Enzymatic regulation of phosphatidylcholine synthesis via protein ubiquitination

Pulmonary surfactant is a critical surface-active substance consisting of dipalmitoylphosphatidylcholine (DPPtdCho) and key apoproteins that are produced and secreted into the airspace from alveolar type II epithelial cells. Deficiency of the surfactant leads to severe lung atelectasis, ventilatory impairment, and gas-exchange abnormalities. The generation of DPPtdCho in cells occurs via two integral routes: the de novo and remodeling pathways. The interplay between these pathways has not been investigated. Overexpression of the remodeling enzyme, acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT1), in epithelia decreases de novo PtdCho synthesis without significantly altering cellular phospholipid mass; this occurs through increased degradation of cholinephosphotransferase (CPT1), the terminal enzyme of the de novo pathway. CPT1 is degraded by multi-ubuiquitination and trafficking via the lysosomal pathway. When expressed in lung epithelia, CPT1 mutants harboring arginine substitutions at multiple carboxyl-terminal lysine residues exhibited proteolytic resistance to effects of LPCAT1 overexpression. Cellular expression of these CPT1 mutants also restores de novo PtdCho synthesis to levels normally observed in lung epithelia. Further studies demonstrate that the SCF (Skip-Cullen-F-box) ubiquitin E3 ligase component, β-TrCP, was sufficient to degrade CPT1. Similar to CPT1, LPCAT1 levels are also regulated at the level of protein stability. However, LPCAT1 is a polyubiquitinated enzyme processed within the proteasome. Similar to CPT1, β-TrCP is the putative E3 ubiquitin ligase subunit responsible for LPCAT1 ubiquitination. β-TrCP appears to dock and ubiquitinate LPCAT1 within its amino-terminus. Collectively, these observations indicate the presence of cross-talk between the phospholipid remodeling and de novo pathways; this involves tight regulation by site-specific ubiquitination of indispensable regulatory enzymes catalyzed by SCF ubiquitin E3 ligase members that mechanistically provide homeostatic control of cellular phospholipid content.