Regulation of Skp2 levels by the Pim-1 protein kinase.
The Pim-1 protein kinase plays an important role in regulating both cell growth and survival and enhancing transformation by multiple oncogenes. The ability of Pim-1 to regulate cell growth is mediated, in part, by the capacity of this protein kinase to control the levels of the p27, a protein that is a critical regulator of cyclin-dependent kinases that mediate cell cycle progression. To understand how Pim-1 is capable of regulating p27 protein levels, we focused our attention on the SCF(Skp2) ubiquitin ligase complex that controls the rate of degradation of this protein. We found that expression of Pim-1 increases the level of Skp2 through direct binding and phosphorylation of multiple sites on this protein. Along with known Skp2 phosphorylation sites including Ser(64) and Ser(72), we have identified Thr(417) as a unique Pim-1 phosphorylation target. Phosphorylation of Thr(417) controls the stability of Skp2 and its ability to degrade p27. Additionally, we found that Pim-1 regulates the anaphase-promoting complex or cyclosome (APC/C complex) that mediates the ubiquitination of Skp2. Pim-1 phosphorylates Cdh1 and impairs binding of this protein to another APC/C complex member, CDC27. These modifications inhibit Skp2 from degradation. Marked increases in Skp2 caused by these mechanisms lower cellular p27 levels. Consistent with these observations, we show that Pim-1 is able to cooperate with Skp2 to signal S phase entry. Our data reveal a novel Pim-1 kinase-dependent signaling pathway that plays a crucial role in cell cycle regulation.
Cen
Bo
B
Department of Medicine, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
Mahajan
Sandeep
S
Zemskova
Marina
M
Beharry
Zanna
Z
Lin
Ying-Wei
YW
Cramer
Scott D
SD
Lilly
Michael B
MB
Kraft
Andrew S
AS
D000818:Animals; D015703:Antigens, CD; D064191:Apc3 Subunit, Anaphase-Promoting Complex-Cyclosome; D015820:Cadherins / Q000235:genetics / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D002478:Cells, Cultured; D004591:Electrophoresis, Polyacrylamide Gel; D005434:Flow Cytometry; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D010766:Phosphorylation; D011485:Protein Binding; D051573:Proto-Oncogene Proteins c-pim-1 / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D016196:S Phase / Q000235:genetics / Q000502:physiology; D044786:S-Phase Kinase-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D054875:Ubiquitination
D015703:Antigens, CD; D064191:Apc3 Subunit, Anaphase-Promoting Complex-Cyclosome; C577366:CDC27 protein, human; C513384:CDH1 protein, human; D015820:Cadherins; D018797:Cell Cycle Proteins; D044786:S-Phase Kinase-Associated Proteins; D051573:Proto-Oncogene Proteins c-pim-1