Proteasome Inhibition Sensitizes Liposarcoma to MDM2 Inhibition with Nutlin-3 by Activating the ATF4/CHOP Stress Response Pathway.

Liposarcoma is the most commonly occurring soft tissue sarcoma and is frequently characterized by amplification of chromosome region 12q13-15 harboring the oncogenes MDM2 and CDK4. This unique genetic profile makes liposarcoma an attractive candidate for targeted therapeutics. While CDK4/6 inhibitors are currently employed for treatment of several cancers, MDM2 inhibitors have yet to attain clinical approval. Here, we report the molecular characterization of the response of liposarcoma to the MDM2 inhibitor nutlin-3. Treatment with nutlin-3 led to upregulation of two nodes of the proteostasis network: the ribosome and the proteasome. CRISPR/Cas9 was used to perform a genome-wide loss of function screen that identified PSMD9, which encodes a proteasome subunit, as a regulator of response to nutlin-3. Accordingly, pharmacological studies with a panel of proteasome inhibitors revealed strong combinatorial induction of apoptosis with nutlin-3. Mechanistic studies identified activation of the ATF4/CHOP stress response axis as a potential node of interaction between nutlin-3 and the proteasome inhibitor carfilzomib. CRISPR/Cas9 gene editing experiments confirmed that ATF4, CHOP, and the BH3-only protein, NOXA, are all required for nutlin-3 and carfilzomib-induced apoptosis. Furthermore, activation of the unfolded protein response using tunicamycin and thapsigargin was sufficient to activate the ATF4/CHOP stress response axis and sensitize to nutlin-3. Finally, cell line and patient-derived xenograft models demonstrated combinatorial effects of treatment with idasanutlin and carfilzomib on liposarcoma growth in vivo. Together, these data indicate that targeting of the proteasome could improve the efficacy of MDM2 inhibitors in liposarcoma.