Succination of Keap1 and activation of Nrf2-dependent antioxidant pathways in FH-deficient papillary renal cell carcinoma type 2

Abstract

Fumarate hydratase (FH) is a tumor suppressor, but how it acts is unclear. Two reports in this issue of Cancer Cell reveal that FH deficiency leads to succination of Keap1, stabilization of Nrf2, and induction of stress-response genes including HMOX1, which is important for the survival of FH-deficient cells.

Figure 1. A proposed update of the Keap1 ‘Hinge and Latch’ model of Nrf2 degradation

A) Cartoon model of Keap1 dimer bound to Nrf2. In the active state (left) two DC domains are oriented to simultaneously bind the Nrf2 high affinity (**) and low affinity (*) sites. Right panel illustrates the inactive state induced by Cys¹⁵¹ and Cys²⁸⁸ modification. According to our model, Cys¹⁵¹ modification causes rotation to a linear conformation and the release of Cul3. We propose that Cys²⁸⁸ modification misorients the DC domain with respect to the IVR. B) Potential structural context of proposed ‘hinge.’ Two Keap1-related structures with BTB and IVR domains have been reported: KLHL11 (pdb accession code: 3i3n) and Gigaxonin (pdb accession code: 3hve). These two proteins exhibit the same domain organization as Keap1: BTB, IVR followed by a domain made up of kelch repeats. KLHL11 (faded tones) differs from Gigaxonin (deeper colors) by a rotation of the IVR domain (cyan) with respect to the BTB domain (blue) (the two states are marked by arrows). We propose that Keap1 exists in two conformations corresponding to these two structures, and that rotation from one to the other (‘hinge’) is brought about by Cys¹⁵¹ modification. The ‘hinge’ motion causes a change in the putative Cul3 binding site (green dots), which we determined by superimposing a Cul1-bound BTB domain (pdb accession code: 1ldk). Thus, the hinge motion may be linked to Cul3 release. C) Potential structural context of proposed ‘latch.’ Left: Surface representation of a hypothetical degradation competent, V-shaped Keap1 dimer interacting with both Nrf2 binding motifs. Right: Surface representation of a hypothetical degradation incompetent, linear Keap1 dimer that cannot bind both Nrf2 motifs simultaneously. Keap1 DC domains (from pdb accession code: 2flu, red) were placed relative to the IVR domains (cyan) using the following considerations: i) EM density pictures suggesting a general positioning of the DC domain with respect to the IVR, ii) identification of a conserved DC surface patch, which included Cys²⁸⁸, that was placed near the IVR, and iii) maintaining the ‘latch’ distance of Nrf2 DC-binding motifs located on either side of a helix (wheat). The orientation of the DC domain toward a position unfavorable for binding the Nrf2 low affinity site (right) may be triggered by either modification of Cys¹⁵¹ resulting in a rotation from a V-shape (left) to a linear dimer (right), as described in B, or modification of Cys²⁸⁸ resulting in a mis-orientation of the DC domain with respect to the IVR.