PS1231 ACTIVATION OF AUTOPHAGY IN ENDOTHELIAL CELLS ENHANCES HEMATOPOIETIC STEM CELLS

Abstract

Background:Emerging murine studies showed that endothelial cells (ECs) serve as an instructive platform to nurture the hematopoietic stem cell (HSCs). Autophagy, the self‐degradative process responsible for the elimination of cytosolic components, has recently been demonstrated to have important effects upon EC function. But no previous study had explore the role of autophagy in the process of ECs to support HSCs. Our previous work reported that abnormal bone marrow (BM) ECs were involved in the pathogenesis of poor graft function (PGF), a serious complication with pancytopenia after allogeneic hematopoietic stem cell transplantation (allo‐HSCT), and improving dysfunctional BM ECs could promote hematopoiesis ( BBMT 2013; BMT 2016; Blood 2016), but the explicit mechanism need to be further clarified. Given the important role of autophagy in ECs, we hypothesized that up‐regulation of autophagy in ECs might enhance HSCs.Aims:The current study was performed to explore whether autophagy would affect the ability of ECs to support HSCs. Moreover, to evaluate whether autophagy in BM ECs play a role in the pathogenesis of PGF, and investigate the effects of autophagic regulation on ECs and thereby promoting hematopoiesis.Methods:In vitro regulation of autophagy by rapamycin (the autophagy activators) or hydroxychloroquine (HCQ, the autophagy inhibitor) were administrated to HUVECs. To explore whether autophagy would affect the ability of ECs to support HSCs in vitro, BM CD34+ cells from healthy donors were cocultured with cultivated BM ECs. Colony‐forming unit (CFU) and the apoptosis of co‐cultured HSCs were analyzed. Subsequently, a prospective case‐control study was conducted to evaluate the autophagy levels in BM ECs and CFU plating efficiency in HSCs from PGF patients, and their matched good graft function (GGF) patients. The expression levels of autophagy‐related markers (LC3, Beclin1, and P62), and intracellular autophagosomes were detected by immunohistochemical staining, flow cytometry, western blot and transmission electron microscopy. Furthermore, to explore whether autophagic regulations would affect the number and ability of BM ECs from PGF patients, including the ability to support HSCs in vitro.Results:In vitro upregulation of autophagy by rapamycin quantitatively and functionally improved HUVECs, which could further significantly improve CFU plating efficiency in BM HSCs co‐cultured with HUVECs, whereas inhibition of autophagy by HCQ produce opposite effects. In the prospective case‐control study, defective autophagy was found in BM ECs from PGF patients, characterized by decreased intracellular autophagosomes and autophagic vacuoles, decreased expression of LC3‐II and Beclin1, and high level of P62, when compared to GGF patients. The co‐culture of BM CD34+ cells with BM ECs showed significant deficient CFU plating efficiency, and increased apoptosis of CD34+ cells in PGF patients. Subsequently, in vitro upregulation of autophagy by rapamycin quantitatively and functionally improved BM ECs from PGF patients. Furthermore, rapamycin significant improved CFU plating efficiency, and decreased apoptosis in BM HSCs co‐cultured with BM ECs from PGF patients.Summary/Conclusion:Taken together, our results suggest that activation of autophagy in ECs could enhance their HSCs‐supporting capacity which indicating a new mechanism of HSC regulation. Moreover, defective autophagy may be involved in the pathogenesis of PGF, and rapamycin might provide a novel clinical intervention for PGF patients post‐HSCT in the future.