A Highly Potent Apomorphine Derivative Enhancing Neurite Outgrowth via Nrf2 Activation

Abstract

Apomorphine (APO), a dopamine agonist, activates nuclear factor erythroid 2-related factor 2 (Nrf2) and exerts antioxidant effects, making it a promising candidate for neuroprotection against oxidative stress. This study evaluated neuroplasticity-enhancing properties of newly synthesized APO derivatives, focusing on their ability to promote neurite outgrowth in PC12 cells under nerve growth factor (NGF) stimulation. D55, an APO derivative, retains the hydroxyl group at APO's 11th position while substituting the 10th with an ethoxy group. D55 exhibited the highest potency (EC₅₀ = 0.5661 nM), significantly enhancing neurite outgrowth. APO demonstrated the highest efficacy (Emax ~10-fold increase), while edaravone (Eda) required higher concentrations (EC₅₀ = 22.5 nM) for moderate effects (Emax ~4-fold increase). D30, in which the 11th hydroxyl was replaced with a methoxy group, had no effect. Neurite outgrowth-promoting effects of APO, D55, and Eda were significantly attenuated by Nrf2 siRNA knockdown, confirming that their neuroplasticity effects are Nrf2-mediated. These findings confirm that D55 is a highly potent Nrf2-activating compound with strong neuroprotective potential, providing new insights into its therapeutic applications for neurodegenerative diseases associated with oxidative stress.

Keywords: Nrf2 activation; apomorphine derivatives; edaravone; neurite outgrowth; neurodegenerative diseases; therapeutic agents.

Figure 1

Chemical structures of apomorphine, D30, D55, and edaravone. (A) Aporphine framework. (B) Apomorphine (APO) has two hydroxy groups at positions 10 and 11. (C) D30 possesses a hydroxy group at position 10 and a methoxy group at position 11 in the aporphine alkaloid framework. (D) In D55, an ethoxy group occupies position 10, with a hydroxy group at position 11 in the aporphine alkaloid framework. (E) Edaravone (Eda) is a free radical scavenger, but its structure does not contain an aporphine alkaloid framework.

Figure 2

Potentiation of neurite outgrowth by apomorphine, D55, and edaravone. (A) Representative photographs in PC12 cells. Control: NGF (2.5 ng/mL) alone, apomorphine, D55, and edaravone: NGF (2.5 ng/mL) + APO (1 µM), D55 (0.1 µM), or Eda (10 µM). (B) Effects of APO and derivatives on NGF-induced neurite outgrowth in PC12 cells. APO (0.001, 0.01, 0.1, and 1 µM), D55 (0.001, 0.01, and 0.1 µM), and Eda (0.001, 0.01, 0.1, 1, and 10 µM) potentiated neurite outgrowth in PC12 cells, in a concentration-dependent manner. All data represent the mean ± S.E.M. (n = 7–8). ** p < 0.01, **** p < 0.0001 as compared with the control group (one-way ANOVA).

Figure 3

EC₅₀s for each Compound. EC₅₀s were calculated for (A) APO, (B) D55, and (C) Eda. The EC₅₀ for (D) D30 could not be calculated because it did not promote neurite outgrowth. Control: NGF (2.5 ng/mL) alone.

Figure 4

Nrf2 enhances nerve growth factor (NGF)-induced neurite outgrowth by APO, D55, and Eda. Potentiating effects of (A) APO (1 µM), (B) D55 (0.1 µM), or (C) Eda (10 µM) on NGF-induced neurite outgrowth were significantly antagonized by treatment with Nrf2 siRNA, but not negative siRNA. Neither Nrf2 siRNA nor negative siRNA alone altered NGF (2.5 ng/mL)-induced neurite outgrowth. All data represent the mean ± S.E.M. (n = 8). * p < 0.05, *** p < 0.001, **** p < 0.0001 as compared with the APO (1 µM), D55 (0.1 µM), or Eda (10 µM) group (two-way ANOVA). NS = not significant, p > 0.05.