Enhanced proteasome-dependent degradation of the CDK inhibitor p27kip1 in immortalized lymphocytes from Alzheimer's dementia patients

Abstract

Cyclin-dependent kinase inhibitor p27(kip1) (p27), a critical determinant for cell cycle progression, is an important regulation target of mitogenic signals. We have recently reported the existence of a molecular link between decreased p27 levels and enhanced phosphorylation of pRb protein and proliferation of immortalized lymphocytes from Alzheimer's disease (AD) patients. These cell cycle disturbances might be considered systemic manifestations, which mirror changes thought to occur in the brain, where post-mitotic neurons have been shown to display various cell cycle markers prior to degeneration. This work was undertaken to delineate the molecular mechanisms underlying the p27 down-regulation associated with AD. To this end, we evaluated the p27 protein stability in control and AD lymphoblasts. Half-life of p27 protein was markedly reduced in lymphoblasts from AD patients compared with that in control cells. The increased phosphorylation of p27 at Thr187, rather than changes in the 26S proteasome activity, is likely responsible for the enhanced degradation of p27 in AD cells. The serum-induced enhanced proliferation of AD lymphoblasts and decreased levels of p27 were abrogated by calmodulin (CaM) antagonists. The findings presented here suggest that Ca(2+)/CaM-dependent overactivation of PI3K/Akt signaling cascade in AD cells, plays an important role in regulating p27 abundance by increasing its degradation in the ubiquitin-proteasome pathway.

HMG-CoA Reductase Inhibitor Simvastatin Inhibits Cell Cycle Progression at the G1/S Checkpoint in Immortalized Lymphocytes from Alzheimer's Disease Patients Independently of Cholesterol-Lowering Effects

“…Figure 1 shows a time course analysis of the effect of increasing doses of simvastatin on rates of proliferation, upon serum stimulation, of lymphoblasts from control and AD patients. In agreement with previous reports from this laboratory (de las Cuevas et al, 2003;Muñ oz et al, 2005Muñ oz et al, , 2007, the proliferative activity of AD lymphoblasts was significantly higher than that of control cells. Treatment of cells with simvastatin resulted in a significant inhibition of cell growth of both control and AD lymphoblasts in a dosedependent manner.…”

“…pRb hyperphosphorylation releases E2F transcription factors, thus contributing to the expression of several growth and cell cycle regulatory genes with functional E2F binding sites in their promoters (Stevaux and Dyson, 2002). In agreement with previous reports (de las Cuevas et al, 2003;Muñ oz et al, 2005Muñ oz et al, , 2007 lymphoblasts from AD patients showed a higher degree of phosphorylation of pRb. The addition of SIM had no effect on the phosphorylation status of pRb in control cells, but it significantly reduced the levels of the hyperphosphorylated form of pRb in AD lymphoblasts.…”

“…This issue has been addressed by carrying out a comparative study of the influence of statin treatment on the proliferative activity of EBV-immortalized lymphocytes from late-onset AD patients and age-matched nondemented individuals. Previous work from this laboratory indicates that differences in the proliferative activity and cell cycle regulatory proteins are also found between untransformed lymphocytes from control and AD subjects Muñ oz et al, 2007).…”

“…Conversely, EpsteinBarr virus (EBV) infection in vitro causes transformation of B cells and generates B-lymphoblastoid cell lines that resemble activated B cells (Neitzel, 1986). In fact, we have previously demonstrated identical cellular response to serum addition or withdrawal in peripheral lymphocytes or EBVtransformed lymphocytes from control and AD patients Muñ oz et al, 2007). Taken together, these reports support a rationale for the use of peripheral cells, and in particular EBV lymphoblasts from AD patients as a model to further understand disease biology, progression, and therapeutic actions.…”

Altered Calmodulin Degradation and Signaling in Non-Neuronal Cells from Alzheimer’s Disease Patients

“…Changes in CaM availability and/or in the intrinsic functional properties of the molecule were linked to disruption of Ca 2+ homeostasis in immortalized lymphocytes from late-onset AD patients [14]. Interestingly, the distinct Ca 2+ response of AD was associated with enhanced cell proliferation compared to control lymphoblasts from age-matched individuals [16,17]. These features were considered as peripheral signs of the disease, as current evidence relates the process of neuronal apoptosis occurring in AD to the aberrant reentry of differentiated neurons into the cell cycle [18][19][20].…”