RasGRF Couples Nox4-Dependent Endoplasmic Reticulum Signaling to Ras

Area of Science:

• Cellular Biology
• Molecular Signaling
• Endothelial Cell Function

Background:

• Endoplasmic reticulum (ER) stress is a cellular condition that triggers adaptive responses like the unfolded protein response (UPR).
• Reactive oxygen species (ROS) generated at the ER may play a role in homeostatic signaling via Ras during ER stress.
• The precise mechanisms of focal signaling pathways in response to ER stress require elucidation.

Purpose of the Study:

• To identify the mechanisms responsible for focal Ras signaling in endothelial cells during ER stress.
• To investigate the role of NADPH oxidase (Nox4) and reactive oxygen species (ROS) in ER stress-induced signaling.
• To determine the upstream mediators linking ER stress to Ras activation.

Main Methods:

• Utilized endothelial cells and induced ER stress with tunicamycin.
• Investigated the localization of Nox4 and Ras activation using specific cellular markers.
• Examined the role of RasGRF1 and RasGRF2 in ER stress signaling pathways.
• Assessed the impact of Nox4 knockdown on calcium levels and UPR activation.

Main Results:

• ER stress induced tunicamycin activates Nox4 and Ras focally on the ER, not the plasma membrane.
• Ras activation downstream of Nox4 is essential for the unfolded protein response.
• Vascular endothelial growth factor (VEGF) treatment demonstrated localized ROS and Ras signaling at the plasma membrane.
• RasGRF1 and RasGRF2 were identified as novel upstream mediators linking Nox4 to Ras activation.
• Nox4 knockdown prevented ER stress-induced increases in cytosolic calcium and blocked Ras activation and UPR.

Conclusions:

• ER stress initiates a localized signaling module on the ER surface.
• This module involves Nox4-dependent calcium mobilization.
• The pathway directs Ras activation via ER-associated, calcium-responsive RasGRF, crucial for the UPR.