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 9

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, B) The mRNA levels for Bcl-2 (A) and Bcl-XL (B) were measured by quantitative PCR, and the relative concentrations are shown as ratio normalized to β-actin. Samples were run in triplicate, and mRNA from 3 independent experiments was included. Relative gene expression was analyzed using the 2–standard curve method. Compared with WT cells (lanes 1), the mRNA levels for Bcl-2 and Bcl-XL (lanes 2) were significantly reduced in cyclin I–null podocytes. Restoring cyclin I in null cells by stable infection normalized transcripts for Bcl-2 and Bcl-XL (lanes 3). In contrast, the absence of p35 in p35-null cells had no effect on Bcl-2 and Bcl-XL mRNA levels (lane 4). (C, D) To determine whether cyclin I or p35 altered the protein levels for certain Bcl-2 family proteins, Western blot analyses were performed in WT, cyclin I–null, and p35-null podocytes grown under physiological, nonstressed conditions. GAPDH and β-actin served as loading controls. Compared with WT cells (C, lanes 1, 2), Bcl-2 and Bcl-XL, but not Bax, protein levels were reduced in cyclin I–null podocytes (C, lanes 3, 4). Levels for Bcl-2 and Bcl-XL were normalized upon infection with cyclin I (lane 5), but not GFP (lane 6). In contrast, in the absence of p35, only Bcl-2 protein expression was strongly reduced compared with WT podocytes (D). No effects on Bcl-XL and Bax protein levels were observed. Data shown represent mean + SD.