Dependence of Peroxisome Proliferator-activated Receptor Ligand-induced Mitogen-activated Protein Kinase Signaling on Epidermal Growth Factor Receptor Transactivation

Abstract

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that function as ligandactivated transcription factors regulating lipid metabolism and homeostasis. In addition to their ability to regulate PPAR-mediated gene transcription, PPAR␣ and ␥ ligands have recently been shown to induce activation of mitogen-activated protein kinases (MAPKs), which in turn phosphorylate PPARs, thereby affecting transcriptional activity. However, the mechanism for PPAR liganddependent MAPK activation is unclear. In the current study, we demonstrate that various PPAR␣ (nafenopin) and ␥ (ciglitazone and troglitazone) agonists rapidly induced extracellular signal-regulated kinase (Erk) and/or p38 phosphorylation in rat liver epithelial cells (GN4). The selective epidermal growth factor receptor (EGFR) kinase inhibitors, PD153035 and ZD1839 (Iressa), abolished PPAR␣ and ␥ agonist-dependent Erk activation. Consistent with this, PPAR agonists increased tyrosine autophosphorylation of the EGFR as well as phosphorylation at a putative Src-specific site, Tyr 845 . Experiments with the Src inhibitor, PP2, and the antioxidant N-acetyl-L-cysteine revealed critical roles for Src and reactive oxygen species as upstream mediators of EGFR transactivation in response to PPAR ligands. Moreover, PPAR␣ and ␥ ligands increased Src autophosphorylation as well as kinase activity. EGFR phosphorylation, in turn, led to Ras-dependent Erk activation. In contrast, p38 activation by PPAR␣ and ␥ ligands occurred independently of Src, oxidative stress, the EGFR, and Ras. Interestingly, PPAR␣ and ␥ agonists caused rapid activation of proline-rich tyrosine kinase or Pyk2; Pyk2 as well as p38 phosphorylation was reduced by intracellular Ca 2؉ chelation without an observable effect on EGFR and Erk activation, suggesting a possible role for Pyk2 as an upstream activator of p38. In summary, PPAR␣ and ␥ ligands activate two distinct signaling cascades in GN4 cells leading to MAPK activation.

Troglitazone inhibits cell proliferation by attenuation of epidermal growth factor receptor signaling independent of peroxisome proliferator-activated receptor γ

“…First, treatment of rat liver epithelial cells with Cig and TGZ induces EGFR-Erk1/2 activation absolutely independent of PPARγ [26], whereas inhibition of EGFR kinase activity or overexpression of the dominant negative Ras mutant blocks PPARγ agonistinduced Erk1/2 phosphorylation [25]. Second, EGFRdependent Erk1/2 activation induced by 15d-PG-J2 can be blocked by EGFR kinase inhibitor or PP2, an Src family protein kinase inhibitor [37], but not by PPARγ antagonists [38].…”

“…Previous studies have shown that PPARγ agonists can cause rapid MAPKs activation or Akt inhibition in multiple cell types [25,[39][40][41]], yet their interrelationship has not been fully addressed. We have demonstrated that, like many other non-natural ligands that transactivate EGFR signaling [42], TGZ can cause rapid, EGFRdependent transient Erk1/2 activation, which has an absolute requirement for Grb2 binding to EGFR.…”

“…A few studies have suggested that PPARγ ligands might cross-activate the EGFR-Erk1/2 pathway [25,26]. However, molecular mechanisms for this activation are still being explored.…”

“…Earlier studies suggested that transient TGZ-activated Erk1/2 phosphorylation was sensitive to EGFR kinase inhibition [25], but it is unclear whether the proximal adaptor proteins Grb2 and/or Shc are required to transduce TGZ signaling. To address this question, we generated were transiently co-transfected with PPRE3-tk-luc reporter (0.2 mg) and phRL-CMV reference plasmid (3 ng) in triplicate.…”

Regulation of food intake by oleoylethanolamide

“…This suggests that, in addition to its transcriptional actions, PPAR-a also engages non-transcriptional mechanisms, possibly analogous to those recruited by receptors for estrogen hormones and vitamin D. Such effects include activation of phosphoinositol-3-kinase [62], NOS [63] and guanylyl cyclase [64]. Whether PPAR-a initiates similar or different molecular events remains to be determined, but initial evidence suggests that PPAR-a agonists can rapidly transactivate epidermal growth factor receptor, extracellular regulated kinase and p38-MAP kinase in vitro [65].…”