Go-protein α-Subunits Activate Mitogen-activated Protein Kinase via a Novel Protein Kinase C-dependent Mechanism

113

Extracellular signal-regulated kinase 1 (ERK1) 1 and ERK2 (p44 ERK and p42 ERK ) are key cellular components that control cell proliferation and differentiation (1). The regulation of ERK through G protein-coupled receptors (GPCRs) is a complicated process. Various signaling components play different roles in ERK activation depending on the GPCR and cell types involved (2).There has been substantial progress in the understanding of cellular events that link the activation of GPCR and ERK (for review, see Ref. 1). These signaling events can be classified into several distinct pathways. They include: (i) Ras-dependent activation of ERK via transactivation of receptor-tyrosine kinases (RTKs) such as EGFR, (ii) Ras-independent ERK activation via protein kinase C (PKC) that converges with RTK signaling at the Raf level (iii) activation or inhibition of ERK via the cAMP/ protein kinase A (PKA) pathway, in which the direction of regulation depends on the type of Raf involved, and (iv) the recently substantiated ␤-arrestin-mediated pathway proven in certain classes of GPCRs (3). However, it should be mentioned that these signaling pathways are deduced from the limited sets of individual receptors or cell types, and more extensive and systemic studies are needed for these signaling models to be generalized (4). Among all the dopamine receptor subtypes characterized, it is generally accepted that D 2 R and D 3 R are related to schizophrenia. Possibly because of high similarity in their amino acid composition (46% overall amino acid homology and 78% identity in the transmembrane domains) (5), D 2 R and D 3 R share most signaling pathways such as adenylyl cyclase, extracellular acidification, mitogenesis, ERK activation, inhibition of dopamine synthesis, and ion channel regulation (K ϩ , Ca 2ϩ ) (for review, see Ref. 6). Furthermore, recent studies show that although D 3 R is more densely expressed in the limbic area, mesencephalic dopaminergic neurons express both D 2 R and D 3 R (7-9). Because D 2 R and D 3 R are virtually the same in functional aspect and are expressed in the same cells, it can be speculated that signaling routes or regulatory mechanisms for functions of effectors could be different. To address this issue, we decided to focus on a specific cellular function and conduct a detailed study on signaling mechanisms in order to see if their signaling or regulatory pathways differ. ERK activation was selected as a model experimental system, because GPCRmediated ERK regulation consists of multiple complex steps, which are relatively well established. D 2 R-mediated ERK activation has been reported from different cell types. It is pertussis toxin (PTX)-sensitive and partially blocked by the dominant negative mutant of Ras, N17Ras, in

Section: Characterization Of D 2 R-and D 3 R-mediated Activation Of Ementioning

confidence: 76%

“…The activation of ERK through the G o protein is different from the G i protein (36); in G␣ o , PMA-sensitive PKCs are involved in a Ras-independent manner. In many cases, G q , which mediates PLC activation, is involved in the PKC-induced activation of ERK.…”

Section: Roles Of Mek Ras and Raf-1 In The D 2 R-and D 3 R-mediatedmentioning

confidence: 99%

Comparative Studies of Molecular Mechanisms of Dopamine D2 and D3 Receptors for the Activation of Extracellular Signal-regulated Kinase

Beom

Cheong

Torres

et al. 2004

113

“…*, p Ͻ 0.05 versus basal. (5) activation and thus stimulate MAPK activtity (12). Additionally, PKC can mediate p38 MAPK activation by an unidentified mechanism (18).…”

Section: Discussionmentioning

confidence: 99%

“…ERK1/2 activation is mediated via a number of signaling molecules including growth factor receptor tyrosine kinases (9 -11) and protein kinase C (PKC) (12,13). Growth factor receptor tyrosine kinases activate ERK1/2 through the Shc/ Grb2/SOS pathway via activation of Ras (9 -11, 14).…”

mentioning

confidence: 99%

Effect of Contraction on Mitogen-activated Protein Kinase Signal Transduction in Skeletal Muscle

Ryder¹,

Fahlman²,

Wallberg-Henriksson³

et al. 2000

142

134

“…These data suggest that G␤␥ subunits can activate Ras using mechanisms that are similar to those employed by receptor tyrosine kinases and that Shc is implicated as a go-between that relays message from G␤␥-coupled receptors to proteintyrosine kinase receptors. On the other hand, Ras-independent signaling pathways involve the G␣ subunits of G o and G qcoupled receptors and are PKC-dependent (5).…”

Section: Discussionmentioning

confidence: 99%

Activation of Stress-activated Protein Kinase/c-Jun NH2-terminal Kinase and p38 Kinase in Calphostin C-induced Apoptosis Requires Caspase-3-like Proteases but Is Dispensable for Cell Death

Ozaki

Tani

Ikemoto

et al. 1999

Apoptosis was induced in human glioma cell lines by exposure to 100 nM calphostin C, a specific inhibitor of protein kinase C. Calphostin C-induced apoptosis was associated with synchronous down-regulation of Bcl-2 and Bcl-x L as well as activation of caspase-3 but not caspase-1. The exposure to calphostin C led to activation of stress-activated protein kinase/c-Jun NH 2 -terminal kinase (SAPK/JNK) and p38 kinase and concurrent inhibition of extracellular signal-regulated kinase (ERK). Upstream of ERK, Shc was shown to be activated, but its downstream Raf1 and ERK were inhibited. The pretreatment with acetyl-Tyr-Val-Ala-Asp-aldehyde, a relatively selective inhibitor of caspase-3, or benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (z-VAD.fmk), a broad spectrum caspase inhibitor, similarly inhibited calphostin C-induced activation of SAPK/JNK and p38 kinase as well as apoptotic nuclear damages (chromatin condensation and DNA fragmentation) and cell shrinkage, suggesting that caspase-3 functions upstream of SAPK/JNK and p38 kinase, but did not block calphostin C-induced surface blebbing and cell death. On the other hand, the inhibition of SAPK/JNK by transfection of dominant negative SAPK/JNK and that of p38 kinase by SB203580 induced similar effects on the calphostin Cinduced apoptotic phenotypes and cell death as did z-VAD.fmk and acetyl-Tyr-Val-Ala-Asp-aldehyde, but the calphostin C-induced PARP cleavage was not changed, suggesting that SAPK/JNK and p38 kinase are involved in the DNA fragmentation pathway downstream of caspase-3. The present findings suggest, therefore, that the activation of SAPK/JNK and p38 kinase is dispensable for calphostin C-mediated and z-VAD.fmk-resistant cell death.It has been expected that signal transduction pathways involving specific protein kinases are involved in mediating apoptosis. Extracellular signal-regulated kinase (ERK) 1 is most strongly stimulated by activation of protein-tyrosine kinase receptors (1) and also activated by both Ras-dependent (2-4) and Ras-independent signalings (5) in response to activation of G protein-coupled receptors, and common intermediates in intracellular signaling cascades are involved in diverse cellular functions including growth and differentiation (6). Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate results in the activation of Raf1 (7) and ERK (8 -11) within minutes, suggesting the involvement of PKC in the signaling pathway leading to ERK activation. In addition, Ras functions as an essential transducer of various physiological signals leading to cell growth and proliferation in a PKC-dependent or PKC-independent manner (12), and activated Ras also renders cells susceptible to apoptosis after depression of PKC activity (13,14).SAPK/JNK and p38 kinase have been proposed to mediate apoptosis, but a number of reports have challenged the notion that the activation of SAPK/JNK and/or p38 kinase is sufficient to induce apoptosis (15-21), and the integration and balance of SAPK/JNK and p38 pathways probably contribute to commitment to a...