006; Yeromin et al. 2006). Presently, three isoforms of Orai (Orai1–3) and two of STIM (STIM1–2) are accepted as major molecular components of the CRAC channel. In the following years, many studies further characterized the properties of Orai–STIM channels (Luik et al. 2006; Soboloff et al. 2006 b; Brandman et al. 2007; Ong et al. 2007; Parvez et al. 2008; Fahrner et al. 2009; Lur et al. 2009; Park et al. 2009; Wang et al. 2010; Bogeski et al. 2010 b; Darbellay et al. 2011; Singaravelu et al. 2011; Hou et al. 2012; Kar et al. 2012; Ng et al. 2012; Zhang et al. 2014; Kar & Parekh, 2015; Miederer et al. 2015). With the molecular components of CRAC channels known, the next task was to understand the function and regulation of Orai and STIM. Not surprisingly, it was soon shown that Orai and STIM are essential in many organs and tissues; they are determinants of immune responses, the cardiovascular system, skin, muscle and blood for example. For the interested reader, we recommend the following review papers (Smyth et al. 2006; Soboloff et al. 2006 a; Hogan & Rao, 2007; Frischauf et al. 2008; Oh‐hora & Rao, 2008; Cahalan, 2009; Cahalan & Chandy, 2009; Di Capite & Parekh, 2009; Dirksen, 2009; Feske, 2009; Shuttleworth, 2009; Vig & Kinet, 2009; Feske, 2010; Hogan et al. 2010; Lee et al. 2010; Parekh, 2010, 2011; Qu et al. 2011; Beech, 2012).

Within the skin, Orai–STIM channels were identified in keratinocytes where they are involved in migration and differentiation as well as in pathological conditions such as atopic dermatitis (Jans et al. 2013; Numaga‐Tomita & Putney, 2013; Vandenberghe et al. 2013; Wilson et al. 2013; Brun et al. 2014). The functional importance of Orai and STIM in melanocytes and in melanoma is increasingly gaining recognition.

Calcium channels in melanocytes and melanoma

Melanocytes are cells that originate from the neural crest. They reside in the epidermal basement layer of the skin or in the eyes where they produce melanin pigment that protects against UV irradiation. Melanocyte malignant transformation leads to melanoma, which is the deadliest form of skin cancer. Once spread into distant organs, metastatic melanoma significantly reduces patient survival (Balch et al. 2009; Garbe et al. 2011). Unfortunately, melanoma incidence is steadily rising and it has been estimated that in the USA 73,870 new cases will be diagnosed in 2015 (4.5% of all US cancers). Around 10,000 patients will succumb to the disease within the same time period (http://seer.cancer.gov/statfacts/html/melan.html).

The recent advances in the treatment of melanoma, achieved by targeted therapies mostly aiming at the mitogen‐activated protein kinase (MAPK) pathway (e.g. vemurafenib, cobimetinib, dabrafenib, trametinib) or immunotherapeutic agents (e.g. the monoclonal antibodies against CTLA‐4 (ipilimumab) and PD‐1 (nivolumab, pembrolizumab)), significantly improved the prognosis for many melanoma patients (Schadendorf et al. 2015). Nevertheless, not all patients respond to treatments and resistance often occurs; therefore, it is still highly important to understand how and why melanocytes transform into melanoma and spread to distant organs. Besides the genetic mutations such as BRAF and NRAS commonly found in cutaneous melanomas, which are clearly linked with the malignant transformation of melanocytes, it has been suggested that the extracellular environment plays an important role in tumour cell growth and metastasis (Villanueva & Herlyn, 2008; Schadendorf et al. 2015). The Ca²⁺ gradient within the skin epidermis is a perfect example for such an environmental cancer modulation. For example, keratinocytes use the Ca²⁺ gradient to control their stage of differentiation by expressing the Ca²⁺ sensing receptor on their surface (Bikle et al. 1996, 2012; Hofer & Brown, 2003). Melanocytes, as all other cells in an organism, also need Ca²⁺ ions as regulators of important cellular functions. For this purpose cells possess a complex molecular machinery which allows them to process Ca²⁺ ions with strict spatial and temporal control. This Ca²⁺‐handling molecular machinery includes Ca²⁺ channels, pumps, transporters and chelators (Clapham, 2007).

