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GROWTH, DIFFERENTIATION, AND APOPTOSIS
13-mediated transformation and apoptosis are permissively dependent on basal ERK activity
Departments of Pharmacology and Anesthesiology, University of Illinois, Chicago, Illinois 60612
Submitted 25 March 2003 ; accepted in final form 1 May 2003
We previously reported that the
-subunit of heterotrimeric
G13 protein induces either mitogenesis and neoplastic
transformation or apoptosis in a cell-dependent manner. Here, we analyzed
which signaling pathways are required for G
13-induced
mitogenesis or apoptosis using a novel mutant of G
13. We
have identified that in human cell line LoVo, the mutation encoding
substitution of Arg260 to stop codon in mRNA of G
13 subunit
produced a mutant protein (G
13-T) that lacks a COOH terminus
and is endogenously expressed in LoVo cells as a polypeptide of 30 kDa. We
found that G
13-T lost its ability to promote proliferation
and transformation but retained its ability to induce apoptosis. We found that
full-length G
13 could stimulate Elk1 transcription factor,
whereas truncated G
13 lost this ability.
G
13-dependent stimulation of Elk1 was inhibited by
dominant-negative extracellular signal-regulated kinase (MEK) but not by
dominant-negative MEKK1. Similarly, MEK inhibitor PD-98059 blocked
G
13-induced Elk1 stimulation, whereas JNK inhibitor
SB-203580 was ineffective. In Rat-1 fibroblasts, G
13-induced
cell proliferation and foci formation were also inhibited by dominant-negative
MEK and PD-98059 but not by dominant-negative MEKK1 and SB-203580. Whereas
G
13-T alone did not induce transformation, coexpression with
constitutively active MEK partially restored its ability to transform Rat-1
cells. Importantly, full-length but not G
13-T could
stimulate Src kinase activity. Moreover, G
13-dependent
stimulation of Elk1, cell proliferation, and foci formation were inhibited by
tyrosine kinase inhibitor, genistein, or by dominant-negative Src kinase,
suggesting the involvement of a Src-dependent pathway in the
G
13-mediated cell proliferation and transformation.
Importantly, truncated G
13 retained its ability to stimulate
apoptosis signal-regulated kinase ASK1 and c-Jun terminal kinase, JNK.
Interestingly, the apoptosis induced by G
13-T was inhibited
by dominant-negative ASK1 or by SB-203580.
mitogen-activated kinase; effector mutant; Src kinase
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