Enhanced endoplasmic reticulum stress in bone marrow angiogenic progenitor cells in a mouse model of long-term experimental type 2 diabetes

Aims/hypothesis: Bone marrow-derived circulating angiogenic cells (CACs) play an important role in vascular repair. In diabetes, compromised functioning of the CACs contributes to the development of diabetic retinopathy; however, the underlying mechanisms are poorly understood. We examined whether endoplasmic reticulum (ER) stress, which has recently been linked to endothelial injury, is involved in diabetic angiogenic dysfunction. Methods: Flow cytometric analysis was used to quantify bone marrow-derived progenitors (Lin⁻/c-Kit⁺/Sca-1⁺/CD34⁺) and blood-derived CACs (Sca-1⁺/CD34⁺) in 15-month-old Lepr^db (db/db) mice and in their littermate control (db/+) mice used as a model of type 2 diabetes. Markers of ER stress in diabetic (db/db) and non-diabetic (db/+) bone marrow-derived early outgrowth cells (EOCs) and retinal vascular density were measured. Results: The numbers of bone-marrow progenitors and CACs were significantly reduced in db/db mice. Vascular density was markedly decreased in the retinas of db/db mice, and this was accompanied by vascular beading. Microglial activation was enhanced, as was the production of hypoxia inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF). The production of ER stress markers (glucose-regulated protein-78 [GRP-78], phosphorylated inositol-requiring enzyme-1α [p-IRE-1α], phosphorylated eukaryotic translation initiation factor-2α [p-eIF2α], activating transcription factor-4 [ATF4], C/EBP homologous protein [CHOP] and spliced X-box binding protein-1 [XBP1s]) was significantly increased in bone marrow-derived EOCs from db/db mice. In addition, mouse EOCs cultured in high-glucose conditions demonstrated higher levels of ER stress, reduced colony formation, impaired migration and increased apoptosis, all of which were largely prevented by the chemical chaperone 4-phenylbutyrate. Conclusions/interpretation: Taken together, our results indicate that diabetes increases ER stress in bone marrow angiogenic progenitor cells. Thus, targeting ER stress may offer a new approach to improving angiogenic progenitor cell function and promoting vascular repair in diabetes.