HIV Tat-Mediated Induction of Human Brain Microvascular Endothelial Cell Apoptosis Involves Endoplasmic Reticulum Stress and Mitochondrial Dysfunction

Endoplasmic reticulum (ER) stress triggered under hyperglycemic, hypoxic, and oxidative conditions has been implicated in cellular dysfunction through activation of the unfolded protein response (UPR). Recent clinical studies have documented that the release of soluble cellular and host factors following HIV infection in the central nervous system (CNS) results in induction of the ER stress response. Herein, we demonstrate that exposure of human brain microvascular endothelial cells (HBMECs) to HIV transactivator protein Tat101 resulted in early induction of several major ER stress regulators including ER chaperones Bip/GRP78 and ER stress sensors ATF6, p-PERK, and downstream mediators p-eIF2α and ATF4. Upregulation of the ER stress mediators was accompanied by decreased cell viability and increased apoptosis as evidenced by MTT and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assays, respectively. Pretreatment of HBMECs with either ER inhibitor or knockdown of the effector C/EBP homologous protein (CHOP) resulted in increased cell viability and abrogation of apoptosis following Tat exposure. Notably, Tat-mediated activation of the UPR response involved reactive oxygen species. Furthermore, treatment of Tat also resulted in mitochondrial dysfunction, evidenced by decrease in Bcl2/Bax ratio, dysfunction of mitochondrial membrane potential, and release of cytochrome c, all of which could be partially reversed by the ER stress inhibitor. The current study demonstrates that exposure of HBMECs to Tat induces multiple stress responses, including ER stress and mitochondrial dysfunction which in turn lead to apoptosis.