Cell Growth & Differentiation, Vol 8, Issue 5 565-570, Copyright © 1997 by American Association of Cancer Research
Phosphatidylinositol 3-kinase inhibitor wortmannin blocks mitogenic activation of the transferrin receptor gene promoter in late G1
WK Miskimins, F King and R Miskimins
University of South Dakota School of Medicine, Department of Biochemistry and Molecular Biology, Vermillion 57069, USA.
Expression of the transferrin receptor is necessary for cells to progress
through S-phase. The transferrin receptor gene promoter is activated as a
delayed event following growth factor stimulation of quiescent fibroblasts.
Serum stimulation in the presence of vanadate leads to superactivation of
the transferrin receptor promoter, suggesting a role for tyrosine
phosphorylation. Wortmannin, a potent inhibitor of phosphatidylinositol
3-kinase, a tyrosine kinase-regulated enzyme, blocks mitogen-dependent
activation of the transferrin receptor promoter. Furthermore, wortmannin
was able to block activation of this promoter when added several hours
after serum stimulation of quiescent cells. This suggests that
phosphatidylinositol 3-kinase may be required in mid to late G1 and that it
is directly involved in a pathway leading to activation of the transferrin
receptor promoter. This is further supported by the finding that the
transferrin receptor promoter is much less responsive to mitogenic
stimulation in cells that have been stably transfected with a dominant
negative form of the phosphatidylinositol 3-kinase regulatory subunit.
Activation of S6 kinase, an event known to be downstream of
phosphatidylinositol 3-kinase activation, appears not to be involved in
activation of the transferrin receptor promoter since no effect was
observed by treatment of cells with rapamycin.