Disabling the Protease DDI2 Attenuates the Transcriptional Activity of NRF1 and Potentiates Proteasome Inhibitor Cytotoxicity.
Proteasome inhibition is used therapeutically to induce proteotoxic stress and trigger apoptosis in cancer cells that are highly dependent on the proteasome. As a mechanism of resistance, inhibition of the cellular proteasome induces the synthesis of new, uninhibited proteasomes to restore proteasome activity and relieve proteotoxic stress in the cell, thus evading apoptosis. This evolutionarily conserved compensatory mechanism is referred to as the proteasome-bounce back response and is orchestrated in mammalian cells by nuclear factor erythroid derived 2-related factor 1 (NRF1), a transcription factor and master regulator of proteasome subunit genes. Upon synthesis, NRF1 is cotranslationally inserted into the endoplasmic reticulum (ER), then is rapidly retrotranslocated into the cytosol and degraded by the proteasome. In contrast, during conditions of proteasome inhibition or insufficiency, NRF1 escapes degradation, is proteolytically cleaved by the aspartyl protease DNA damage inducible 1 homolog 2 (DDI2) to its active form, and enters the nucleus as an active transcription factor. Despite these insights, the cellular compartment where the proteolytic processing step occurs remains unclear. Here we further probed this pathway and found that NRF1 can be completely retrotranslocated into the cytosol where it is then cleaved and activated by DDI2. Furthermore, using a triple-negative breast cancer cell line MDA-MB-231, we investigated the therapeutic utility of attenuating DDI2 function. We found that DDI2 depletion attenuated NRF1 activation and potentiated the cytotoxic effects of the proteasome inhibitor carfilzomib. More importantly, expression of a point-mutant of DDI2 that is protease-dead recapitulated these effects. Taken together, our results provide a strong rationale for a combinational therapy that utilizes inhibition of the proteasome and the protease function of DDI2. This approach could expand the repertoire of cancer types that can be successfully treated with proteasome inhibitors in the clinic.
['Northrop|Amy|A|', 'Vangala|Janakiram R|JR|', 'Feygin|Alex|A|', 'Radhakrishnan|Senthil K|SK|0000-0002-5211-9498']
D000818:Animals D017209:Apoptosis D057055:Aspartic Acid Proteases D045744:Cell Line, Tumor D003600:Cytosol D057809:HEK293 Cells D006801:Humans D051379:Mice D041681:NIH 3T3 Cells D051266:Nuclear Respiratory Factor 1 D009842:Oligopeptides D046988:Proteasome Endopeptidase Complex D061988:Proteasome Inhibitors D021381:Protein Transport D015533:Transcriptional Activation
DDI2 transcription NRF1 proteasome inhibitor cancer proteasome genes