The role of Ca²⁺ in melanoma was established in the late 70s and early 80s when researchers established a link between melanoma growth and migration and inhibitors of voltage gated Ca²⁺ channels (CaV) (Parsons et al. 1983). Of note, CaV inhibitors have been shown to also block other Ca²⁺ channels (Bogeski et al. 2010 a). Probably the first detailed study on Ca²⁺ channels, their expression, and electrophysiological characterization in melanoma cells was conducted in 1997 by Allen and colleagues (Allen et al. 1997). In this study, the authors identified and characterized two types of Ca²⁺ channels in four melanoma cell types; the CaV channels and the so‐called Ca²⁺ release activated Ca²⁺ (CRAC) channels, whose molecular identity was elusive at the time. Since then, a number of studies examined the functional importance of different Ca²⁺ channels and pumps in melanocytes and melanoma (Guo et al. 2012; Stanisz, 2013; Bellono & Oancea, 2014; Saul et al. 2014). In the late nineties, a third major group of Ca²⁺ channels known as transient receptor potential (TRP) channels was shown to play a role in melanocytes and melanoma (Duncan et al. 1998). TRP channels belong to a superfamily of close to 30 channels which are responsible for transporting not only Ca²⁺ but also Mg²⁺ and Na⁺ (Montell, 2005; Pedersen et al. 2005). In melanocytes, the melastatin TRP (TRPM) subfamily is represented by four members, including TRPM1, TRPM2, TRPM7 and TRPM8 (Guo et al. 2012). In fact, TRPM1 was originally cloned from melanocytes and was later shown to serve as a tumour suppressor in melanoma (Duncan et al. 1998, 2001; Deeds et al. 2000). TRPM1 is now established as an ion channel required for melanin synthesis and as a tumour marker since its expression levels inversely correlate with melanoma stage and prognosis (Miller et al. 2004; Erickson et al. 2009; Oancea et al. 2009; Oancea & Wicks, 2011; Bellono & Oancea, 2014). Other TRPM family members as well as TRPA1, TRPV1 and TRPML3 channels have been identified and demonstrated to control melanocyte and/or melanoma function (Atoyan et al. 2009; Choi et al. 2009; Oehler et al. 2012; Pucci et al. 2012; Bellono et al. 2013). Since this review focuses mostly on the Orai–STIM Ca²⁺ channels, we will not go into further details regarding the CaV and the TRP channels and suggest several publications which describe the role of ion channels in melanocytes and melanoma in more detail (Devi et al. 2009; Oancea et al. 2009; Lu et al. 2010; Oancea & Wicks, 2011; Guo et al. 2012; Saul et al. 2014; Ho & Lee, 2015).

Orai and STIM in cancer: a brief overview

Orai and STIM proteins, and thereby the CRAC channels, are important determinants of Ca²⁺ homeostasis and have been reported to be regulated in different tissues and cells, but also in several cancers (Yang et al. 2009; Feng et al. 2010; Flourakis et al. 2010; Chen et al. 2011; Hou et al. 2011; McAndrew et al. 2011; Aytes et al. 2012; Holzmann et al. 2013; Motiani et al. 2013 a; Dragoni et al. 2014; Kim et al. 2014; Kondratska et al. 2014; Stanisz et al. 2014; Holzmann et al. 2015; Hooper et al. 2015; Wang et al. 2015; Wong & Chang, 2015; Xu et al. 2015). The most extensively examined proteins in this regard are the predominant isoforms Orai1 and STIM1. Less is known about the role of the other CRAC channel components Orai2, Orai3 and STIM2 (Aytes et al. 2012; Faouzi et al. 2013; Motiani et al. 2013 b; Sobradillo et al. 2014; Stanisz et al. 2014). According to current published reports, Orai1 and STIM1 are upregulated in most cancers, suggesting an increased SOCE as a result. In some studies, this was proven experimentally, but in others this important parameter was, unfortunately, not quantified. Having in mind that the relative expression between Orai and STIM determines SOCE amplitude (Hoover & Lewis, 2011) rather than the absolute levels (e.g. upregulation of Orai1 alone would lead to decreased SOCE) such an experimental examination of not only SOCE but also of resting Ca²⁺ levels is absolutely critical. This information is essential in our understanding of the role of Orai and STIM in cancer development, but also for cell growth and metastasis. Interestingly, functionally relevant single nucleotide polymorphisms in Orai and STIM have also been reported in some cancers (Chang et al. 2014).

