Lysophosphatidic acid upregulates the epidermal growth factor receptor in human airway smooth muscle cells

Human airway smooth muscle cells treated with lysophosphatidic acid (LPA) and epidermal growth factor (EGF) exhibit synergistic stimulation of mitogenesis (Ediger TL and Toews ML. J Pharmacol Exp Ther 294: 1076-1082, 2000). The effects of LPA treatment of human airway smooth muscle cells on EGF receptor (EGFR) regulation have now been investigated. LPA treatment for 12-24 h resulted in a twofold increase in I-125-EGF binding and EGFR protein levels as assessed by Western blot analysis. Competition binding assays indicated single-site binding with an affinity of 3 nM, and the affinity was not changed by LPA treatment. EGFR upregulation was blocked by cycloheximide and actinomycin D, suggesting that LPA influences transcriptional regulation of EGFR expression. Inhibitor studies revealed a prominent role for activation of mitogen-activated protein kinase and p70 ribosomal S6 kinase. Both synergism and EGFR upregulation increased with increased cell density, whereas EGFR expression in control cells decreased. The similar requirements for exposure time, LPA concentrations, and cell confluence suggest that EGFR upregulation may be one contributing factor to the synergistic stimulation of mitogenesis seen with LPA plus EGF.