The function of S100A8 and S100A9 in the differentiation of AML: A Canadian Study

Myeloid-Related Proteins (MRPs), S100A8 and S100A9, are members of the S100 calcium-binding protein family. These proteins have been shown to be overly expressed in Acute Myeloid Leukemia (AML) ¹ and S100A8 in particular can contribute to poor prognosis in AML patients². Once secreted, S100A8 and S100A9 induce inflammatory and immune responses via interaction with a range of receptors including Toll-Like Receptor (TLR4)³. The potential functions of S100A8 and S100A9 in the development and progression of leukemia is yet to be elucidated.

Malika Laouedj  and colleagues from the Centre de Recherche du CHU de Quebec-Universite Laval, Qubec, Canada published results from their study on the role of S100A8 and S100A9 in AML in Blood on 30 January 2017.⁴

In this study, the biological functions of S100A8 and S100A9 and its effect on survival were investigated using mouse models of AML as well as cell samples of patients with primary AML.  

The key results of the study are:

• Compared to poorly differentiated AML, high expression of S100A9 and S100A8 observed in both myelomonocytic and monocytic AML cells.
• Elevated levels of S100A8/S100A9 heterodimer in the plasma of AML patients (n = 6) compared to healthy individuals (n = 10); P < 0.01
• S100A8/A9 concentrations gradually increased as leukemia progressed and correlated with an increase of leukemic cells in the Peripheral Blood (PB); correlation co-efficient = 0.9767; P = 0.0003
• Compared to control (n = 8) , anti-S100A8 (n = 10) significantly extended survival and delayed leukemia symptoms in treated mice; median survival  = 31 vs 39.5 days, P = 0.010
• In mice treated with anti-S100A8,  significant reduction of leukemic cells in the PB compared to the control; 47.6 ± 10.9% vs 86.3± 2.6 %, P = 0.009
• Anti-S100A8 treatment increased mature myeloid phenotype
• Median survival significantly increased in mice treated with Recombinant Murine (rm) S100A9 protein (n = 10) compared to control (n = 6); 41 vs 32 days, P = 0.0045
• No Adverse Effects (AEs) observed with rmS100A9 and anti-S100A8
• Cellular differentiation induced by rmS100A9 blocked upon addition of rmS100A8; P < 0.05
• Significant decrease in rhS100A9 induced CD14+ cells, upon neutralization of TLR-4; P < 0.001
• S100A9, S100A8 and TLR4 more expressed in myelomonocytic and monocytic AML cells compared to undifferentiated cells

In summation, S100A9 and S100A8 can regulate myeloid differentiation in AML.  S100A9 can induce the differentiation of AML cells via TLR-4 and also prolonged survival with no adverse effects in AML models. Additionally, S100A8 can regulate S100A9 activity and also sustain AML immature phenotypes.

The authors concluded by stating that their study could pave way for an alternative treatment approach aimed at cell differentiation of myelomonocytic and monocyic AMLs. The authors further suggested that a differentiated targeted therapy on the SA1008/SA100A9 and TLR-4 pathway could have a significant impact on the clinical outcomes of AML patients.

 Abstract

S100A8 and S100A9 are calcium-binding proteins predominantly expressed by neutrophils and monocytes, and play key roles in both normal and pathological inflammation. Recently, both proteins were found to promote tumor progression through the establishment of pre-metastatic niches and to inhibit antitumor immune responses. Although S100A8 and S100A9 have been studied in solid cancers, their functions in hematological malignancies remain poorly understood. However, S100A8 and S100A9 are highly expressed in acute myeloid leukemia (AML), and S100A8 expression has been linked to a poor prognosis in AML. We identified a small subpopulation of cells expressing S100A8 and S100A9 in AML mouse models and in primary human AML samples. In vitro and in vivo analyses revealed that S100A9 induces AML cell differentiation, whereas S100A8 prevents differentiation induced by S100A9 activity and maintains AML immature phenotype. Treatment with recombinant S100A9 proteins increased AML cell maturation, induced growth arrest, and prolonged survival in an AML mouse model. Interestingly, anti-S100A8 antibody treatment had effects similar to S100A9 therapy in vivo, suggesting that high ratios of S100A9 over S100A8 are required to induce differentiation. Our in vitro studies on the mechanisms/pathways involved in leukemic cell differentiation revealed that binding of S100A9 to toll-like receptor 4 (TLR4) promotes activation of p38 mitogen-activated protein kinase, extracellular signal-regulated kinases 1 and 2, and Jun N-terminal kinase signaling pathways, leading to myelomonocytic and monocytic AML cell differentiation. These findings indicate that S100A8 and S100A9 are regulators of myeloid differentiation in leukemia and have therapeutic potential in myelomonocytic and monocytic AMLs.