A third mode of ouabain signaling. Focus on "Regulation of ERK1/2 by ouabain and Na-K-ATPase-dependent energy utilization and AMPK activation in parotid acinar cells"

ERK activation is required for hydrostatic pressure-induced tensile changes in engineered articular cartilage

The objective of this study was to identify ERK 1/2 involvement in the changes in compressive and tensile mechanical properties associated with hydrostatic pressure treatment of self-assembled cartilage constructs. In study 1, ERK 1/2 phosphorylation was detected by immunoblot, following application of hydrostatic pressure (1 h of static 10 MPa) applied at days 10-14 of self-assembly culture. In study 2, ERK 1/2 activation was blocked during hydrostatic pressure application on days 10-14. With pharmacological inhibition of the ERK pathway by the MEK1/ERK inhibitor U0126 during hydrostatic pressure application on days 10-14, the increase in Young's modulus induced by hydrostatic pressure was blocked. Furthermore, this reduction in Young's modulus with U0126 treatment during hydrostatic pressure application corresponded to a decrease in total collagen expression. However, U0126 did not inhibit the increase in aggregate modulus or GAG induced by hydrostatic pressure. These findings demonstrate a link between hydrostatic pressure application, ERK signalling and changes in the biomechanical properties of a tissue-engineered construct.

Ca2+ influx through reverse mode Na+/Ca2+ exchange is critical for vascular endothelial growth factor-mediated extracellular signal-regulated kinase (ERK) 1/2 activation and angiogenic functions of human endothelial cells

VEGF is a key angiogenic cytokine and a major target in anti-angiogenic therapeutic strategies. In endothelial cells (ECs), VEGF binds VEGF receptors and activates ERK1/2 through the phospholipase γ (PLCγ)-PKCα-B-Raf pathway. Our previous work suggested that influx of extracellular Ca(2+) is required for VEGF-induced ERK1/2 activation, and we hypothesized that this could occur through reverse mode (Ca(2+) in and Na(+) out) Na(+)-Ca(2+) exchange (NCX). However, the role of NCX activity in VEGF signaling and angiogenic functions of ECs had not previously been described. Here, using human umbilical vein ECs (HUVECs), we report that extracellular Ca(2+) is required for VEGF-induced ERK1/2 activation and that release of Ca(2+) from intracellular stores alone, in the absence of extracellular Ca(2+), is not sufficient to activate ERK1/2. Furthermore, inhibitors of reverse mode NCX suppressed the VEGF-induced activation of ERK1/2 in a time- and dose-dependent manner and attenuated VEGF-induced Ca(2+) transients. Knockdown of NCX1 (the main NCX isoform in HUVECs) by siRNA confirmed the pharmacological data. A panel of NCX inhibitors also significantly reduced VEGF-induced B-Raf activity and inhibited PKCα translocation to the plasma membrane and total PKC activity in situ. Finally, NCX inhibitors reduced VEGF-induced HUVEC proliferation, migration, and tubular differentiation in surrogate angiogenesis functional assays in vitro. We propose that Ca(2+) influx through reverse mode NCX is required for the activation and the targeting of PKCα to the plasma membrane, an essential step for VEGF-induced ERK1/2 phosphorylation and downstream EC functions in angiogenesis.

AMPK activation enhances PPARα activity to inhibit cardiac hypertrophy via ERK1/2 MAPK signaling pathway

Activation of adenosine monophosphate-activated protein kinase (AMPK) has been shown to inhibit cardiac hypertrophy through peroxisome proliferators-activated receptor-α (PPARα) signaling pathway, but the detailed mechanism remains unclear. A rat model of cardiac hypertrophy created by transaortic constriction (TAC) was used to investigate the mechanism involved in regulation of PPARα activity by AMPK. It was observed that treatment with AICAR (5-aminoimidazole 1 carboxamide ribonucleoside), an AMPK activator, significantly inhibited cardiac hypertrophy in vivo and in vitro. Phosphorylated extracellular signal regulated protein kinase (phospho-ERK1/2) and phospho-p38 mitogen-activated protein kinase (MAPK) protein levels were significantly up-regulated, while PPARα protein level was down-regulated in TAC rats. AICAR treatment reversed the changes of PPARα and phospho-ERK1/2, but increased phospho-p38 MAPK protein level in TAC rats. Similar changes of PPARα and phospho-ERK1/2 protein levels were observed in the hypertrophied cardiomyocytes induced by phenylephrine treatment. Epidermal growth factor (EGF, ERK1/2 activator), but not SB203580 (p38 inhibitor) blocked the up-regulation of PPARα protein level induced by AICAR. Luciferase assay showed that AICAR increased PPARα transcriptional activity which was abrogated by EGF, but not by SB203580. These results demonstrate that AMPK activation enhances the activity of PPARα to inhibit cardiac hypertrophy through ERK1/2, but not p38 MAPK, signaling pathway.

Signaling mechanisms that link salt retention to hypertension: endogenous ouabain, the Na(+) pump, the Na(+)/Ca(2+) exchanger and TRPC proteins

Salt retention as a result of chronic, excessive dietary salt intake, is widely accepted as one of the most common causes of hypertension. In a small minority of cases, enhanced Na(+) reabsorption by the kidney can be traced to specific genetic defects of salt transport, or pathological conditions of the kidney, adrenal cortex, or pituitary. Far more frequently, however, salt retention may be the result of minor renal injury or small genetic variation in renal salt transport mechanisms. How salt retention actually leads to the increase in peripheral vascular resistance (the hallmark of hypertension) and the elevation of blood pressure remains an enigma. Here we review the evidence that endogenous ouabain (an adrenocortical hormone), arterial smooth muscle α2 Na(+) pumps, type-1 Na/Ca exchangers, and receptor- and store-operated Ca(2+) channels play key roles in the pathway that links salt to hypertension. We discuss cardenolide structure-function relationships in an effort to understand why prolonged administration of ouabain, but not digoxin, induces hypertension, and why digoxin is actually anti-hypertensive. Finally, we summarize recent observations which indicate that ouabain upregulates arterial myocyte Ca(2+) signaling mechanisms that promote vasoconstriction, while simultaneously downregulating endothelial vasodilator mechanisms. In sum, the reports reviewed here provide novel insight into the molecular mechanisms by which salt retention leads to hypertension.