title: Changes in sodium pump expression dictate the effects of ouabain on cell growth.
abstract: Here we show that ouabain-induced cell growth regulation is intrinsically coupled to changes in the cellular amount of Na/K-ATPase via the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway. Ouabain increases the endocytosis and degradation of Na/K-ATPase in LLC-PK1, human breast (BT20), and prostate (DU145) cancer cells. However, ouabain stimulates the PI3K/Akt/mTOR pathway and consequently up-regulates the expression of Na/K-ATPase in LLC-PK1 but not BT20 and DU145 cells. This up-regulation is sufficient to replete the plasma membrane pool of Na/K-ATPase and to stimulate cell proliferation in LLC-PK1 cells. On the other hand, ouabain causes a gradual depletion of Na/K-ATPase and an increased expression of cell cycle inhibitor p21(cip), which consequently inhibits cell proliferation in BT20 and DU145 cells. Consistently, we observe that small interfering RNA-mediated knockdown of Na/K-ATPase is sufficient to induce the expression of p21(cip) and slow the proliferation of LLC-PK1 cells. Moreover, this knockdown converts the growth stimulatory effect of ouabain to growth inhibition in LLC-PK1 cells. Mechanistically, both Src and caveolin-1 are required for ouabain-induced activation of Akt and up-regulation of Na/K-ATPase. Furthermore, inhibition of the PI3K/Akt/mTOR pathway by rapamycin completely blocks ouabain-induced expression of Na/K-ATPase and converts ouabain-induced growth stimulation to growth inhibition in LLC-PK1 cells. Taken together, we conclude that changes in the expression of Na/K-ATPase dictate the growth regulatory effects of ouabain on cells.
authors.affiliation: Department of Physiology and Pharmacology, University of Toledo College of Medicine, Toledo, Ohio 43614, USA.
authors.forename: Jiang
authors.initials: J
authors.lastname: Tian
authors.forename: Xin
authors.initials: X
authors.lastname: Li
authors.forename: Man
authors.initials: M
authors.lastname: Liang
authors.forename: Lijun
authors.initials: L
authors.lastname: Liu
authors.forename: Joe X
authors.initials: JX
authors.lastname: Xie
authors.forename: Qiqi
authors.initials: Q
authors.lastname: Ye
authors.forename: Peter
authors.initials: P
authors.lastname: Kometiani
authors.forename: Manoranjani
authors.initials: M
authors.lastname: Tillekeratne
authors.forename: Runming
authors.initials: R
authors.lastname: Jin
authors.forename: Zijian
authors.initials: Z
authors.lastname: Xie