J Biol Chem. 2003 Sep 26;278(39):37032-40. Epub 2003 Jul 11. Related Articles,
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Oxidized lipoproteins inhibit surfactant phosphatidylcholine synthesis via
calpain-mediated cleavage of CTP:phosphocholine cytidylyltransferase.

Zhou J, Ryan AJ, Medh J, Mallampalli RK.

Department of Internal Medicine, The University of Iowa College of Medicine,
Iowa City, Iowa 52242, USA.

We investigated effects of pro-atherogenic oxidized lipoproteins on
phosphatidylcholine (PtdCho) biosynthesis in murine lung epithelial cells
(MLE-12). Cells surface-bound, internalized, and degraded oxidized low density
lipoproteins (Ox-LDL). Ox-LDL significantly reduced [3H]choline incorporation
into PtdCho in cells by selectively inhibiting the activity of the
rate-regulatory enzyme, CTP:phosphocholine cytdylyltransferase (CCT). Ox-LDL
coordinately increased the cellular turnover of CCTalpha protein as determined
by [35S]methionine pulse-chase studies by inducing the calcium-activated
proteinase, calpain. Forced expression of calpain or exposure of cells to the
calcium ionophore, A23187, increased CCTalpha degradation, whereas
overexpression of the endogenous calpain inhibitor, calpastatin, attenuated
Ox-LDL-induced CCTalpha degradation. The effects of Ox-LDL on CCTalpha
breakdown were attenuated in calpain-deficient cells. In vitro calpain
digestion of CCTalpha isolated from cells transfected with truncated or
internal deletion mutants indicated multiple cleavage sites within the CCTalpha
primary structure, leading to the generation of a 26-kDa (p26) fragment.
Calpain hydrolysis of purified CCTalpha generated p26, which upon NH2-terminal
sequencing localized a calpain attack site within the CCTalpha amino terminus.
Expression of a CCTalpha mutant where the amino-terminal cleavage site and a
putative carboxyl-terminal hydrolysis region were modified resulted in an
enzyme that was significantly less sensitive to proteolytic cleavage and
restored the ability of cells to synthesize surfactant PtdCho after Ox-LDL
treatment. Thus, these results provide a critical link between proatherogenic
lipoproteins and their metabolic target, CCTalpha, resulting in impaired
surfactant metabolism.

PMID: 12857760 [PubMed - indexed for MEDLINE]

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