PYCR1 inhibition in bone marrow stromal cells enhances bortezomib sensitivity in multiple myeloma cells by altering their metabolism.

Despite significant advancements, multiple myeloma (MM) remains incurable, largely due to drug resistance. Our previous research has demonstrated that proline metabolism plays a role in MM progression and that inhibiting PYCR1, the final enzyme in proline synthesis, enhances bortezomib sensitivity in MM cells. Given the high expression of PYCR1 in bone marrow stromal cells (BMSCs), we sought to investigate the effects of PYCR1 inhibition in BMSCs and its indirect influence on MM cell metabolism and viability. Culturing MM cells in conditioned medium (CM) of PYCR1-silenced BMSC significantly impaired oxidative phosphorylation and sensitised MM cells to bortezomib. Analysis of the CM secretome revealed a reduction in activin A release. Proline and activin A supplementation were able to counteract MM sensitivity to bortezomib. Combination therapy of the PYCR1 inhibitor pargyline and bortezomib reduced tumour load in a 3D model and reduced serum activin A levels in 5TGM1-bearing mice. This study demonstrates the contribution of stromal cell metabolism to MM progression. Inhibiting PYCR1 in BMSCs leads to less activin A release, limits oxidative phosphorylation in MM cells and enhances bortezomib efficacy.