The autophagy protein RUBCNL/PACER represses RIPK1 kinase-dependent apoptosis and necroptosis.

ABSTRACT

Mesenchymal stem cells (MSCs) are used in cell therapy; nonetheless, their application is limited by their poor survival after transplantation in a proinflammatory microenvironment. Macroautophagy/autophagy activation in MSCs constitutes a stress adaptation pathway, promoting cellular homeostasis. Our proteomics data indicate that RUBCNL/PACER (RUN and cysteine rich domain containing beclin 1 interacting protein like), a positive regulator of autophagy, is also involved in cell death. Hence, we screened MSC survival upon various cell death stimuli under loss or gain of function of RUBCNL. MSCs were protected from TNF (tumor necrosis factor)-induced regulated cell death when RUBCNL was expressed. TNF promotes inflammation by inducing RIPK1 kinase-dependent apoptosis or necroptosis. We determine that MSCs succumb to RIPK1 kinase-dependent apoptosis upon TNF sensing and necroptosis when caspases are inactivated. We show that RUBCNL is a negative regulator of both RIPK1-dependent apoptosis and necroptosis. Furthermore, RUBCNL mutants that lose the ability to regulate autophagy, retain their function in negatively regulating cell death. We also found that RUBCNL forms a complex with RIPK1, which disassembles in response to TNF. In line with this finding, RUBCNL expression limits assembly of RIPK1-TNFRSF1A/TNFR1 complex I, suggesting that complex formation between RUBCNL and RIPK1 represses TNF signaling. These results provide new insights into the crosstalk between the RIPK1-mediated cell death and autophagy machineries and suggest that RUBCNL, due to its functional duality in autophagy and apoptosis/necroptosis, could be targeted to improve the therapeutic efficacy of MSCs. Abbreviations BAF bafilomycin A1; CASP3 caspase 3; Caspases cysteine-aspartic proteases; cCASP3 cleaved CASP3; CQ chloroquine; CHX cycloheximide; cPARP cleaved poly (ADP-ribose) polymerase; DEPs differential expressed proteins; ETO etoposide; MEF mouse embryonic fibroblast; MLKL mixed lineage kinase domain-like; MSC mesenchymal stem cell; MTORC1 mechanistic target of rapamycin kinase complex 1; Nec1s necrostatin 1s; NFKB/NF-kB nuclear factor of kappa light polypeptide gene enhancer in B cells; PLA proximity ligation assay; RCD regulated cell death; RIPK1 receptor (TNFRSF)-interacting serine-threonine kinase 1; RIPK3 receptor-interacting serine-threonine kinase 3; RUBCNL/PACER RUN and cysteine rich domain containing beclin 1 interacting protein like; siCtrl small interfering RNA nonsense; siRNA small interfering RNA; TdT terminal deoxynucleotidyl transferase; Tm tunicamycin; TNF tumor necrosis factor; TNFRSF1A/TNFR1 tumor necrosis factor receptor superfamily, member 1a.