Cyclin I activates Cdk5 and regulates expression of Bcl-2 and Bcl-XL in postmitotic mouse cells
Paul T. Brinkkoetter, … , James M. Roberts, Stuart J. Shankland
Paul T. Brinkkoetter, … , James M. Roberts, Stuart J. Shankland
Published September 1, 2009
Citation Information: J Clin Invest. 2009;119(10):3089-3101. https://doi.org/10.1172/JCI37978.
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Research Article Nephrology

Cyclin I activates Cdk5 and regulates expression of Bcl-2 and Bcl-XL in postmitotic mouse cells

Abstract

Cyclin I is an atypical cyclin because it is most abundant in postmitotic cells. We previously showed that cyclin I does not regulate proliferation, but rather controls survival of podocytes, terminally differentiated epithelial cells that are essential for the structural and functional integrity of kidney glomeruli. Here, we investigated the mechanism by which cyclin I safeguards against apoptosis and found that cyclin I bound and activated cyclin-dependent kinase 5 (Cdk5) in isolated mouse podocytes and neurons. Cdk5 activity was reduced in glomeruli and brain lysates from cyclin I–deficient mice, and inhibition of Cdk5 increased in vitro the susceptibility to apoptosis in response to cellular damage. In addition, levels of the prosurvival proteins Bcl-2 and Bcl-XL were reduced in podocytes and neurons from cyclin I–deficient mice, and restoration of Bcl-2 or Bcl-XL expression prevented injury-induced apoptosis. Furthermore, we found that levels of phosphorylated MEK1/2 and ERK1/2 were decreased in cyclin I–deficient podocytes and that inhibition of MEK1/2 restored Bcl2 and Bcl-XL protein levels. Of interest, this pathway was also defective in mice with experimental glomerulonephritis. Taken together, these data suggest that a cyclin I–Cdk5 complex forms a critical antiapoptotic factor in terminally differentiated cells that functions via MAPK signaling to modulate levels of the prosurvival proteins Bcl-2 and Bcl-XL.

Authors

Paul T. Brinkkoetter, Paul Olivier, Jimmy S. Wu, Scott Henderson, Ronald D. Krofft, Jeffrey W. Pippin, David Hockenbery, James M. Roberts, Stuart J. Shankland

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Figure 11

Cyclin I, but not p35, differentially regulates specific Bcl-2 family proteins.

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Cyclin I, but not p35, differentially regulates specific Bcl-2 family pr...
(A) Reducing Cdk5 expression by siRNA decreased the protein levels of Bcl-2 and Bcl-XL. No effect was observed in control podocytes transfected with control siRNA. (B) Inhibiting Cdk5 activity by roscovitine (50 μM) reduced the protein expression of Bcl-2 and Bcl-XL compared with vehicle (DMSO). GAPDH and β-actin served as loading controls. (C) To prove that regulation of Bcl-2 and Bcl-XL underlies the prosurvival effect of cyclin I, protein expression of Bcl-2 or Bcl-XL was restored in cyclin I–null podocytes by retroviral infection. There was a 3- to 4-fold increase in apoptosis following UV-C irradiation in cyclin I–null cultured podocytes infected with GFP compared with injured cyclin I WT cells. UV-C–induced apoptosis was markedly reduced in cyclin I–null podocytes stably infected with Bcl-2 or Bcl-XL. (D) Accordingly, caspase-3 cleavage products were also decreased in cyclin I–null podocytes infected with Bcl-2 or Bcl-XL 6 hours after UV-C irradiation compared with GFP-infected podocytes. (E) Infecting cyclin I–null podocytes with constitutively active MEK1 (MEK-DD) mutants also increased the expression of Bcl-2 and Bcl-XL linking regulation of the MEK-ERK pathway by cyclin I–Cdk5 to the observed regulation of Bcl-2 and Bcl-XL expression. Data shown represent mean + SD.