Tb. Kinane et al., G-ALPHA(I-2) MEDIATES RENAL LLC-PK1 GROWTH BY A RAF-INDEPENDENT ACTIVATION OF P42 P44 MAP KINASE/, American journal of physiology. Renal, fluid and electrolyte physiology, 41(2), 1997, pp. 273-282
The protooncogene G alpha(i-2) plays a pivotal role in signaling pathw
ays that control renal cell growth and differentiation. Mitogen-activa
ted protein kinases (MAPKs) are potential downstream effecters for G a
lpha(i-2) in these pathways. In predifferentiated LLC-PK1 renal cells,
the temporal maximal expression of G alpha(i-2) coincided with maxima
l activation of MAPK((p42/p44)). By contrast, pertussis toxin treatmen
t of these cells inhibited cell growth and reduced MAPK((p42/p44)) act
ivity by 30%. These findings reflected upstream activation of MAPK kin
ase (MEK1), as transient transfection of cells with a plasmid encoding
a constitutively active form of MEK1 increased MAPK((p42/p44)) activi
ty and cell growth, whereas treatment with PD-098059, an inhibitor of
MEK1 activity, reduced MAPK((p42/p44)) activity and cell growth. Expre
ssion of a guanosinetriphosphatase (GTPase)-deficient G alpha(i-2) in
these cells increased MAPK((p42/p44)) activity and correspondingly red
uced cell doubling time from 24 to 10 h without altering the activity
of Raf-1 or c-Jun/stress-activated protein kinases (SAPKs). By contras
t, expression of a GTPase-deficient G alpha(i-3) in these cells reduce
d both their cell doubling time by 30% and MAPK((p42/p44)) activity by
60%. As the known MEKK isoforms (MEKK1, -2, and -3) can also activate
SAPKs, these findings suggest the GTP-charged G alpha(i-2) subunit tr
ansduces growth signals in renal cells via activation of MAPK((p42/p44
)) and that such activation may be linked to pathways containing novel
MEKK isoforms that preferentially activate MEKs.