RESUMO
Members of the Bcl-2-protein family are key controllers of apoptotic cell death. The family is divided into antiapoptotic (including Bcl-2 itself, Bcl-xL, Mcl-1, etc.) and proapoptotic members (Bax, Bak, Bim, Bim, Puma, Noxa, Bad, etc.). These proteins are well known for their canonical role in the mitochondria, where they control mitochondrial outer membrane permeabilization and subsequent apoptosis. However, several proteins are recognized as modulators of intracellular Ca2+ signals that originate from the endoplasmic reticulum (ER), the major intracellular Ca2+-storage organelle. More than 25 years ago, Bcl-2, the founding member of the family, was reported to control apoptosis through Ca2+ signaling. Further work elucidated that Bcl-2 directly targets and inhibits inositol 1,4,5-trisphosphate receptors (IP3Rs), thereby suppressing proapoptotic Ca2+ signaling. In addition to Bcl-2, Bcl-xL was also shown to impact cell survival by sensitizing IP3R function, thereby promoting prosurvival oscillatory Ca2+ release. However, new work challenges this model and demonstrates that Bcl-2 and Bcl-xL can both function as inhibitors of IP3Rs. This suggests that, depending on the cell context, Bcl-xL could support very distinct Ca2+ patterns. This not only raises several questions but also opens new possibilities for the treatment of Bcl-xL-dependent cancers. In this review, we will discuss the similarities and divergences between Bcl-2 and Bcl-xL regarding Ca2+ homeostasis and IP3R modulation from both a molecular and a functional point of view, with particular emphasis on cancer cell death resistance mechanisms.
Assuntos
Apoptose , Neoplasias , Humanos , Sobrevivência Celular , Apoptose/fisiologia , Retículo Endoplasmático , Mitocôndrias/metabolismo , Sinalização do Cálcio/fisiologia , Neoplasias/tratamento farmacológico , Neoplasias/metabolismoRESUMO
Anti-apoptotic Bcl-2-family members not only act at mitochondria but also at the endoplasmic reticulum, where they impact Ca2+ dynamics by controlling IP3 receptor (IP3R) function. Current models propose distinct roles for Bcl-2 vs. Bcl-xL, with Bcl-2 inhibiting IP3Rs and preventing pro-apoptotic Ca2+ release and Bcl-xL sensitizing IP3Rs to low [IP3] and promoting pro-survival Ca2+ oscillations. We here demonstrate that Bcl-xL too inhibits IP3R-mediated Ca2+ release by interacting with the same IP3R regions as Bcl-2. Via in silico superposition, we previously found that the residue K87 of Bcl-xL spatially resembled K17 of Bcl-2, a residue critical for Bcl-2's IP3R-inhibitory properties. Mutagenesis of K87 in Bcl-xL impaired its binding to IP3R and abrogated Bcl-xL's inhibitory effect on IP3Rs. Single-channel recordings demonstrate that purified Bcl-xL, but not Bcl-xLK87D, suppressed IP3R single-channel openings stimulated by sub-maximal and threshold [IP3]. Moreover, we demonstrate that Bcl-xL-mediated inhibition of IP3Rs contributes to its anti-apoptotic properties against Ca2+-driven apoptosis. Staurosporine (STS) elicits long-lasting Ca2+ elevations in wild-type but not in IP3R-knockout HeLa cells, sensitizing the former to STS treatment. Overexpression of Bcl-xL in wild-type HeLa cells suppressed STS-induced Ca2+ signals and cell death, while Bcl-xLK87D was much less effective in doing so. In the absence of IP3Rs, Bcl-xL and Bcl-xLK87D were equally effective in suppressing STS-induced cell death. Finally, we demonstrate that endogenous Bcl-xL also suppress IP3R activity in MDA-MB-231 breast cancer cells, whereby Bcl-xL knockdown augmented IP3R-mediated Ca2+ release and increased the sensitivity towards STS, without altering the ER Ca2+ content. Hence, this study challenges the current paradigm of divergent functions for Bcl-2 and Bcl-xL in Ca2+-signaling modulation and reveals that, similarly to Bcl-2, Bcl-xL inhibits IP3R-mediated Ca2+ release and IP3R-driven cell death. Our work further underpins that IP3R inhibition is an integral part of Bcl-xL's anti-apoptotic function.
Assuntos
Apoptose , Sinalização do Cálcio , Receptores de Inositol 1,4,5-Trifosfato , Proteína bcl-X , Cálcio/metabolismo , Células HeLa , Humanos , Receptores de Inositol 1,4,5-Trifosfato/genética , Proteína bcl-X/metabolismoRESUMO
Recently, a functional IP3R ortholog (CO.IP3R-A) capable of IP3-induced Ca2+ release has been discovered in Capsaspora owczarzaki, a close unicellular relative to Metazoa. In contrast to mammalian IP3Rs, CO.IP3R-A is not modulated by Ca2+, ATP or PKA. Protein-sequence analysis revealed that CO.IP3R-A contained a putative binding site for anti-apoptotic Bcl-2, although Bcl-2 was not detected in Capsaspora owczarzaki and only appeared in Metazoa. Here, we examined whether human Bcl-2 could form a complex with CO.IP3R-A channels and modulate their Ca2+-flux properties using ectopic expression approaches in a HEK293 cell model in which all three IP3R isoforms were knocked out. We demonstrate that human Bcl-2 via its BH4 domain could functionally interact with CO.IP3R-A, thereby suppressing Ca2+ flux through CO.IP3R-A channels. The BH4 domain of Bcl-2 was sufficient for interaction with CO.IP3R-A channels. Moreover, mutating the Lys17 of Bcl-2's BH4 domain, the residue critical for Bcl-2-dependent modulation of mammalian IP3Rs, abrogated Bcl-2's ability to bind and inhibit CO.IP3R-A channels. Hence, this raises the possibility that a unicellular ancestor of animals already had an IP3R that harbored a Bcl-2-binding site. Bcl-2 proteins may have evolved as controllers of IP3R function by exploiting this pre-existing site, thereby counteracting Ca2+-dependent apoptosis.