It is well known that Ca²⁺ is needed for proliferation and migration of various cell types (Clapham, 2007). Hence, changes in Ca²⁺ homeostasis can undoubtedly affect cancer cell biology. However, it is still unclear if alterations in Ca²⁺ homeostasis can contribute to malignant transformation and/or change the metastatic potential. For more on this issue, please refer to a recent review by Hoth (2016). A more in‐depth look into the role of Orai and STIM in different cancers will bring researchers one step further in their quest for using Orai–STIM Ca²⁺ channels as anticancer drug targets. In this regard the melanocyte–melanoma system, which we describe below, provides important hints.

Orai and STIM in melanocytes and melanoma

The first evidence regarding the expression and functional role of Orai and STIM in mouse melanoma cells was reported in 2010 (Feldman et al. 2010). The authors showed that Orai1 and STIM1 are expressed in B16BL6 mouse cells where they control SOCE. Moreover, SOCE was shown to be essential for the activation of protein kinase B (PKB/AKT), which in turn is able to control proliferation and apoptosis in malignant cells. The authors demonstrate that pharmacological inhibition of either PKB/AKT or SOCE, as well as silencing of STIM1, caused a reduction in cell growth and increased cell death. Hence, they proposed that SOCE might be a potential target for melanoma. Interestingly, the authors could detect a much higher SOCE in malignant B16BL6 cells compared to their non‐malignant counterparts; however, they could not assign the difference to altered Orai1 and STIM1 expression because both cell types had comparable protein levels. Since the plasma membrane Ca²⁺‐ATPase (PMCA) was also refuted as a cause for the different SOCE amplitudes, Feldman and colleagues tested mitochondrial Ca²⁺ buffering as a possible regulation mechanism and found that, indeed, mitochondrial Ca²⁺ uptake after SOCE was much higher in the malignant cells. This allowed the Orai1 channels to remain open for a longer time period and thereby efficiently activate PKB/AKT. A study published by the same group 2 years later expanded on these observations and suggested that lipid rafts are essential for proper SOCE function and activation of PKB/AKT in malignant B16BL6 cells (Fedida‐Metula et al. 2012).

Although these first studies were valuable they were performed in mouse cell lines, which are physiologically different from human skin cells (Herlyn & Fukunaga‐Kalabis, 2010). Hence, we examined Orai and STIM in primary human melanocytes and showed that all three Orai isoforms as well as the two STIMs are expressed in these cells (Stanisz et al. 2012). At the mRNA level, Orai1 and STIM2 were the predominant isoforms expressed and this was an interesting finding given that cells of neural origin usually utilize Orai2 and STIM2 as the major channel components. Why would melanocytes, which also originate from the neural crest, switch from Orai2 to Orai1 remains to be answered. It should be noted that Orai and STIM expression levels did not depend on the skin type since we did not observe any significant difference across a variety of human donors displaying distinct skin types and pigmentation (Stanisz et al. 2012). To analyse the functional role of Orai and STIM in this study, we used endothelin‐1, which is a hormone released upon UV radiation and known to activate SOCE. Measurements of melanin content and tyrosinase activity suggested that Orai–STIM channels control melanin synthesis and are thereby involved in adaptive tanning. This hypothesis, while needing confirmation, indicates a potential use for modulators of Orai and STIM in skin pigmentation control.

