Adaptation of Camp Signaling System in SH-SY5Y Neuroblastoma Cells Following Expression of a Constitutively Active Stimulatory G Protein Alpha, Q227L Gsα
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EXPERIMENTAL and MOLECULAR MEDICINE, Vol. 33, No. 1, 37-45, March 2001 Adaptation of cAMP signaling system in SH-SY5Y neuroblastoma cells following expression of a constitutively active stimulatory G protein alpha, Q227L Gsα Ik-Soon Jang1 and Yong-Sung Juhnn1,2 cAMP system adapts at the post-receptor level to a sustained activation of the system by differential ex- 1 Department of Biochemistry and Cancer Research Institute, pression of the isoforms of AC, PDE, and PKA in SH- Seoul National University College of Medicine, Seoul 110-799, SY5Y neuroblastoma. We also showed that an in- Korea crease in cellular cAMP concentration might medi- 2 Corresponding author: Tel, +82-2-740-8247; Fax, +82-2-744-4534; ate the observed changes in the cAMP system. E-mail, [email protected] Keywords: adenylate cyclase, cAMP-dependent protein Accepted 23 March, 2000 kinase, 3’,5’-cyclic-nucleotide phosphodiesterase, Pro- tein isoforms, reverse transcriptase polymerase chain Abbreviations: AC, adenylate cyclase; ACI, AC isoform I; dbcAMP, reaction dibutyryl cAMP; GAPDH, glyceraldehyde-3-phosphate dehydroge- nase; Gsα, alpha subunit of stimulatory G protein; PDE, cyclic nucleotide phosphodiesterase; PKA, cAMP-dependent protein Introduction kinase; RT-PCR, reverse transcription-polymerase chain reaction Signal transducing heterotrimeric GTP-binding proteins (G protein) relay signals from activated receptors to intracellular effectors to elicit cellular responses (Gilman, Abstract 1987). Binding of signaling molecules such as hormones Heterotrimeric GTP-binding proteins (G protein) are and neurotransmitters to the receptors makes the receptor known to participate in the transduction of signals undergo conformational change. This conformational from ligand activated receptors to effector mole- change leads to activation of G proteins by promoting cules to elicit cellular responses. Sustained activa- the exchange of GDP for GTP on the Gα subunit and tion of cAMP-G protein signaling system by agonist finally the dissociation of Gα-GTP from its Gβγ dimer results in desensitization of the pathway at receptor (Bourne et al., 1990). Both the dissociated Gα-GTP and levels, however it is not clear whether such receptor Gβγ subunits are able to mediate physiological responses responses induce other changes in post-receptor by interacting with numerous effectors, including adenyl- signaling path that are associated with maintenance ate cyclase, phospholipases, phosphodiesterases, pro- of AMP levels, i.e. cAMP-forming adenylate cyclase tein kinases, and ion channels (Neer, 1995). The G pro- (AC), cAMP-degrading cyclic nucleotide phosphodi- tein signal is terminated as the GTP on the Gα subunit esterase (PDE) and cAMP-dependent protein kinase is hydrolyzed by intrinsic GTPase which is activated by (PKA). Experiments were performed to determine the regulator of G protein-signaling (RGS) family (Zheng the expression of AC, PDE, and PKA isoforms in SH- et al., 1999). SY5Y neuroblastoma cells, in which cAMP system The cAMP system is one of the most intensively studi- was activated by expressing a constitutively acti- ed G protein signaling systems. Many extracellular signals vated mutant of stimulatory G protein (Q227L Gsα). including epinephrine and acetylcholine bind to and acti- Expression of ACI mRNA was increased, but levels vate their seven transmembrane receptors, which couple of ACVIII and ACIX mRNA were decreased. All of the to stimulatory G proteins (Gsα) or inhibitory G proteins 4 expressed isoforms of PDE (PDE1C, PDE2, PDE to regulate adenylate cyclase (AC, EC 4.6.1.1.) activity. 4A, and PDE4B) were increased in mRNA expres- Activated AC catalyses the formation of cAMP, the key sion; the levels of PKA RIα, RIβ, and RIIβ were regulator of cAMP system, from ATP. However, the cellular increased moderately, however, those of RIIα and Cα concentration of cAMP is also regulated by cyclic nucleo- were increased remarkably. The activities of AC, tide phosphodiesterases (PDE, EC 3.1.4.17.), which PDE and PKA were also increased in the SH-SY5Y degrade cAMP into adenosine 5’-phosphate. An in- cells expressing Q227L Gsα. The similar changes in crease in cAMP concentration results in an activation of expression and activity of AC, PDE and PKA were cAMP dependent protein kinase (PKA, EC 2.7.10.