Expression of MPL in Leukemia Stem Cells and Its Role in Stemness Maintainance

Osteoblast cells play an important role in bone marrow niche. The interaction between osteoblast and leukemia cells promotes leukemia development, which is mediated by some cytokines including TPO. It has become evident that TPO-MPL signaling is essential for the quiescence and self-renewal of hematopoietic stem cells, however, its expression pattern and the role in leukemia stem cells have not been reported. This study was aimed to determine the expression of MPL in acute myeloid leukemia (AML) and investigate the role of MPL in the leukemia stem cells' quiescence, drug resistance and self renewal.

The expression levels of CD34, CD38 and MPL were detected by flow cytometry in bone marrow cells from 57 newly diagnoses AML patients. The correlation between MPL and CD34, CD38 expression in AML patients were analyzed. The results showed that expression of MPL in AML patients was higher significantly than that in 13 normal donors (P<0.05). Expression of MPL in CD34 positive AML patients was obviously higher than that in CD34 negative AML patients (P<0.01). MPL was higher expressed in CD34⁺ cells than that in CD34^- cells significantly (P<0.0001). We also detected the expression of MPL in different populations of leukemia cells in AML1-ETO9a mouse leukemia model established in our lab. We found that the ratios of MPL positive cells in Lin^-c-kit⁺ and Lin^-c-kit⁺sca-1⁺ populations were significantly higher than that in total leukemia cells. In addition, in chemotherapy treated AML1-ETO9a mice, the proportion of Lin^-c-kit⁺MPL⁺ leukemia cells were increased 23.5 folds than that in untreated leukemia mice, which indicates that MPL⁺Lin^-c-kit⁺ LSCs population could be enriched by chemotherapy. Furthermore, MPL⁺ and MPL^- cells in Lin^-c-kit⁺ leukemia population were sorted by flow cytometry and the colony formation and quiescence state were determined. The results showed that MPL⁺Lin-c-kit⁺ cells produced significantly more colonies in the second round of colony formation (p<0.05) than MPL^-Lin^-c-kit⁺ cells. The G0 phase accumulation of MPL⁺Lin^-c-kit⁺ cells was significantly higher than that of MPL^-Lin^-c-kit⁺ cells (p<0.01). Above results indicate that MPL⁺ leukemia cells display more clonogenic potential and maintain quiescence.

These data demonstrate that as a receptor of TPO, MPL is highly expressed in leukemia stem cells and MPL positive leukemia stem cells could be enriched by chemotherapy. MPL positive leukemia stem cells exhibit more clonogenic potential, quiescence and drug resistance. It suggests TPO-MPL mediated interaction of osteoblast and leukemia cells take a role in the stemness of leukemia stem cells.