Chapter Four - Type 3 IP3 receptors: The chameleon in cancer
Introduction
Calcium (Ca2+) is a universal second messenger involved in a plethora of physiological processes including neuronal transmission, muscle contraction, gene transcription, secretion, cellular motility, cell proliferation, and apoptosis (Bultynck and Parys, 2018; Parys and Bultynck, 2018a, Parys and Bultynck, 2018b; Sammels et al., 2010). Ca2+ signals in the cell can arise from the Ca2+ release from intracellular stores (i.e. endoplasmic reticulum (ER), mitochondria, Golgi apparatus, etc.) and the entry of Ca2+ from the extracellular environment (Bootman and Bultynck, 2019). These mechanisms rely on different kinds of Ca2+ channels, pumps, and exchangers and are tightly regulated by a myriad of proteins (Chen et al., 2019; Lewis, 2019; Parys and Vervliet, 2020; Prole and Taylor, 2019).
The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a ubiquitous Ca2+ channel mediating Ca2 + release from the ER, the main intracellular Ca2+ storage organelle. IP3Rs open in response to IP3, which is produced through hydrolysis of phosphatidyl inositol 4,5-bisphosphate (PIP2) following phospholipase C (PLC) activation elicited by extracellular stimuli (Berridge, 2009; Berridge et al., 1984). Three isoforms of the IP3R exist (IP3R1, IP3R2 and IP3R3, respectively encoded by the genes ITPR1, ITPR2, and ITPR3 in human) with strong sequence homology (Foskett et al., 2007). Their relative expression levels vary not only according to tissue type, but also to cellular location and stage of development (Ivanova et al., 2014; Taylor et al., 1999; Vermassen et al., 2004). This allows the IP3R to meet very specific needs of the cell. The channels are assembled in homo- and heterotetramers (Monkawa et al., 1995) and display subtle differences in affinity toward IP3, Ca2+ and adenosine triphosphate (ATP) (Patel et al., 1999). Of note, heterotetramers do not simply exhibit a blend of the behavior of the constitutive subunits (Alzayady et al., 2013; Chandrasekhar et al., 2015, Chandrasekhar et al., 2016). For example, the activity of channels formed from coupled IP3R1 and IP3R2 heterodimers was predominantly dictated by IP3R2 (Alzayady et al., 2013) while IP3R2/IP3R3 complexes exhibited unique properties (Chandrasekhar et al., 2016). At the monomer level, IP3Rs display three functional regions: the ligand binding domain, the central, modulatory domain and the C-terminal region. The central, modulatory domain displays more variability among the isoforms while the N-terminal ligand binding domain and the C-terminal region are the most conserved (Prole and Taylor, 2019). The C-terminal region contains the transmembrane channel domain and determinants for tetramer formation (Boehning et al., 2001; Mignery and Sudhof, 1990). All three domains contain interaction sites for an array of modulatory proteins (Ivanova et al., 2014, Ivanova et al., 2019a; Parys and De Smedt, 2012; Parys and Vervliet, 2020; Prole and Taylor, 2016; Yule et al., 2010).
The IP3R opens upon binding of IP3 whereby the isoforms exhibit different binding affinities for IP3 (IP3R2 > IP3R1 > IP3R3) (Hattori et al., 2004; Miyakawa et al., 1999; Tu et al., 2005). Recent work revealed that four IP3 molecules ought to bind to a tetrameric IP3R in order to open the channel (Alzayady et al., 2016; Taylor and Konieczny, 2016). It is well known that global Ca2+ signals are initiated through Ca2+ puffs (Bootman et al., 1997). These puffs arise from the simultaneous opening of a small cluster of IP3Rs (Thomas et al., 2000). Fascinatingly, not all IP3Rs within a cell are equally sensitive to IP3 for generating Ca2+ puffs. Recent work indicated that IP3Rs located close to ER-plasma membrane junctions are “licensed” to respond to IP3 and generate puffs (Prole and Taylor, 2019; Thillaiappan et al., 2017). The mechanisms that render this subpopulation of IP3Rs licensed and thus to be responsive to IP3 for the generation of Ca2+ puffs remain to be elucidated.
The regulation of the IP3Rs is further fine-tuned through Ca2+-dependent modulation of their ability to flux Ca2+ in response to IP3 (Finch et al., 1991; Kaftan et al., 1997). The effect of Ca2 + on IP3R activity is bell-shaped, with a stimulatory effect of cytosolic [Ca2+] in the submicromolar range and an inhibitory effect of high cytosolic [Ca2+] (Bezprozvanny et al., 1991; Finch et al., 1991; Iino, 1990). In addition, the channel is regulated by Ca2+ levels in the lumen of the ER, decreasing its affinity toward IP3 when ER-Ca2+ levels are low (Missiaen et al., 1992, Missiaen et al., 1994; Nunn and Taylor, 1992; Parys et al., 1993a). This mechanism helps to prevent excessive depletion of the ER, protecting the cell from harmful consequences. Moreover, IP3R activity is regulated by ATP, where its activity is increased by concentrations at the submillimolar level while higher concentrations have an inhibitory effect (Bezprozvanny and Ehrlich, 1993; Iino, 1991). Although all isoforms are modulated by ATP, some isoform-specific characteristics have been observed (Maes et al., 2000; Tu et al., 2005). ATP promotes Ca2+ release of IP3R2 and IP3R3 by increasing their unitary open probability (the effect on IP3R2 is observed only at low IP3 concentrations) while it increases IP3R1 activity by improving its Ca2+ sensitivity (Betzenhauser et al., 2008; Wagner and Yule, 2012).