-) as observed in the SH-SY5Y cells treated with dbcAMP well as ion channels and PKA, in turn, phosphorylates for 6 days. Consequently, it is concluded that the target proteins in order to regulate various metabolism 38 Exp. Mol. Med. Vol. 33(1), 37-45, 2001 and gene expression, resulting in numerous cellular re- vation of cAMP system resulting in adaptation at the sponses, including proliferation, differentiation, and for- post-receptor level. mation of memory. When the cAMP signaling system is activated continu- ously by agonists, the desensitization of the pathway is Materials and Method induced at the receptor level by various mechanisms such as receptor phosphorylation, uncoupling of receptor and Materials G proteins, down-regulation of the receptors (Ferguson Materials were obtained as follows: isoproterenol, bovine and Caron, 1998). Sustained activation of cAMP system serum albumin (BSA), Ethylene glycol-bis(beta-aminoethyl is also achieved, without activation of receptors, by ex- ether)-N,N,N',N'-tetraacetic acid (EGTA), N-(2-hydroxy- pression of constitutively activated mutants of stimulatory ethyl)piperazine-N'-(2-ethanesulfonic acid) (HEPES), and α G protein (Gs ), and by treatment with forskolin or vari- 3-isobutyl-1-methylxanthine (IBMX) were purchased from ous cAMP analogues. Such sustained activation of cAMP Sigma Chemicals; agarose, reverse transcriptase (Super- system can influence the cAMP-forming AC, cAMP-de- script II RNase H(−) Reverse transcriptase) were from grading PDE and cAMP-binding PKA activity. Therefore, GIBCO BRL; Taq polymerase was from TaKaRa; HA it is quite possible that the adaptation of cAMP system antibody against hemagglutinin peptide, GTP, FuGENE to the sustained activation can induce the downstream 6 Transfection Reagent, phosphocreatine, and creatine effectors such as AC, PDE, and PKA. However, the phosphokinase were from Roche Diagnostics. All other adaptation of AC, PDE, and PKA expression to a reagents were of analytical grade. sustained activation of cAMP signaling system has not been clearly recognized. Cell culture At least nine genes for mammalian AC have been SH-SY5Y human neuroblastoma cells (CRL-2266) were cloned, and each isoform seems to have different kinetic purchased from American Type Culture Collection, and and regulatory properties, including the response to cells were cultured in Dulbecco’s modified Eagle’s calcium ions, beta gamma subunits of the G protein, medium containing 10% fetal calf serum, 100 IU/ml and protein kinase C (Sunahara et al., 1996). In mammals, penicillin, and 50 µg/ml streptomycin in a humidified PDE are encoded by at least 19 different genes, and atmosphere containing 5% CO in air at 37oC. each PDE gene contains several distinct transcriptional 2 units that give rise to proteins with subtle structural Establishment of SH-SY5Y cells expressing Q227L differences, bringing the number of the PDE proteins Gsα close to 50 (Conti and Jin, 2000). Each PDE isoform α α seems to have different kinetic properties such as sub- A constitutively activated mutant of Gs , Q227L Gs strate specificity and it responds differentially to diverse containing hemagglutinin (HA) epitope was kindly provid- regulators such as calcium/calmodulin and cyclic GMP. ed from Dr. H. Bourne (University of California San In addition, the expression of PDE isoforms varies in dif- Francisco; Levis and Bourne, 1992). The gene was sub- ferent tissues (Beavo, 1995). PKA is composed of distinct cloned in pcDNA3 vector (Invitrogen), and the plasmid catalytic (C) and regulatory (R) subunits, and the holo- was transfected into SH-SY5Y neuroblastoma cells using enzyme contains two C subunits bound to homo- or FuGENE 6 Transfection Reagent. Transfected cells were µ heterodimers of R subunits. At present, 4 different R grown in the medium containing 500 g/ml G418. The α subunits and three different C subunits have been iden- clones expressing the mutant Gs were selected by ana- tified as separate gene products (Scott, 1991). The iso- lyzing mRNA expression with reverse transcription-poly- form multiplicity of AC, PDE and PKA, is suggested merase chain reaction (RT-PCR) using primers specific α to play an important role in the regulation of the for HA tag region of Q227L Gs . The primers used for α cAMP signaling system by intracellular targeting, cross- amplifying Gs were as follows; for transfected Q227L α talk between a wide variety of other signaling systems, Gs 5'-GACGTGCCGGATTACGCGTCC and 5'-ATGA- α and flux-controlled sensitivity (Houslay and Milligan, AGTACTTGGCCCGG, for total Gs 5'-GCTGGAGAG- 1997). TCTGGCAAAAGCACC and 5'-ATGAAGTACTTGGCC- The present experiment was performed to examine how CGG. The expression of the mutant protein was also cAMP signaling system adapts at post-receptor levels to analyzed by Western blot using anti-HA antibody for α a sustained activation of cAMP system. The expression detection of HA tag region of Q227L Gs or RM anti- levels of isoforms of AC, PDE, and PKA were analyzed body for detection of both the endogenous and exogen- α in SH-SY5Y neuroblastoma cells of which cAMP system ous Gs protein as described (Yang et al., 1999). was activated by expressing a constitutively activated mutant of Gsα (Q227L Gsα)