Given the important role of Orai and STIM in healthy melanocytes, we next examined the function of these channels in human melanoma (Stanisz et al. 2014). Our expression analyses showed no significant differences between melanoma cells and primary melanocytes or between different melanoma cell lines. Moreover, SOCE was also not overtly different between healthy and malignant cells, which did not fit with the findings of Feldman et al. (2010). This, however, does not prove that SOCE is not involved in malignant transformation of melanocytes or has no additional role in melanoma. In fact, further experiments showed that the melanoma phenotype is dependent on Orai‐ and STIM‐mediated SOCE. Inhibition of the channel by pharmacological inhibitors, or by silencing Orai1 and/or STIM2, caused melanoma cells to grow faster, but reduced their invading potential. This correlated with the upregulation of proteins such as microphthalmia‐associated transcription factor (MITF) that indicate a proliferative melanoma cell phenotype and the downregulation of slow‐cycling cell markers such as JARID1B and Brn2 (Hoek & Goding, 2010; Roesch et al. 2013; Stanisz et al. 2014). Immunostaining of human melanoma patient samples confirmed the expression of Orai1 and STIM2 in vivo. Higher expression levels near the melanoma invasive front further strengthened the hypothesis that Orai1 and STIM2 are involved in tumour invasion.

Another study on Orai and STIM in melanoma, published by Umemura and colleagues, showed that Orai1 and STIM1 are expressed in both melanoma and melanocytes (Umemura et al. 2014). However, none of the other three CRAC channel components were detected in melanoma cells or in melanocytes. Moreover, the authors observed high SOCE in metastatic melanomas, with much lower SOCE in primary melanomas and healthy melanocytes, an observation that was not in full agreement with the work by Feldman et al. or our own observations. Umemura et al. next investigated the effects of Orai–STIM channels on melanoma proliferation and invasion by using gene silencing and pharmacological inhibitors. Reduction of SOCE strongly suppressed cell migration in vitro and the formation of tumour colonies in vivo. However, the effects on cellular proliferation were mild; for example, silencing of Orai1 did not cause any change in cell growth in two out of three examined melanoma cell lines. Moreover, an inhibitor of SOCE (YM58483) slightly suppressed cell growth but mostly at concentrations higher than that required to block the channels, indicating possible off‐target effects. Interestingly, the authors identified the extracellular‐signal‐regulated kinases (ERK) signalling pathway to be regulated by SOCE and to affect phenotypic changes (Umemura et al. 2014). This suggests that there is a link between SOCE and MAPK signalling, a key pathway involved in melanoma pathobiology.

Recently, another study focusing on Orai and STIM in melanoma suggested that SOCE is involved in invadopodia formation and cancer invasion (Sun et al. 2014). Sun et al. came to this conclusion by counting invadopodia and measuring extracellular matrix degradation in melanoma cells treated with Ca²⁺ chelators (BAPTA‐AM and EGTA), pharmacological inhibitors of Orai1 (2‐APB and SKF96365), and by silencing Orai1 and STIM1 expression. The authors did not perform expression analyses on the other three CRAC channel components, namely Orai2, Orai3 and STIM2, and thereby did not consider the possibility that STIM2 might be more functionally important than STIM1 as suggested by our study (Stanisz et al. 2014). In another set of experiments, the authors tested the effects of SOCE on invadopodia formation by overexpressing STIM1 but not Orai1. To activate SOCE, the authors starved the cells and reintroduced 10% fetal calf serum (FCS) in the measuring bath. They examined the possible link between SOCE and invadopodia formation by screening for kinase expression and identified a master regulator of invasive cell phenotypes, the SRC kinase, to be regulated by cytosolic Ca²⁺ and hence, by Orai1 and STIM1. Moreover, they showed that inhibition of SOCE reduced the localization of membrane type 1 metalloprotease (MT1‐MMP) to the plasma membrane and thereby explained how Orai1 and STIM1 can promote invasion. Next, Sun and colleagues questioned the role of STIM1 in melanoma metastasis. For this purpose, the authors used 1205Lu melanoma cells, which are highly metastatic and form lung metastases in immunosuppressed mice. The cells were first stably transfected with STIM1 shRNA and then they were tested for proliferation changes. Contrary to previously published findings, the authors did not detect any difference in proliferation between control and STIM1 silenced cells. Next, the authors asked if the specific pattern of serum induced Ca²⁺ oscillations were necessary for invadopodia formation; this was tested by stimulating the cells with the sarco(endo)plasmic reticulum Ca²⁺‐ATPase (SERCA) inhibitor thapsigargin (Tg) or ionomycin which both induce sustained Ca²⁺ entry. Quantification of invadopodia formation showed that Tg and ionomycin caused a reduction in invadopodia formation in contrast to the serum‐induced activation of SOCE. Although this is an intriguing observation, the authors did not further analyse the mechanism behind the opposite effects of drug versus serum‐induced SOCE in melanoma invasion.