Assuntos
Sinalização do Cálcio , Evolução Molecular , Receptores de Inositol 1,4,5-Trifosfato/metabolismo , Proteínas Proto-Oncogênicas c-bcl-2/metabolismo , Células HEK293 , Humanos , Receptores de Inositol 1,4,5-Trifosfato/química , Receptores de Inositol 1,4,5-Trifosfato/genética , Filogenia , Ligação Proteica , Proteínas de Protozoários/química , Proteínas de Protozoários/genética , Proteínas de Protozoários/metabolismo , Homologia de SequênciaRESUMO
Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs), intracellular calcium (Ca2+) release channels, fulfill key functions in cell death and survival processes, whose dysregulation contributes to oncogenesis. This is essentially due to the presence of IP3Rs in microdomains of the endoplasmic reticulum (ER) in close proximity to the mitochondria. As such, IP3Rs enable efficient Ca2+ transfers from the ER to the mitochondria, thus regulating metabolism and cell fate. This review focuses on one of the three IP3R isoforms, the type 3 IP3R (IP3R3), which is linked to proapoptotic ER-mitochondrial Ca2+ transfers. Alterations in IP3R3 expression have been highlighted in numerous cancer types, leading to dysregulations of Ca2+ signaling and cellular functions. However, the outcome of IP3R3-mediated Ca2+ transfers for mitochondrial function is complex with opposing effects on oncogenesis. IP3R3 can either suppress cancer by promoting cell death and cellular senescence or support cancer by driving metabolism, anabolic processes, cell cycle progression, proliferation and invasion. The aim of this review is to provide an overview of IP3R3 dysregulations in cancer and describe how such dysregulations alter critical cellular processes such as proliferation or cell death and survival. Here, we pose that the IP3R3 isoform is not only linked to proapoptotic ER-mitochondrial Ca2+ transfers but might also be involved in prosurvival signaling.
Assuntos
Receptores de Inositol 1,4,5-Trifosfato/metabolismo , Neoplasias/metabolismo , Animais , Sinalização do Cálcio , Morte Celular , Sobrevivência Celular , Retículo Endoplasmático/metabolismo , Humanos , Mitocôndrias/metabolismo , Neoplasias/patologiaRESUMO
Type 3 Inositol 1,4,5-trisphosphate (IP3) receptors (IP3R3s) have been identified as anti-oncogenic channels by propelling pro-apoptotic Ca2+ signals to mitochondria. Yet, recent studies (Rezuchova et al, Cell Death Dis, 2019; Ueasilamongkol et al, Hepathology, 2019; Guerra et al, Gut, 2019) revealed that IP3R3 upregulation drives oncogenesis via ER-mitochondrial Ca2+ crosstalk, adding complexity to IP3R3's role in cancer.
Assuntos
Carcinogênese/metabolismo , Receptores de Inositol 1,4,5-Trifosfato/metabolismo , Animais , Carcinogênese/patologia , Sobrevivência Celular , Humanos , Modelos Biológicos , Neoplasias/metabolismo , Neoplasias/patologiaRESUMO
The pro- and antiapoptotic proteins belonging to the B-cell lymphoma-2 (Bcl-2) family exert a critical control over cell-death processes by enabling or counteracting mitochondrial outer membrane permeabilization. Beyond this mitochondrial function, several Bcl-2 family members have emerged as critical modulators of intracellular Ca2+ homeostasis and dynamics, showing proapoptotic and antiapoptotic functions. Bcl-2 family proteins specifically target several intracellular Ca2+-transport systems, including organellar Ca2+ channels: inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs), Ca2+-release channels mediating Ca2+ flux from the endoplasmic reticulum, as well as voltage-dependent anion channels (VDACs), which mediate Ca2+ flux across the mitochondrial outer membrane into the mitochondria. Although the formation of protein complexes between Bcl-2 proteins and these channels has been extensively studied, a major advance during recent years has been elucidating the complex interaction of Bcl-2 proteins with IP3Rs. Distinct interaction sites for different Bcl-2 family members were identified in the primary structure of IP3Rs. The unique molecular profiles of these Bcl-2 proteins may account for their distinct functional outcomes when bound to IP3Rs. Furthermore, Bcl-2 inhibitors used in cancer therapy may affect IP3R function as part of their proapoptotic effect and/or as an adverse effect in healthy cells.