Furthermore, IP3Rs are modulated by their redox state. Reactive oxygen species (ROS) cause oxidation of the channel to sensitize Ca2+ release and promote mitochondrial Ca2+ uptake (Bansaghi et al., 2014). Redox regulation by ROS or thiol-modifying agents occurs via oxidation of cysteine residues in the N-terminal domain of IP3R (Joseph et al., 2018; Khan et al., 2013). Thiol-modifying reagents, such as thimerosal, modulate the IP3R in an isoform-specific fashion. While low micromolar concentrations of thimerosal induce IP3R-medicated Ca2+ release, concentrations higher than 10 μM displayed an inhibitory effect (Parys et al., 1993b). Thimerosal acts synergistically with Ca2+ to generate its stimulatory effect (Bultynck et al., 2004) on IP3R1 and IP3R2, but not IP3R3 (Bultynck et al., 2004; Khan et al., 2013; Missiaen et al., 1998). This effect is mediated through the stabilization of the channel in an active conformation, characterized by an interaction between the suppressor domain and IP3-binding domain of the N-terminal region (Bultynck et al., 2004). Redox regulation of IP3Rs in living cells could be particularly relevant at ER-mitochondria contact sites (Booth et al., 2016).
Finally, an increasing number of proteins interacting with the IP3R are being identified not only as regulatory, but also as scaffolding proteins, structural proteins and motor proteins (Parys and De Smedt, 2012; Parys and Vervliet, 2020; Prole and Taylor, 2016).
Dysregulation of IP3Rs and Ca2+ signaling has been described to alter cell functions and has been indeed linked to several diseases. Moreover, recent studies also revealed several disease-causing mutations in ITPR genes that affected IP3R-channel function, resulting in abnormal ER Ca2+ fluxes (Hisatsune and Mikoshiba, 2017; Kerkhofs et al., 2018; Terry et al., 2018). In cancer, remodeling of the Ca2+-signaling toolkit, including alterations at the level of the IP3R, impacts several oncogenic hallmarks (Ivanova et al., 2017; Kania et al., 2017; Monteith et al., 2017; Morciano et al., 2018; Roberts-Thomson et al., 2019). Moreover, several oncogenes and tumor suppressors can directly target IP3Rs to modulate its properties in such a way that oncogenesis, malignant cell survival and chemotherapeutic resistance are favored (Akl and Bultynck, 2013; Bittremieux et al., 2016). Particularly, changes in IP3R3 function or expression have been associated with oncogenesis and cell death resistance. Downregulation of IP3R3 has been proposed to be key in oncogenesis as the channel is implicated in transferring proapoptotic Ca2+ signals to mitochondria. Yet, it seems that this may not be a universal concept among all cancer types, since IP3R3 is upregulated in several cancer types and contributes to malignant cell survival and behavior, adding complexity to IP3R3 in cancer (Sneyers et al., 2019). In this review, we present an up-to-date view on the complex role of IP3R3 channels in cancer organized in three points (i) the role of IP3R3-mediated Ca2+ signaling in cell death and survival; (ii) the dysregulation of IP3R3 expression in cancers; and (iii) the consequence of altered IP3R3-mediated Ca2+ release in cancer.
Section snippets
Ca2+ signaling at the mitochondria associated-ER membranes
IP3Rs play an important role in the Ca2+ transfer from the ER to the mitochondria, and subsequent maintenance of mitochondrial bioenergetics (Marchi et al., 2017). The relationship between ER Ca2+ and mitochondrial metabolism is highly dependent on IP3R-mediated Ca2+ release. It has been demonstrated that ER Ca2+ depletion induced by thapsigargin, an inhibitor of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), give rise to Ca2+ leakage all over the ER membrane but is unable to increase
Expression of IP3R3 in cancer cells and tissues
IP3Rs fulfill key functions in cell death and survival processes. We described in the previous section the pivotal role of IP3Rs in ER-mitochondria Ca2+ transfers, with a marked interest in the third isoform. As the IP3R3 has been particularly linked to apoptosis, alteration of its expression or function has been suggested to be responsible for tumor development.
The role of IP3R3 in cancer: Tumor suppression versus oncogenesis
Although IP3R3 has been reported to be involved in cancer, opposing roles for the channel have surfaced over the years. A vast literature indicates a tumor suppressor role for IP3R3 by promoting proapoptotic ER-mitochondrial Ca2+ signaling, whereby IP3R3 downregulation or inhibition promotes oncogenesis and cell death resistance. However, several studies reported a dependence of cancer cells toward IP3R3 for their survival, whereby IP3R3 upregulation could account for uncontrolled cell growth
Conclusions
We aimed to provide a balanced view on the roles and implication of IP3R3 in cancer, whereby IP3R3 is associated with both anti- and procancer features. Indeed, in some cancers, loss of IP3R3 results in apoptosis resistance enabling the survival of malignant cells despite cellular damage and genomic aberrations. Here, loss of IP3R3 provides cancer cells with a growth advantage and restoring IP3R3 expression could promote cancer cell death by enhancing apoptotic sensitivity. In other cancers
Acknowledgments
The work was supported by Grants from the Research Foundation—Flanders (FWO) (Grants 6.057.12, G.0819.13, G.0C91.14, G.0A34.16, and G.0901.18) and by the Research Council of the KU Leuven (Grants 14/101 and C14/19/099). N.R. is recipient of a postdoctoral fellowship of the FWO and F.S. is holder of a Ph.D. fellowship of the FWO. G.B. and J.B.P. are part of the FWO Scientific Research Network CaSign (W0.019.17N).
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