Regulation of epidermal-growth-factor-receptor signal transduction by cis-unsaturated fatty acids. Evidence for a protein kinase C-independent mechanism

Abstract

The effect of acute treatment with non-esterified fatty acids (NEFA) on transmembrane signalling has been investigated in three different cell lines. In EGFR T17 cells, pretreatment with cis-unsaturated (oleic and palmitoleic acids) NEFA, but not with saturated or trans-unsaturated NEFA, inhibited the epidermal-growth-factor (EGF)-induced increases in cytosolic [Ca2+], membrane potential and Ins(1,4,5)P3 generation. The blocking effect was found to be time- and dose-dependent and rapidly reversible after washout. However, oleic acid treatment did not block either binding of 125I-EGF to its receptor or EGF-induced autophosphorylation of the EGF receptor. The mechanism of action of NEFA could not be attributed to protein kinase C activation, since (i) down-regulation of the enzyme by long-term treatment with phorbol esters did not prevent blockade by oleic acid, and (ii) the effects of acutely administered phorbol ester and oleic acid were additive. In this cell line, signalling at bradykinin and bombesin receptors was also impaired by oleic acid. In A431 cells, oleic acid also blocked signal transduction at the EGF and B2 bradykinin receptors. Finally, in PC12 cells, oleic acid blocked the Ca2+ influx mediated by the activation of B2 bradykinin receptors. In conclusion: (1) NEFA block signal transduction by interfering with receptor-phospholipase C or phospholipase C-substrate interaction without preventing ligand binding; (2) NEFA do not act by a protein kinase C-mediated mechanism; (3) the effect of NEFA is dependent on their configuration rather than hydrophobicity or chain length; (4) this effect is evident in several different cell lines and receptor systems.