The impact of natural and synthetic PPARy ligands on endoplasmic reticulum stress and the unfolded protein response

Peroxisome Proliferator-Activated Receptor-y (PPARy) is a ligand-activated nuclear hormone receptor. PPARy is activated by naturally-occurring (eg. oxidized low-density lipoprotein (oxLDL), and 15-deoxy-delta-12,14-prostaglandinJ2 (15dPGJ2)) and pharmacological compounds (eg. the anti-hyperglycemic agent Rosiglitazone).

However, disturbance of ER functions by PPARy ligands causes ER stress, and activation of the unfolded protein response (UPR). In the current study, the impacts of natural and synthetic PPARy ligands on ER stress/UPR were investigated.

Treatment (0.5h) of HL-1 cardiomyocytes or MM6 monocytes with Rosiglitazone (0- 10µM)/15dPGJ2 (0-3µM) disrupted ER homeostasis via inhibition of the ER 'housekeeping Ca2+ pump' SERCA2b. Inhibition of SERCA2b-catalysed Ca2+ pumping activity led within 4h to unchecked Ca2+ leakage from the ER, and activation of UPR transcription factor XBP-1. Within 24-72h, this caused up-regulation of UPR genes, Bip and SERCA2b. However, severe ER stress impairs the ability of BiP/SERCA2b to restore ER homeostasis; hence, UPR can lead to apoptosis via activation of proapoptotic CHOP/caspase pathways. Interestingly, this was seen at lower Rosiglitazone levels in HL-l than in MM6 cells. Thus, Rosiglitazone and 15dPGJ2 can cause apoptosis, particularly in HL-1 cells, via a mechanism involving ER stress/UPR.

oxLDL intenalization by monocyte/macrophages leads to incorporation of cholesterol molecules into the ER membrane, SERCA2b inhibition, unchecked Ca2+  leakage from the ER, ER stress and UPR activation. Different monocyte/macrophage subsets were investigated: with regard to activation of XBP-I; upregulation of UPR target genes (BiP/CHOP); and apoptosis/cell viability, the effects of oxLDL (1-40µg/ml; 24h) exhibited a consistent pattern: non-polarised monocyte/macrophages were less sensitive to oxLDL than M2-polarised macrophages. Such enhanced susceptibility of anti-inflammatory M2-macrophages could, over time, result in a shift similar to that seen in obese individuals in vivo; i.e. relative increases in proportion of non-M2 cells within each macrophage population, and thus contribute to the development of a proinflammatory milieu in the tissues of obese individuals.