The most recent paper on Orai and STIM in melanoma was published by Hooper et al.; the authors examined and compared the role of CRAC channels in melanoma cells previously characterized as invasive and non‐invasive, mostly based on Wnt5a expression levels (Hooper et al. 2015). SOCE was induced either via receptor stimulation (histamine) or by Tg or ionomycin treatment. Interestingly, the authors found that the invasive Wnt5a expressing cells had a very small SOCE when compared to the non‐invasive melanomas. This finding is not in a full agreement with the observations of some of the above‐mentioned studies which reported that invasive melanoma has much higher SOCE than primary melanoma cell lines or melanocyte cell lines. Expression analysis, however, showed no overt differences in the abundance of Orai1 and STIM1, or of the other three CRAC channel components Orai2, Orai3 and STIM2 at the protein as well as the mRNA level. Of note, at the mRNA level it appears that Orai2 and Orai3 are more highly expressed than Orai1 in almost all examined melanoma cells. This interesting observation was not further examined. Remarkably, even the transient overexpression of Orai1 and STIM1 could not reinstate SOCE in the invasive cells. Hence, the authors went on to explore the molecular mechanism(s) responsible for the inhibition of the Orai–STIM driven Ca²⁺ entry. Because the cells examined were characterized by the expression pattern of Wnt5a, they were able to demonstrate that by manipulating Wnt5a levels, they could also affect SOCE. In invasive cells, silencing of Wnt5a led to SOCE recovery while in non‐invasive cells, overexpression of Wnt5a reduced the Ca²⁺ entry. In addition, based on the fact that Orai1 is inhibited by phosphorylation and that some melanomas have high protein kinase C (PKC) activity, the authors examined the role of this kinase in their cellular model. Their results demonstrate that high Wnt5a levels correlate with high PKC activity, which phosphorylates and inhibits the channel. Accordingly, by using PKC inhibitors or by mutating the phosphorylation sites in Orai1, the authors were able to recover SOCE in the highly invasive melanoma cells (Hooper et al. 2015).

Concluding remarks and future perspectives

The Orai–STIM research field in melanocytes and melanoma is still in its infancy, yet findings already point to their important role in normal skin function and malignancy. As shown in Table 1, all studies examining the role of Orai and STIM in melanocytes and melanoma demonstrate the important functional role of these channels. These studies show that SOCE regulates proliferation, migration and invasion as well as melanin synthesis in melanocytes and melanoma. Moreover, they identify several key signalling pathways involved in disease which are controlled by Ca²⁺ and hence, by Orai and STIM. However, the findings are far from being in a full agreement and need careful dissection and confirmation in the future. The most obvious reasons for the discrepant findings might be the dynamic nature of the calcium signalling system and its dependency on endogenous and exogenous parameters such as the heterogeneity of melanoma cell lines, varying culturing conditions, and different experimental techniques. While additional work is required in order to fully explain how Orai and STIM regulate melanocyte and melanoma biology, it is clear that their functional role in these cells is important and further knowledge in this area is very much needed.

Table 1.

Summary of the major findings regarding Orai and STIM functional effects in melanocytes and melanoma

