Hypoxia increases multiple myeloma stem cells by modulating notch signalling

BACKGROUND Multiple myeloma (MM) is a haematological tumor characterized by malignant plasma cells accumulation in the bone marrow (BM). Notch pathway receptors and ligands are upregulated in MM. Hypoxia is a low oxygen condition resulting in HIF-1a-mediated hypoxic response resulting in tumor neoangiogenisis and progression. Recent reports indicate that HIF-1a may positively regulate Notch signalling and enhance the expression of downstream genes. In MM, hypoxia can sustain MM stem cell (MM-SC) population. METHODS OPM2 and H929 MM cells underwent a hypoxia mimic treatment with 100μM Cobalt Chloride (CoCl2). HIF-1a and Notch pathway members were analyzed by Western Blot or qRT-PCR. A Notch reporter assay was performed in OPM2 upon CoCl2 treatment. MM-SCs analysis was performed by flow cytometry for CD138 and methylcellulose clonogenic assay. Notch inhibition was obtained by 25mM DAPT. RESULTS CoCl2-mediated hypoxia sustained MM-SC niche by increasing CD138neg population. Moreover, it increased HIF-1a and the Notch pathway members, Jag1 and ICN2, at protein and gene level, resulting in an increased Notch-activated transcription. DAPT-mediated Notch inhibition was able to revert hypoxia-mediated amplification of MM-SCs indicating that Notch pathway acts downstream hypoxia in MM-SC self-renewal. DISCUSSION Our results concerning hypoxia ability to amplification MM-SC population and to induce Notch pathway activation in MM cells by upregulating Notch receptors and ligands is consistent with results in other types of cancer. For the first time we demonstrate that Notch pathway is a hypoxia downstream effector in supporting the amplification of the MM-SC population . CONCLUSION Our results indicate that hypoxia can activate Notch pathway in MM and sustain MM-SC niche.