Abstract 268: Differential Activation and Subcellular Targeting of Erk1/2 and Akt in Response to β-Adrenergic Receptor-Mediated Transactivation of Epidermal Growth Factor Receptor in Cardiomyocytes

Abstract

β-adrenergic receptors (βAR) are critical regulators of cardiac function whose dysregulation during heart failure are associated with diminished cardiac function, however βAR-mediated EGFR transactivation has been shown to relay cardioprotection via unknown mechanisms. We hypothesized that EGFR transactivation may result in differential activation and subcellular targeting of prosurvival kinases known to be downstream of EGFR, namely ERK1/2 and Akt. Thus, ERK1/2 and Akt phosphorylation and subcellular distribution was assessed in rat neonatal cardiomyocytes (RNCM). Treatment of RNCM with the βAR agonist isoproterenol (Iso) resulted in significant phosphorylation of both ERK1/2 (P-ERK) and Akt (P-Akt), in the cytosolic, plasma membrane and nuclear fractions. EGFR inhibition with AG1478 resulted in complete ablation of Iso-induced P-ERK in all fractions, as did MEK1/2 inhibition with PD184352. Total ERK levels did not change in any fraction under any condition, which along with the PD184352 data suggests Iso-mediated EGFR-dependent effects on ERK1/2 activity at different cellular locations is reliant upon MEK1/2 trafficking. While Akt phosphorylation in response to Iso-mediated EGFR transactivation was not sensitive to EGFR inhibition in the cytosol, the P-Akt response was completely abrogated by AG1478 in the plasma membrane and nuclear fractions. The PI3K inhibitor LY-294002 blocked Iso-induced Akt phosphorylation in all fractions, confirming reliance upon PI3K activity for Iso-mediated Akt activation. Additionally, total Akt levels remained constant over all treatments except at the plasma membrane, where AG 1478 reduced T-Akt, suggesting that Akt recruitment and PI3K activity each contribute to an increase in plasma membrane-associated P-Akt, whereas increased nuclear P-Akt in response to Iso-induced EGFR signaling depends solely on PI3K activity. In all, these results demonstrate differential impact of βAR-mediated EGFR transactivation on the subcellular activation and targeting of cardiomyocyte ERK1/2 and Akt. Further understanding of the downstream consequences of these effects in response to βAR-mediated EGFR transactivation could lead to improved therapies for the treatment of heart failure.