MS4A3 Serves As a Biomarker Reflecting the Lineage Differentiation and Metabolic Status of Human Acute Myeloid Leukemia Cells

Numerous molecularly targeted drugs have been developed for human blood cancers and have dramatically improved the prognosis of chronic leukemia and malignant lymphoma. On the other hand, although Flt3 inhibitors have covered a subset of patients, conventional anticancer therapy is still the mainstay of treatment for acute myeloid leukemia (AML). As a result, there has been no notable improvement in the outcomes and long-term survival of AML patients. Therefore, the identification of novel molecules involved in AML and new therapeutic strategies targeting these molecules are desirable to address the unmet needs in the field of hematology.

The aim of this study is to identify a biomarker that reflects the differential and metabolic characteristics of AML cells and to develop new methods to control the progression of AML. We have previously identified membrane-spanning 4-domains, subfamily A member 3 (Ms4a3) as a molecule specifically expressed on bone marrow progenitor cells with myelo-monocytic differentiation potential (BBRC 2018). Subsequently, MS4A3 expression was reported to be useful in fate mapping models tracing monocyte and granulocyte lineage (Cell 2019). Furthermore, a recent study shows that MS4A3 expression promotes CML stem cell differentiation and is associated with treatment sensitivity (Blood 2022). However, the expression and functions of MS4A3 in AML have remained elusive.

We analyzed human acute leukemia patients by flow cytometry and found that MS4A3 is specifically expressed on AML cells, especially those with monocyte lineage characteristics, but not at all in lymphocytic leukemia. In addition, MS4A3 expression levels correlated with monocyte lineage differentiation and active cell cycle status in human AML cell lines. We then sought to elucidate the biological functions of MS4A3 by exploring its associated molecules. To this end, we exploited our original proteomics method using the ancestral BirA for Proximity-Dependent Biotin Identification (AirID), which enables the identification of proteins that physiologically interact with MS4A3 in live AML cells. As a result, we found that mTOR-related proteins, innate immune inflammatory proteins, and cell cycle-related proteins are located in close proximity to MS4A3. This result suggests that MS4A3 expression might serve as a biomarker for the active status of AML cells, because mTOR, inflammation and cell cycle pathways are closely related to activated metabolism. In this context, it is noteworthy that the addition of mTORC1 inhibitors to culture media suppressed MS4A3 expression as well as proliferation of AML cells. Based on these findings, we hypothesized that MS4A3 serves as a reliable biomarker reflecting the metabolic status of AML.

This hypothesis was further supported by the finding that exogenously induced expression of endothelial cell-selective adhesion molecule (ESAM) significantly upregulated MS4A3 expression in AML cells. Furthermore, deletion of ESAM significantly decreased MS4A3 expression in murine HSCs. Our previous studies have shown that ESAM expression level indicates the active status of AML stem cells as well as healthy hematopoietic stem cells (HSCs) (Blood 2009, J Immunol 2012, Stem Cells 2021). In fact, several studies have identified ESAM as one of the AML stem cell-related antigens. Therefore, we assert that MS4A3 is localized in the ESAM-mediated signaling pathway and plays some roles in the growth of AML cells.

Taken together, these data show that MS4A3 is an important molecule to investigate the nature of AML cells. In addition, information on intracellular molecules that may interact with MS4A3 would provide new insights for the development of new therapeutic strategies.