Abnormal adhesion of particular hematopoietic stem and progenitor cell subpopulations in the bone marrow, driven by aberrant interaction between VCAM-1 and VLA-4, ultimately contributes to leukemogenesis Free

Introduction: The interplay between Vascular Cell Adhesion Molecule-1 (VCAM-1) and Very Late Antigen-4 (VLA-4, α4β1 integrin) is established to have a key role in the adhesion of hematopoietic stem and progenitor cells (HSPC) to the bone marrow cellular network. Under normal conditions, this adhesion ensures the preservation of HSPC stemness and dormancy, thus supporting their long-term hematopoietic potential.Abnormal changes in this mechanism could be involved in leukemia propagation via such pathways as enhanced cell retention, chemo-resistance, and re-arrangement of the cellular network in the bone marrow niche. Various HSPC subpopulations may display different degrees of VCAM-1/VLA-4 interaction that could shape their role in disease progression. Our recent study has disclosed abundant presence of multi-potent lymphoid progenitor (MLP) and granulocyte-monocyte progenitor (GMP) HSPC subpopulations, found to be dominant in AML progression, suggesting their direct involvement in this process. However, the underlying mechanism remains unclear. The present study has aimed to investigate potential impacts of aberrations in the VCAM-1/VLA-4 interaction, at the level of these HSPC subpopulations, on the development of acute myeloid leukemia (AML).

Methods: In this study, a unique ex-vivo “patient-in-a-dish” platform, created at our lab, was utilized to explore activation of adhesion components in mesenchymal stem cells and HSPCs obtained from AML patients and HDs as well as to evaluate differential chemo-resistance of AML HSPC subpopulations. Quantitative proteomics and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed to identify changes in protein pathways in MLP and GMP HSPC subpopulations of bone marrow samples derived from AML patients and healthy donors (HD). Effects of VLA-4 inhibition with natalizumab on these subpopulations were assessed with flow cytometry. Interactions between VCAM-1 and VLA-4, cell migration, invasion and proliferation were evaluated.

Results:The present study has revealed disruption of the VCAM-1/VLA-4 interaction in leukemia, characterized by significantly decreased VLA-4 levels in MLP and GMP subpopulations of HSPCs obtained from AML patients relative to those recorded in HD HSPCs (24.6±11% and 18.62±16% versus 54.55±18.8% and 76.80±16.2%, respectively; p=0.05). Quantitative proteomics and KEGG pathway enrichment analyses demonstrated the following distinct protein patterns in GMP/MLP HSPC subpopulations derived from AML patients. Adhesion pathways were significantly upregulated in these two subpopulations in samples obtained from leukemia patients compared to those derived from HDs. Cell migration assays demonstrated elevated transendothelial migration of these two leukemic HSPC subpopulations (AML MLP: 78.3±15%, GMP: 69.24±11%; HD MLP: 22.55±4%, GMP: 36.55±13%; p<0.05). The colony-forming unit assay and cell cycle analysis showed increased proliferation of these subpopulations (AML MLP: 48.28±23%, GMP: 53±4.5%; HD MLP: 12±24%, GMP: 18±16.3%; p<0.05). Similar to the leukemia setting, the VLA-4 inhibition with natalizumab in HD HSPC subpopulations resulted in adhesion reduction and elevated cell proliferation, pointing to the involvement of the aforementioned interaction in AML development. VCAM-1 expression had been previously reported to be linked to therapy resistance in cancer, indicating that alterations in adhesion mechanisms could affect treatment outcomes. In the present study, chemo-resistance assays, using the patient-in-a-dish platform, demonstrated that impaired VCAM-1 and integrin signaling in MLP and GMP subpopulations protected them from cytotoxic effects.Conclusions: The current study demonstrates the contribution of disrupted VCAM-1/VLA-4 interaction to impaired adhesion of AML MLP and GMP HSPC subpopulations to the bone marrow cellular network. This has led to their exiting the state of dormancy, followed by the loss of stemness, increased proliferation capacity, higher migration rate and enhanced chemo-resistance, ultimately intensifying leukemogenesis. Hence, focusing on the preservation of the VCAM-1/VLA-4 interaction in the bone marrow niche of AML patients could pave the way to a new therapeutic strategy allowing improved leukemia treatment and relapse prevention.