Phosphatidic Acid Increases Epidermal Growth Factor Receptor Expression by Stabilizing mRNA Decay and by Inhibiting Lysosomal and Proteasomal Degradation of the Internalized Receptor

Overexpression of epidermal growth factor receptor (EGFR) is one of the frequent mechanisms implicated in cancer progression, and so is the overexpression of the enzyme phospholipase D (PLD) and its reaction product, phosphatidic acid (PA). However, an understanding of how these signaling molecules interact at the level of gene expression is lacking. Catalytically active PLD enhanced expression of EGFR in human breast cancer cells. Overexpression of the PLD2 isoform increased EGFR mRNA and protein expression. It also negated an EGFR downregulation mediated by small interfering RNA targeting EGFR (siEGFR). Several mechanisms contributed to the alteration in EGFR expression. First was the stabilization of EGFR transcripts as PLD2 delayed mRNA decay, which prolonged their half-lives. Second, RNase enzymatic activity was inhibited by PA. Third, protein stabilization also occurred, as indicated by PLD resistance to cycloheximide-induced EGFR protein degradation. Fourth, PA inhibited lysosomal and proteasomal degradation of internalized EGFR. PLD2 and EGFR colocalized at the cell membrane, and JAK3 phosphorylation at Tyr980/Tyr981 followed receptor endocytosis. Further, the presence of PLD2 increased stabilization of intracellular EGFR in large recycling vesicles at ~15 min of EGF stimulation. Thus, PLD2-mediated production of PA contributed to the control of EGFR exposure to ligand through a multipronged transcriptional and posttranscriptional program during the out-of-control accumulation of EGFR signaling in cancer cells.