Sustained erk-2 activation in rat glomerular mesangial cells:
differential regulation by protein phosphatases.
Schramek, Herbert, Michael Schumacher, and Walter Pfaller.
Department of Physiology, University of Innsbruck, A-6020
Innsbruck, Austria
APStracts 3:0069F, 1996.
Sustained growth factor-induced stimulation of ERK-2 in rat glomerular
mesangial cells: differential regulation by protein phosphatases. -
The duration of extracellular signal-regulated protein kinase (ERK)
activation is critical for cell signaling decisions and probably
determines whether a stimulus elicits proliferation or
differentiation. We studied the intracellular signals regulating
sustained ERK-2 activity in glomerular mesangial cells (GMC),
utilizing combinations of GMC mitogens of different potency.
Incubation of GMC with both endothelin-1 (ET-1) and platelet-derived
growth factor BB (PDGF BB) led to a long lasting, monophasic increase
in ERK-2 activity. In contrast, when ET-1 was administered together
with epidermal growth factor (EGF), a less pronounced and shorter
activation occured. Long-term stimulation of ERK-2 was accompanied by
an increase in p45 MEK activity, which again was more pronounced when
ET-1 was administered together with PDGF BB as compared to EGF. In
the presence of actinomycin D (Act.D), an inhibitor of RNA synthesis,
ERK-2 activity induced by ET-1 and PDGF BB but not by ET-1 and EGF
remained elevated more than 6-fold throughout the whole incubation
period of 6 hours. The effect of Act.D on ET-1- and PDGF BB-induced
ERK-2 activation was mimicked by the protein phosphatase inhibitor
sodium orthovanadate. In addition, vanadate also unmasked an ET-1-
and EGF-induced ERK-2 activity after 6 h. The serine/threonine
phosphatase inhibitor okadaic acid (OA) did neither alter agonist
-induced ERK-2 activity after 6 h (0.5 nM OA) nor after 10 min or 1 h
(250 nM). Together these results suggest that, in GMC, long-term
activation of the mitogen-activated protein kinase ERK-2 is
differentially regulated depending on the combination of agonists
administered. ET-1- and PDGF BB-induced long-term activation of ERK-2
is regulated by a vanadate-sensitive protein phosphatase(s) and by a
transcriptionally regulated protein(s). In contrast, ET-1- and EGF
-induced sustained ERK-2 stimulation is regulated by a vanadate
-sensitive protein phosphatase(s) but not by a transcriptionally
regulated protein. Agonist-specific and time-dependent stimulation of
ERK-2-regulating protein phosphatases may be critical for the length
of ERK-2 activation in GMC and could thus be of pathophysiological
significance in glomerular diseases associated with alterations in
cell proliferation or cell differentiation.
Received 13 Decmeber 1995; accepted in final form 21 March 1996.
APS Manuscript Number F412-5.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 16 April 96