Glycogen synthase kinase 3 has a limited role in cell cycle regulation of cyclin D1 levels.
BACKGROUND
The expression level of cyclin D1 plays a vital role in the control of proliferation. This protein is reported to be degraded following phosphorylation by glycogen synthase kinase 3 (GSK3) on Thr-286. We recently showed that phosphorylation of Thr-286 is responsible for a decline in cyclin D1 levels during S phase, an event required for efficient DNA synthesis. These studies were undertaken to test the possibility that phosphorylation by GSK3 is responsible for the S phase specific decline in cyclin D1 levels, and that this event is regulated by the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway which controls GSK3.


RESULTS
We found, however, that neither PI3K, AKT, GSK3, nor proliferative signaling activity in general is responsible for the S phase decline in cyclin D1 levels. In fact, the activity of these signaling kinases does not vary through the cell cycle of proliferating cells. Moreover, we found that GSK3 activity has little influence over cyclin D1 expression levels during any cell cycle phase. Inhibition of GSK3 activity by siRNA, LiCl, or other chemical inhibitors failed to influence cyclin D1 phosphorylation on Thr-286, even though LiCl efficiently blocked phosphorylation of beta-catenin, a known substrate of GSK3. Likewise, the expression of a constitutively active GSK3 mutant protein failed to influence cyclin D1 phosphorylation or total protein expression level.


CONCLUSIONS
Because we were unable to identify any proliferative signaling molecule or pathway which is regulated through the cell cycle, or which is able to influence cyclin D1 levels, we conclude that the suppression of cyclin D1 levels during S phase is regulated by cell cycle position rather than signaling activity. We propose that this mechanism guarantees the decline in cyclin D1 levels during each S phase; and that in so doing it reduces the likelihood that simple over expression of cyclin D1 can lead to uncontrolled cell growth.
Yang
Ke
K
Department of Molecular Genetics, The Lerner Research Institute, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland OH, USA. Yangk@ccf.org
Guo
Yang
Y
Stacey
William C
WC
Harwalkar
Jyoti
J
Fretthold
Jonathan
J
Hitomi
Masahiro
M
Stacey
Dennis W
DW
D000818:Animals; D002453:Cell Cycle / Q000502:physiology*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002867:Chromones / Q000494:pharmacology; D056741:Cyclin D; D016213:Cyclins / Q000096:biosynthesis*; D005347:Fibroblasts; D019940:Genes, bcl-1; D038362:Glycogen Synthase Kinase 3 / Q000235:genetics / Q000502:physiology*; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D018021:Lithium Chloride / Q000494:pharmacology; D051379:Mice; D009025:Morpholines / Q000494:pharmacology; D041681:NIH 3T3 Cells; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D010769:Phosphothreonine / Q000737:chemistry; D014176:Protein Biosynthesis; D011494:Protein Kinases / Q000187:drug effects; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016196:S Phase; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D051176:beta Catenin / Q000378:metabolism
C495270:CTNNB1 protein, human; C495267:CTNNB1 protein, mouse; D002867:Chromones; D056741:Cyclin D; D016213:Cyclins; D007976:Leupeptins; D009025:Morpholines; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011993:Recombinant Fusion Proteins; D051176:beta Catenin; D010769:Phosphothreonine; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D011494:Protein Kinases; C546842:MTOR protein, human; C546843:mTOR protein, mouse; D000071679:Glycogen Synthase Kinase 3 beta; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D038362:Glycogen Synthase Kinase 3; D018021:Lithium Chloride; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde; D020123:Sirolimus