Protein phosphatase 2A–negative regulation of the protective signaling pathway of Ca²⁺/CaM‐dependent ERK activation in cerebral ischemia

Abstract

Extracellular-signal-regulated kinase (ERK) undergoes rapid inactivation following the intense activation evoked by cerebral ischemia and reperfusion. However, the precise mechanism of this inactivation has not been elucidated. To investigate how phosphatases regulate the ERK cascade following ischemia, the PP2A inhibitors cantharidin and okadaic acid were administrated to the CA1 subregion of the rat hippocampus. The resulting sustained ERK activity implies that PP2A is a major phosphatase contributing to the rapid inactivation, but not activation, of ERK following cerebral ischemia. The increase in PP2A activity induced by ceramide has a weak effect on the activation of Raf via dephosphorylation of Ser259 in response to ischemia. In contrast, ketamine (Keta) and cyclosporine A (CsA), two chemicals that block calcium signal in ischemia, decrease ERK activity by blocking Raf dephosphorylation of Ser259. We also observed that activation of an upstream protein, Ras-GRF, leads to calcium/calmodulin-dependent activation of the ERK signaling cascade in response to ischemic stimuli. In addition, the activity of cyclic AMP response element-binding protein (CREB) and estrogen receptor alpha (ER alpha), target proteins of ERK and protective elements against ischemic lesion, parallels the activity of ERK. These data indicate that PP2A plays a significant role in blocking the protective effect induced by the ERK kinase pathway and that fast inactivation of ERK is the result of cross talk between calcium/calmodulin-dependent, positively regulated signal cascades and a ceramide-dependent negative signaling pathway.

Systemic Revealing Pharmacological Signalling Pathway Networks in the Hippocampus of Ischaemia-Reperfusion Mice Treated with Baicalin

“…In fact, death of CA1 pyramidal cells has been demonstrated to be an immediate result of ischaemia [34], though it has been reported that the majority of cells at the borders of the infarct were not yet dead after 48 hours [32]. In the current study, it was found that BA acted on calcium ion (Ca 2+ )-dependent signalling cascades, shown to play a neuroprotective role in cerebral ischaemia by signalling Ca 2+ /CaM-dependent ERK activation, and this is consistent with previous findings [35]. In 1999, Lipton noticed that extracellular Ca 2+ concentration sharply declines in the ischemic core immediately after ischaemia with a concomitant rise in extracellular potassium ions (K + ) ( ∼ 70 mM) over 2 hours and returns to near normal concentration over 6 to 24 hours [31].…”

Effects of Estrogen on Beta-Amyloid-Induced Cholinergic Cell Death in the Nucleus Basalis Magnocellularis

“…We found upregulation of two raf kinase inhibiting proteins following Aβ treatment, namely phosphatidylethanolamine-binding protein 1 or Raf kinase inhibitor protein (RKIP) and protein phosphatase 2A (PP2A) [68,69]. The possible inactivation of ERK1/2, p38 or JNK pathways is in contrast to some studies reporting Aβ-induced pathological activation of raf pathways [70,71].…”

Negative Feedback Regulation of Raf/MEK/ERK Cascade After Sublethal Cerebral Ischemia in the Rat Hippocampus

“…Raf-1 phosphorylation at Ser289/296/301 and ERK activity were apparently increased by inhibition of PP2A. Previous studies have demonstrated that ERK can be dephosphorylated by PP2A [18,19]. Therefore, we speculated that PP2A may dephosphorylate Raf-1 at Ser289/296/301 through two pathways: direct dephosphorylation of Ser289/ 296/301 and indirect inhibition of ERK inhibition.…”

“…Phosphorylation of those sites would induce Raf-1 to enter an inactive and desensitized state [15]. The Ser/Thr specific protein phosphatase 2A (PP2A) is also known to play a key role in regulating the Raf/MEK/ERK cascade [16][17][18][19]. It was thought that hyperphosphorylation of Raf-1 can be dephosphorylated by PP2A with the help of peptidylprolyl cis/trans isomerase Pin1 (protein interacting with NIMA -1).…”