Modulation of nongenomic activation of PI3K signalling by tetramerization of N-terminally-cleaved RXRα

Abstract

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

Overview of the structure-based non-genomic effects of the nuclear receptor RXRα

“…RXRα–LXR heterodimers participate in AP1 signaling in keratinocytes [ 37 ]. Recent findings indicate that an N-terminally truncated form of RXRα (tRXRα) produced in cancer cells resides in the cytoplasm, where it promotes the growth of tumor cells [ 38 – 43 ]. Proteolytic cleavage of RXRα, which could reduce RXRα expression or enhance truncated RXRα expression in tumor cells, is also correlated with the development of certain malignancies [ 38 , 39 ].…”

“…This tRXRα (RXRα-Δ80) can interact with the p85α subunit of the PI3K/AKT survival pathway and promote cancer cell proliferation in the majority of cancer cells [ 42 ]. In our study, tRXRα was detected in the cytoplasm while RXRα was detected in the nucleus [ 43 ]. We also found extensive intramolecular interaction between the N terminus and the C terminus (N/C) of full-length RXRα but not that of RXRα-Δ80, which explains why tRXRα can interact with p85α while full-length RXRα cannot [ 43 ].…”

“…In our study, tRXRα was detected in the cytoplasm while RXRα was detected in the nucleus [ 43 ]. We also found extensive intramolecular interaction between the N terminus and the C terminus (N/C) of full-length RXRα but not that of RXRα-Δ80, which explains why tRXRα can interact with p85α while full-length RXRα cannot [ 43 ]. The results of our study suggested that amino acids from 60 to 80 are critical for the RXRα N/C interaction.…”

Dual conformation of the ligand induces the partial agonistic activity of retinoid X receptor α (RXRα)

“…Crystal structures of apo and various modulatorbound ligand-binding domain (LBD) of human RXRa (hRXRa) were reported in the past [16][17][18][19][20][21][22][23][24][25]. They can largely be classified into two types according to their conformation of the AF-2 interface in the C-terminal region; the 'folded' or 'extended' form.…”

“…The agonistbound RXR complexes mostly appear in the 'folded' form [18,20,22], although the existence of this complex in 'extended' form was also found in some cases, such as in an endogenous ligand 9-cis-retinoic acid (9cRA)bound RXR complex, where the LBP of only some subunits are occupied by the ligand [17]. The apo [17,21] and a few other selectively bound ligated RXR structures [17,21,23,24] were found in the 'extended' form. This is also true with the antagonist danthronbound hRXRa-LBD [21].…”