Important Role for Glutamine in Human and Mouse TME Models of CLL ; Towards Development of a PET Tracer As a Novel Diagnostic Tool

The amino acid transporter ASCT2, encoded by the gene SLC1A5, is the main glutamine importer in proliferating human and mouse cells. We recently showed that glutamine is the main fuel of the tricarboxylic acid (TCA) cycle in cells from chronic lymphocytic leukemia (CLL) patients, and that blocking its import attenuated CD40/BCR-induced venetoclax (VEN) resistance ¹. In the current study, we have investigated the role of ASCT2 and the consequences of its inhibition in human and mouse CLL cells and in healthy lymphocytes by flow cytometry, gene expression analysis, extracellular flux analysis, metabolomics, CRISPR/Cas9 gene editing and adoptive-cell transfer experiments. To enable in vivo assessment of ASCT2 in the future with Positron Emission Tomography (PET), we have explored the potential use of ¹⁸F-labelelled ( 2S,2R)-4-fluoroglutamine (4- ¹⁸F]FGln) ² as PET tracer.

Increased expression of ASCT2 upon PMA/ionomycin stimulation was observed in healthy B and T cells. In BCR/CD40-stimulated CLL cells, the increased expression was irrespective of IGHV status. We dissected the main routes of glutamine usage in CD40- and BCR-stimulated CLL cells and found that in addition to its role in the TCA cycle, glutamine import is linked to mTOR signaling. The ASCT2 inhibitor V9302 impaired mTOR signaling and decreased translation of new proteins, including the anti-apoptotic proteins Bcl-XL and Mcl-1. Of relevance for therapeutic approaches, the effector and proliferative capacity of human T cells was preserved upon V9302 treatment. This suggests that blocking ASCT2 will not affect immune responses.

The SLC1A5 gene was deleted by CRISPR/Cas9 technique in murine TCL1 cells ³. Adoptive cell transfer experiments of SLC1A5-KO and WT TCL1 cells injected at 50-50% ratio in recipient mice showed selective outgrowth of SLC1A5-WT cells, with no detection of the SLC1A5-KO in the spleen of overt leukemic mice. This indicates that TCL1 leukemic cells rely on SLC1A5/ASCT2 for growth.

Finally, we explored the high expression of ASCT2 and glutamine addiction in CLL from the diagnostic perspective. With the aim of improving the currently suboptimal [ ¹⁸F]FDG PET for imaging of CLL LN sites ⁴, we successfully synthesized 4-[ ¹⁸F]FGln. We observed uptake in CD40-stimulated CLL cells, which was decreased in the presence of V9302. Experiments to perform PET with 4-[ ¹⁸F]FGln in TCL1 mice to demonstrate an in vivo proof of concept are ongoing.

Overall, these data support a crucial role for ASCT2 and glutamine in leukemia development and VEN resistance in CLL. This provides a basis for targeting the transporter for therapeutic options and/or taking advantage of its function for diagnostic approaches using 4-[ ¹⁸F]FGln as PET tracer.

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