LDL immune complexes stimulate LDL receptor expression in U937 histiocytes via extracellular signal-regulated kinase and AP-1.

We have previously shown that LDL-containing immune complexes (LDL-ICs) induce up-regulation of LDL receptor (LDLR) expression in human macrophages. The present study further investigated the molecular mechanisms leading to LDLR up-regulation by LDL-ICs as well as the signaling pathways involved. Results showed that treatment of U937 histiocytes with LDL-ICs did not increase the precursors and the cleaved forms of sterol-regulatory element binding proteins (SREBPs) 1a and 2, suggesting that SREBPs may not be involved in LDLR up-regulation by LDL-ICs. Promoter deletion and mutation studies showed that the AP-1 binding sites were essential for LDL-IC-stimulated LDLR expression. Electrophoretic mobility shift assays further demonstrated that LDL-ICs stimulated transcription factor AP-1 activity. Studies assessing the signaling pathways involved in LDLR up-regulation by LDL-ICs showed that the up-regulation of LDLR was extracellular signal-regulated kinase (ERK) dependent. In conclusion, the present study shows that LDL-ICs up-regulate LDLR expression via the ERK signaling pathway and the AP-1 motif-dependent transcriptional activation.