Stimulatory Heterotrimeric GTP-binding Protein Inhibits Hydrogen Peroxide-induced Apoptosis by Repressing BAK Induction in SH-SY5Y Human Neuroblastoma Cells

Abstract

Heterotrimeric stimulatory GTP-binding protein (G s ) stimulates adenylate cyclases to activate the cAMP signaling pathway. Although the cAMP pathway has been reported to be involved in apoptosis, the role of the G s -cAMP signaling pathway during reactive oxygen species (ROS)-mediated apoptosis, which is involved in the resistance of cancer cells to chemotherapy and radiation, is not clearly understood. Thus, in this study we aimed to investigate the role of the ␣ subunit of G s (G␣ s The heterotrimeric GTP-binding proteins (G proteins) 2 are composed of ␣, ␤, and ␥ subunits, and the ␣ subunits of G protein (G␣) are classified into four main families: G␣ s , G␣ i , G␣ q , and G␣ 12 . When a signaling molecule, such as a hormone or a neurotransmitter binds to a G protein-coupled receptor (GPCR), the receptor stimulates the replacement of GDP with GTP on the G␣. GTP-bound activated heterotrimer dissociates into an ␣ subunit (G␣-GTP) and a ␤␥ dimer (G␤␥), which can independently regulate effectors including adenylate cyclases, phospholipases, phosphodiesterases, and ion channels. The hydrolysis of GTP to GDP by intrinsic GTPase, a process that is regulated by RGS (regulator of G-protein signaling) proteins, leads to the reassociation of the heterotrimer and the termination of the activation cycle (1-3). G protein signaling systems are involved in regulation of a variety of cellular responses, which include metabolism, neurotransmission, proliferation, differentiation, and apoptosis. Although signaling for cellular proliferation and apoptosis has generally been attributed to growth factor receptors that possess ligand-regulated protein-tyrosine kinase activity, growing evidence now indicates that GPCRs also regulate cellular growth and apoptosis (4 -6).Reactive oxygen species (ROS) are constantly generated under normal conditions as a consequence of aerobic metabolism, and the most common ROS types are superoxide anions (O .2 ), hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals (HO Ϫ ) (7). ROS can react with DNA, proteins, carbohydrates, and lipids in a destructive manner due to their high levels of chemical reactivity. Thus, ROS are considered DNA-damaging agents that increase mutation rates and promote oncogenic transformation, and are implicated in the development of cancer and metastases (8, 9). Moreover, ROS participate in the modulation of apoptosis following treatment with various agents including Fas, ultraviolet, and chemotherapeutic drugs (10 -12). Therefore, the ability of a cancer cell to defend itself against ROS is associated with resistance to chemo-and radiotherapy. Consequently, a more detailed understanding of the mechanisms that regulate ROS-induced apoptosis would contribute to our abilities to develop novel therapeutic strategies directed toward killing resistant cancer cells (13).

Human Neuroglobin Functions as an Oxidative Stress-responsive Sensor for Neuroprotection

“…The functional roles of G␣ i/o and G␣ s in cell survival or death are completely opposite (47)(48)(49). G␣ s stimulates adenylate cyclases to activate the cAMP signaling pathway, whereas G␣ i/o inhibits adenylate cyclases.…”

“…The activation of G s has been shown to increase glutathione and to protect neuronal cells against oxidative stress (49). Recently, it has been reported that G␣ s inhibits hydrogen peroxide-induced apoptosis of SH-SY5Y cells (48); overexpression of a constitutively active G␣ s mutant protected, while G␣ s siRNA augmented hydrogen peroxideinduced apoptosis in SH-SY5Y cells (48). In addition, the protective effect of G␣ s was abolished by co-expressing a constitutively active G␣ i mutant, which antagonizes G␣ s by inhibiting adenylate cyclase (48).…”

“…Recently, it has been reported that G␣ s inhibits hydrogen peroxide-induced apoptosis of SH-SY5Y cells (48); overexpression of a constitutively active G␣ s mutant protected, while G␣ s siRNA augmented hydrogen peroxideinduced apoptosis in SH-SY5Y cells (48). In addition, the protective effect of G␣ s was abolished by co-expressing a constitutively active G␣ i mutant, which antagonizes G␣ s by inhibiting adenylate cyclase (48). Moreover, hydrogen peroxide-induced apoptosis was reduced by treating cells with prostaglandin E 2 , which activates G␣ s , but was augmented by CCPA, which activates G␣ i , causing a decrease in cAMP levels (48).…”

“…In addition, the protective effect of G␣ s was abolished by co-expressing a constitutively active G␣ i mutant, which antagonizes G␣ s by inhibiting adenylate cyclase (48). Moreover, hydrogen peroxide-induced apoptosis was reduced by treating cells with prostaglandin E 2 , which activates G␣ s , but was augmented by CCPA, which activates G␣ i , causing a decrease in cAMP levels (48). Furthermore, it should be also noted that a GDI and a guanine nucleotide exchange factor (GEF) of G␣ i3 regulate autophagy by balancing G protein activity antagonistically (47).…”

Stimulatory heterotrimeric G protein augments gamma ray-induced apoptosis by up-regulation of Bak expression via CREB and AP-1 in H1299 human lung cancer cells

“…Furthermore, we showed that PGE2 augmented gamma ray-induced apoptosis of H1299 cells via Gαs-coupled receptor, as stimulation of beta-adrenergic receptor induced cell death of thymocytes (Gu et al, 2000). However, Gαs protein was reported to inhibit radiation-or hydrogen peroxide-induced apoptosis of SH-SY5Y human neuroblastoma cells (Kim et al, 2007(Kim et al, , 2008. Inhibitory G proteins were found to antagonize the anti-apoptotic effect of Gαs in hydrogen peroxide-treated SH-SY5Y human neuroblastoma cells (Kim et al, 2008) and also to inhibit hydrogen peroxide-induced apoptosis by up-regulation of Bcl-2 via NF-κB in H1299 human lung cancer cells (Seo et al, 2009).…”