p27 Kip1 localization depends on the tumor suppressor protein tuberin

Abstract

p27(Kip1) plays an important role in cell cycle regulation by inhibiting cyclin-CDK complex activity in the nucleus. p27(Kip1) is regulated by its concentration as well as by its subcellular localization. Tuberin, encoded by the tuberous sclerosis tumor suppressor gene TSC2, is a potent negative cell cycle regulator. We show herein, that tuberin induces nuclear p27 localization by inhibiting its 14-3-3-mediated cytoplasmic retention. Tuberin interferes with 14-3-3's counteracting effects on p27-mediated cell cycle arrest. Akt-mediated phosphorylation of p27, but not of tuberin, negatively regulates tuberin's potential to trigger p27 nuclear localization. In G0 cells, tuberin binds p27 triggering downregulation of p27's binding to 14-3-3 and of its cytoplasmic retention. At transition to S phase p27 is phosphorylated by Akt, tuberin/p27 complex levels are downregulated and binding of p27 to 14-3-3 increases triggering cytoplasmic retention of p27. These findings demonstrate p27 localization during the mammalian cell cycle to be under the control of the tumor suppressor tuberin.

Rheb activates AMPK and reduces p27Kip1 levels in Tsc2-null cells via mTORC1-independent mechanisms: implications for cell proliferation and tumorigenesis

“…Rheb depletion induced an evident increase of p27 in both cytoplasmic and nuclear fractions. The increase of p27 in both compartments resembles the one resulting from cell cycle arrest on serum removal in NIH3T3 (Tsc2 þ / þ ) cells (Rosner et al, 2007;Short et al, 2008). Similar results were also observed on overexpression of TSC2 or phosphatidylinositol 3 kinase/Akt inhibition (Rosner et al, 2007;Kim et al, 2009).…”

“…The increase of p27 in both compartments resembles the one resulting from cell cycle arrest on serum removal in NIH3T3 (Tsc2 þ / þ ) cells (Rosner et al, 2007;Short et al, 2008). Similar results were also observed on overexpression of TSC2 or phosphatidylinositol 3 kinase/Akt inhibition (Rosner et al, 2007;Kim et al, 2009). Consistent with the lack of regulation of AMPK activity by mTORC1, we observed that treatment with rapamycin for 2 and 8 h did not alter the nuclear levels of p27 in Tsc2À/À MEFs (Figures 3d and e).…”

“…Alternatively, a combined effect of Rheb on p27 stability and subcellular localization may explain the parallel increase of the cytoplasmic p27 levels on Rheb depletion. Similarly, overexpression of TSC2 in HEK293 cells or inhibition of phosphatidylinositol 3 kinase/Akt in renal carcinoma cells, both known regulators of p27 stability and subcellular localization, induced a parallel increase of the nuclear and cytoplasmic p27 levels (Rosner et al, 2007;Kim et al, 2009).…”

“…As cell cycle inhibition correlates with increased p27 levels (Alkarain and Slingerland, 2004;Rosner et al, 2007), the elevation of the cytoplasmic levels of p27 is likely a consequence of an increased nuclear influx of p27 on Rheb knockdown that translated into reduced cell cycle progression and thus enhanced p27 stability. For instance, resembling our results, it was observed that serum removal, which induces cell cycle arrest in normal cells, had induced an increase in nuclear and cytoplasmic p27 levels in NIH3T3 cells (Rosner et al, 2007;Short et al, 2008). Alternatively, a combined effect of Rheb on p27 stability and subcellular localization may explain the parallel increase of the cytoplasmic p27 levels on Rheb depletion.…”

“…Regulation of p27 localization is complex and may occur at multiple levels. For example, TSC2 increases p27 stability and nuclear localization by a mechanism that is independent of its effect on mTORC1 (Rosner et al, 2006(Rosner et al, , 2007. Conversely, Akt has been shown to phosphorylate p27 at both T157 and T198, and phosphorylation on these residues has been associated with reduced nuclear localization of p27 (Fujita et al, 2002;Vervoorts and Luscher, 2008).…”