					Effects of	Signalling
		CRAC		Effects of	Orai–STIM on	pathways/
		components		Orai–STIM on	migration and	proteins	Other functional
	Cells examined	identified	SOCE/I CRAC	proliferation	invasion	involved	effects
Feldman et al. 2010	Malignant and non‐malignant B16BL6 mouse cells	Orai1 and STIM1	SOCE is detectable and is higher in the malignant cells	ND	ND	PKB/AKT	Inhibition of mitochondrial Ca2+ buffering sensitizes melanoma cells to apoptosis
Stanisz et al. 2012	Primary human melanocytes	Orai1–3, STIM1–2(Orai1 and STIM2 highest expression)	SOCE and I CRAC are detectable	Promote	ND	ND	Orai1 and STIM2 promote melanin synthesis
Stanisz et al. 2014	Human melanoma	Orai1–3, STIM1–2 (Orai1 and STIM2 highest expression)	SOCE is detectable (no differences between melanocytes and different melanoma cell lines)	Suppress	Promote	MITFJARID1BBrn2	Orai1 and STIM2 are highly expressed in invasive tumour fronts in human melanoma patient samples
Umemura et al. 2014	Human melanoma	Orai1, STIM1 (other Orai isoforms and STIM2 not detected)	SOCE is increased in invasive melanoma cell lines vs. primary melanoma cells and melanocytes	Promote (minor effect)	Promote	ERK	Orai1‐ and STIM1‐silencing resulted in fewer lung metastases
	Melanocyte cell line	Low Orai1 expression
Sun et al. 2014	Human melanoma	Orai1, STIM1	SOCE is detectable10% FCS induces Ca2+ oscillations while Tg sustained Ca2+ entry	No effect	Promote (oscillatory SOCE)Suppress (sustained SOCE)	Src	Promotion of plasma membrane localization of MT1‐MMP and invadopodium maturation
Hooper et al. 2015	Human melanoma	Orai1‐3, STIM1‐2 (Orai2, ‐3 higher expression than Orai1; STIM1 higher expression than STIM2)	SOCE and I CRAC are detectable but are much lower in highly invasive melanoma cells	ND	Suppress	Wnt5a, PKC	Wnt5a regulates SOCE amplitude and melanoma invasiveness

ND, not determined. MT1‐MMP, membrane type 1 metalloprotease; Src, proto‐oncogene tyrosine‐protein kinase.

Targeting these channels for the purpose of pigmentation or disease control may be challenging; however, this can be overcome with additional studies focusing on function, dynamics, signalling, mutations and observations with current therapies. Given the recent success of immunotherapies in melanoma patients, the melanoma field could benefit from studies that will examine the role of Orai–STIM channels in the immune‐cancer interplay. Finally, Orai and STIM are only one of the channels involved in melanocyte/melanoma Ca²⁺ homeostasis and they showcase the intricate yet important role of Ca²⁺ in melanocyte physiology and pathology. Our knowledge of these channels and calcium‐related processes is integral to both understanding normal skin biology and for curbing disease.

Additional information

Competing interests

None declared.

Funding

This work is supported by the Medical Faculty of Saarland (HOMFOR excellent research grant to I.B. and HOMFOR research grant to H.S.) and by the Deutsche Forschungsgemeinschaft (DFG) (SFB1027 project C4 and BO3643/3‐1 research grant to I.B.; R3577/3‐1 to A.R.). A.V. and M.H. acknowledge the NIH grant R01 CA047159.

Biographies

Hedwig Stanisz graduated with an MD from Homburg Medical School, University of Saarland, Germany in 2008. Subsequently, she started her dermatology training at the Department of Dermatology at the University Clinics in Homburg, Germany, achieving board certification in Dermatology in 2013. In her doctoral work, which she defended in 2012, she examined the functional role of MCR1 receptor polymorphisms in primary human melanocytes and fibroblasts. Her current research interests are in understanding melanocyte biology with an emphasis on melanogenesis and mitogenesis, but also in examining the role of calcium metabolism in melanomagenesis and melanoma pathobiology.

Adina Vultur received her Ph.D. from the Department of Pathology at Queen's University, Canada in 2005 with a governor general's gold medal award. With an NSERC fellowship, she obtained postdoctoral training at City of Hope, CA, USA, with Dr. Richard Jove, focusing on Src and Stat3 signalling in breast cancer. She then joined Meenhard Herlyn's laboratory at the Wistar Institute, PA, USA, in 2007, where she is now a staff scientist. Dr. Vultur's current goals are to identify novel targets and treatment strategies against malignant melanoma. Her investigations also focus on signal transduction and the role of the microenvironment in tumor heterogeneity, treatment resistance, and metastasis.