Endoplasmic Reticulum Ca2+ Handling and Apoptotic Resistance in Tumor-Derived Endothelial Colony Forming Cells

Truly endothelial progenitor cells (EPCs) can be mobilized from bone marrow to support the vascular network of growing tumors, thereby sustaining the metastatic switch. Endothelial colony forming cells (ECFCs) are the only EPC subtype belonging to the endothelial phenotype and capable of incorporating within neovessels. The intracellular Ca²⁺ machinery plays a key role in ECFC activation and is remodeled in renal cellular carcinoma-derived ECFCs (RCC-ECFCs). Particularly, RCC-ECFCs seems to undergo a drop in endoplasmic reticulum (ER) Ca²⁺ concentration ([Ca²⁺]_ER). This feature is remarkable when considering that inositol-1,4,5-trisphosphate (InsP₃)-dependent ER-to-mitochondria Ca²⁺ transfer regulates the intrinsic apoptosis pathway. Herein, we sought to assess whether: (1) the [Ca²⁺]_ER and the InsP₃-induced ER-mitochondria Ca²⁺ shuttle are reduced in RCC-ECFCs; and (2) the dysregulation of ER Ca²⁺ handling leads to apoptosis resistance in tumor-derived cells. RCC-ECFCs displayed a reduction both in [Ca²⁺]_ER and in the InsP₃-dependent mitochondrial Ca²⁺ uptake, while they expressed normal levels of Bcl-2 and Bak. The decrease in [Ca²⁺]_ER was associated to a remarkable ER expansion in RCC-ECFCs, which is a hallmark of ER stress, and did not depend on the remodeling of the Ca²⁺-transporting and the ER Ca²⁺-storing systems. As expected, RCC-ECFCs were less sensitive to rapamycin- and thapsigargin-induced apoptosis; however, buffering intracellular Ca²⁺ levels with BAPTA dampened apoptosis in both cell types. Finally, store-operated Ca²⁺ entry was seemingly uncoupled from the apoptotic machinery in RCC-ECFCs. Thus, the chronic underfilling of the ER Ca²⁺ pool could confer a survival advantage to RCC-ECFCs and underpin RCC resistance to pharmacological treatment. J. Cell. Biochem. 117: 2260–2271, 2016.