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Cell death in human diseases is often a consequence of disrupted cellular homeostasis. If cell death is prevented without restoring cellular homeostasis, it may lead to a persistent dysfunctional and pathological state. Although mechanisms of cell death have been thoroughly investigated1-3, it remains unclear how homeostasis can be restored after inhibition of cell death. Here we identify TRADD4-6, an adaptor protein, as a direct regulator of both cellular homeostasis and apoptosis. TRADD modulates cellular homeostasis by inhibiting K63-linked ubiquitination of beclin 1 mediated by TRAF2, clAPl and clAP2, thereby reducing autophagy. TRADD deficiency inhibits RlPKl-dependent extrinsic apoptosis and proteasomal stress-induced intrinsic apoptosis. We also show that the small molecules lCCB-19 and Apt-1 bind to a pocket on the N-terminal TRAF2-binding domain of TRADD (TRADD-N), which interacts with the C-terminal domain (TRADD-C) and TRAF2 to modulate the ubiquitination ofRIPKl and beclin 1. Inhibition of TRADD by lCCB-19 or Apt-1 blocks apoptosis and restores cellular homeostasis by activating autophagy in cells with accumulated mutant tau, a-synuclein, or huntingtin. Treatment with Apt-1 restored proteostasis and inhibited cell death in a mouse model of proteinopathy induced by mutant tau(P301S). We conclude that pharmacological targeting of TRADD may represent a promising strategy for inhibiting cell death and restoring homeostasis to treat human diseases.
The accumulation of misfolded proteins in human diseases promotes cell death and progressive pathology by disrupting cellular homeostasis7. As the activation of autophagy is highly effective in reducing the accumulation of misfolded proteins and restoring cellular homeostasis8,9, we sought to identify targets that could be manipulated pharmacologically to restore cellular homeostasis by inducing autophagy and inhibiting apoptosis simultaneously.
Apoptosis inhibitors that activate autophagy
No known target can simultaneously modulate autophagy and cell death; therefore, we designed and conducted a multiplexed cell-based screen to identify small molecule inhibitors of apoptosis mediated by proteasomal stress and RlPK1-dependent apoptosis (RDA) that can also activate autophagy (Extended Data Fig. 1a). This quadruplexed screen of 170,000 compounds identified two active structural analogues, lCCB-17 and lCCB-19, and an inactive close analogue, lCCB-19i (Fig. 1a). An structure-activity relationship study identified an improved derivative called apostatin-1 (Apt-1). lCCB-19 and Apt-1 inhibited Velcade (bortezomib)-induced apoptosis and RDA with a half-maximal inhibitory concentration (lC50) of about 1 pM (Extended Data Fig. 1b, c). Apt-1 showed no...