P58(IPK) inhibition of endoplasmic reticulum stress in human retinal capillary endothelial cells in vitro

PURPOSE: The goal of this research was to determine if P58(IPK), a member of the Hsp40 family that inhibits eukaryotic initiation factor 2α (eIF2α), inhibits endoplasmic reticulum (ER) stress and leads to downregulated expression of vascular endothelial growth factor (VEGF) and decreased apoptosis in human retinal capillary endothelial cells (HRCECs). METHODS: Recombinant vectors were constructed using P58 in adeno-associated virus type 2 (rAAV2-P58 (IPK)) and P58 RNA in the plasmid pGIPZ (pGIPZ-P58(IPK)). The four experimental groups were: (1) non-transfected/non ER stressed control; (2) non-transfected/ER stressed; (3) rAAV2-P58(IPK)-transfected/ER stressed; and (4) pGIPZ- P58(IPK) RNAi transfected/ER stressed. ER stress was induced by treating cells with tunicamycin. Expression of P58(IPK) was determined in transfected cells. Expressions of the following factors were assessed: vascular endothelial growth factor (VEGF), C/EBP homologous protein (CHOP), activating transcription factor 4 (ATF4), and glucose-regulated protein 78 (GRP78). Apoptosis levels were also determined. RESULTS: Significantly increased expression of P58(IPK) was detected in cells transfected with rAAV2-P58(IPK) (0.63±0.02) as compared to those transfected with pGIPZ-P58(IPK) RNAi (0.23±0.01). P58(IPK) expression was not different between the control transfected cells (rAAV2-GFP and pGIPZ-GFP). Following ER stress, expression levels of ATF-4, GRP78, CHOP, and VEGF in cells overexpressing P58(IPK) were not different from those in unstressed control cells. This inhibitory effect of P58(IPK) on the expression of ER stress-related factors was suppressed in cells transfected with pGIPZ-P58(IPK) RNAi. Apoptosis was significantly increased in cells transfected with pGIPZ-P58(IPK) RNAi but not with rAAV2-P58(IPK). CONCLUSIONS: The study demonstrates that P58(IPK) inhibits ER stress and plays an important role in maintaining balance and stability of the ER in HRCECs.