# RAF inhibitors activate the integrated stress response by direct activation of GCN2

## TL;DR

This study shows that RAF inhibitors, used to treat cancer, unexpectedly activate a stress response pathway through a protein called GCN2, which could affect how cancer cells respond to treatment.

## Contribution

The paper reveals that RAF inhibitors directly activate GCN2, a novel off-target effect that triggers the integrated stress response independently of their known mechanisms.

## Key findings

- RAFi activates the UPR and ISR via GCN2, not PERK.
- GCN2 inhibition or knockout blocks RAFi-induced ATF4 and CHOP expression.
- Structural models confirm RAFi binds directly to GCN2's kinase domain.

## Abstract

Paradoxical activation of wild type RAF by chemical RAF inhibitors (RAFi) is a well-understood ‘on-target’ biological and clinical response. In this study, we show that a range of RAFi drive ERK1/2-independent activation of the Unfolded Protein Response (UPR), including expression of ATF4 and CHOP, that requires the translation initiation factor eIF2α. RAFi-induced ATF4 and CHOP expression was not reversed by inhibition of PERK, a known upstream activator of the eIF2α-dependent Integrated Stress Response (ISR). Rather, RAFi exposure activated GCN2, an alternate eIF2α kinase, leading to eIF2α-dependent (and ERK1/2-independent) ATF4 and CHOP expression. The GCN2 kinase inhibitor A-92, GCN2 RNAi, GCN2 knock-out or ISRIB (an eIF2α antagonist) all reversed RAFi-induced expression of ATF4 and CHOP indicating that RAFi require GCN2 to activate the ISR. RAFi also activated full-length recombinant GCN2 in vitro and in cells, generating a characteristic ‘bell-shaped’ concentration-response curve, reminiscent of RAFi-driven paradoxical activation of WT RAF dimers. Activation of the ISR by RAFi was abolished by a GCN2 kinase dead mutation. A M802A GCN2 gatekeeper mutant was activated at lower RAFi concentrations, demonstrating that RAFi bind directly to the GCN2 kinase domain; this is supported by mechanistic structural models of RAFi interaction with GCN2. Since the ISR is a critical pathway for determining cell survival or death, our observations may be relevant to the clinical use of RAFi, where paradoxical GCN2 activation is a previously unappreciated off-target effect that may modulate tumour cell responses.

Three BRAF inhibitors are used to treat melanoma and colorectal cancer. Here, the authors demonstrate that these drugs bind and activate the protein kinase GCN2, a previously unappreciated off-target effect that may modulate tumour cell responses.