Degradation of cyclin D3 independent of Thr-283 phosphorylation

Abstract

Cyclin D3 has been shown to play a major role in the regulation of cell cycle progression in lymphocytes. It is therefore important to understand the mechanisms involved in the regulation of this protein. We have previously shown that both basal and cAMP-induced degradation of cyclin D3 in Reh cells is dependent on Thr-283 phosphorylation by glycogen synthase kinase-3b (GSK-3b). We now provide evidence of an alternative mechanism being involved in the regulation of cyclin D3 degradation. Treatment of lymphoid cells with okadaic acid (OA), an inhibitor of protein phosphatases 1 and 2A (PP1 and PP2A), induces rapid phosphorylation and proteasomal degradation of cyclin D3. This degradation is not inhibited by the GSK-3b inhibitors lithium or Kenpaullone, or by substitution of Thr-283 with Ala on cyclin D3, indicating that cyclin D3 can be degraded independently of Thr-283 phosphorylation and GSK-3b activity. Interestingly, in vitro experiments revealed that PP1, but not PP2A, was able to dephosphorylate cyclin D3 efficiently, and PP1 was found to associate with His-tagged cyclin D3. These results support the hypothesis that PP1 constitutively keeps cyclin D3 in a stable, dephosphorylated state, and that treatment of cells with OA leads to phosphorylation and degradation of cyclin D3 through inhibition of PP1.

The Age-Associated Decline of Glycogen Synthase Kinase 3β Plays a Critical Role in the Inhibition of Liver Regeneration

“…6). It is interesting to note that Lahne et al have previously demonstrated that the degradation of cyclin D3 can be achieved using a cyclin D3 T283A mutant, independently of GSK3␤ activity (21). We think that these differences are associated with the existence of tissue-specific pathways of degradation of cyclin D3.…”

“…Next, we searched for kinases/ phosphatases that might regulate this phosphorylation. It is known that GSK3␤ and p38 phosphorylate cyclin D3 at Thr283 (4,23), while PP1 dephosphorylates and stabilizes cyclin D3 (21). Since another isoform of GSK, GSK3␣, has overlapping functions with GSK3␤, we have also included this protein in The Thr283-ph isoform of cyclin D3 is abundant in young livers.…”

“…Cyclin D1 and cyclin D3 have been shown to be targeted for proteasomal degradation by phosphorylation (23). Recent studies have discovered that the stability of cyclin D3 protein is reduced by glycogen synthase kinase 3␤ (GSK3␤) in human B-lymphocyte Reh cells (23) and by p38 in Molts-4 lymphoblastoid cells and COS cells (4) by phosphorylation at Thr283, while protein phosphatase 1 (PP1) dephosphorylates and stabilizes cyclin D3 in lymphoid Reh cells (21). GSK3␤ is a ubiquitously expressed multifunctional serine/ threonine protein kinase originally identified as a regulator of glycogen synthesis (38).…”

“…In old livers, cyclin D3-cdk4 also phosphorylates an RNA binding protein, CUGBP1, and increases interactions of CUGBP1 with translation initiation complex eIF2 (29). This activation of translational activity of CUGBP1 results in the increased translation of several proteins, including transcription factor C/EBP␤ and histone deacetylase 1 (HDAC1) (32).The expression of D-type cyclins is regulated at several levels, including transcription, translation, and posttranslational modifications (1,21,22). Cyclin D1 and cyclin D3 have been shown to be targeted for proteasomal degradation by phosphorylation (23).…”

“…The expression of D-type cyclins is regulated at several levels, including transcription, translation, and posttranslational modifications (1,21,22). Cyclin D1 and cyclin D3 have been shown to be targeted for proteasomal degradation by phosphorylation (23).…”