Abstract 2505: PYCR1 inhibition in multiple myeloma-associated stroma limits tumor growth

Introduction Multiple myeloma (MM) is a hematological cancer, characterized by the accumulation of monoclonal plasma cells in the bone marrow. It remains an incurable cancer due to drug resistance, wherein the bone marrow microenvironment plays a crucial role. We have previously shown that glutamine-to-proline conversion is upregulated in MM cells upon hypoxic culture. Moreover, inhibition of proline production by blocking its converting enzyme PYCR1 successfully reduced proliferation and viability in vitro. Importantly, PYCR1 inhibition combined with standard-of-care agent bortezomib decreased tumor load in vivo. As PYCR1 is also highly expressed in stromal cells and proline an important component of extracellular matrix proteins, we investigated whether PYCR1 targeting in stromal cells affects MM viability and its structural microenvironment. Material and methods PYCR1 expression was investigated by the use of microarray data from the Heidelberg/Montpellier cohort. For in vitro experiments, the human MM cell line OPM-2 and human stromal cell line HS-5 were used. CD138+ and CD138- fractions from primary patient samples were separated by MACS. To obtain primary stromal cells, the CD138- fraction was plated out in fresh medium. After 48h, the medium was refreshed and all adherent cells were further cultured and used as primary stromal cells. PYCR1 expression in HS-5 cells was reduced through siRNA. Hypoxic culture (<1% O2) was established through chambers. RNA expression was measured by RT-qPCR, while protein expression was measured by western blot. Viability was assessed by CellTiterGlo assay. Results Gene expression analysis shows high RNA expression of PYCR1 in healthy bone marrow plasma cells (BMPCs), plasma cells from monoclonal gammopathy of undetermined significance (MGUS) and MM patients, but also in bone marrow stromal cells (BMSCs). PYCR1 expression was low or absent in other cell types, including T cells, osteoclasts, monocytic and granulocytic cells. On protein level, we also confirmed PYCR1 expression in MM cells (CD138+), CD138- fraction (including stromal cells) and cultured primary stromal cells. Moreover, PYCR1 expression increased upon hypoxic culture in primary stromal cells and stromal cell line HS-5. PYCR1 knockdown in stromal cell line HS-5 did not affect viability of the stromal cells, but its conditioned medium did reduce its protective effects when OPM-2 MM cells were treated with standard-of-care agent bortezomib. Further investigation revealed that knockdown of PYCR1 in HS-5 reduces RNA expression of structural proteins col1a1, col1a2, col3a1, ctgf and acta2 in HS-5. Conclusion PYCR1 is highly expressed in myeloma cells and stromal cells. PYCR1 inhibition in stromal cells reduces its proliferative effect on myeloma cells when combined with bortezomib. Preliminary data indicates a link between PYCR expression in stromal cells and rearrangement of extracellular matrix proteins. Citation Format: Inge Oudaert, Catharina Muylaert, Hatice Satilmis, Sylvia Faict, Kim De Veirman, Elke De Bruyne, Karin Vanderkerken, Eline Menu. PYCR1 inhibition in multiple myeloma-associated stroma limits tumor growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2505.