Cyclin I activates Cdk5 and regulates expression of Bcl-2 and Bcl-XL in postmitotic mouse cells
Paul T. Brinkkoetter, Paul Olivier, Jimmy S. Wu, Scott Henderson, Ronald D. Krofft, Jeffrey W. Pippin, David Hockenbery, James M. Roberts, Stuart J. Shankland
Paul T. Brinkkoetter, Paul Olivier, Jimmy S. Wu, Scott Henderson, Ronald D. Krofft, Jeffrey W. Pippin, David Hockenbery, James M. Roberts, Stuart J. Shankland
View: Text | PDF
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

×

Figure 6

Cyclin I–Cdk5 regulates apoptosis by activating MEK1/2–ERK1/2.

Options: View larger image (or click on image) Download as PowerPoint
Cyclin I–Cdk5 regulates apoptosis by activating MEK1/2–ERK1/2. (A) There...
(A) There were no substantial differences in the phosphorylation status of A-, B- and c-Raf in cyclin I–null (–/–) and WT (+/+) podocytes under nonstressed conditions using several phosphospecific antibodies. (B) Phosphorylation of MEK1/2 on residues Ser217/221 was substantially reduced in cyclin I–null podocytes (lane 2) compared with WT podocytes (lane 1) under physiological, nonstressed conditions. Restoring cyclin I levels in null cells by retroviral infection normalized MEK1/2 phosphorylation (lane 3) to levels comparable to those of WT podocytes. GFP transfection had no effect (lane 4). Total MEK served as a loading control. These results show that MEK1/2 Ser217/221 phosphorylation is cyclin I dependent. (C) Phosphorylation of ERK1/2 on residues Thr202/Tyr204 was reduced in 2 different clones of cyclin I–null podocytes (lanes 3, 4) compared with 2 different WT podocyte clones (lanes 1, 2). Restoring cyclin I levels in null cells by retroviral infection normalized ERK1/2 phosphorylation (lane 5) to levels comparable to those of WT podocytes; GFP infection had no effect. Total ERK1/2 served as loading control. These results show that the cyclin I–dependent activation of MEK1/2 was reflected by an increased phosphorylation of ERK1/2. (D) Reducing Cdk5 expression in cyclin I WT podocytes with siRNA decreased ERK1/2 phosphorylation (lane 2) compared with nontransfected cells. Scrambled siRNA had no effect on ERK1/2 phosphorylation.