The Activation of Macrophage Scavenger Receptors through the Phosphorylation Cascade Generated by Nanomolar Cadmium Doses

Abstract

Nanomolar cadmium concentrations have been shown to increase the risk for atherosclerosis and promote endothelial damage. Atherosclerotic plaque formation is regulated by scavenger receptors, which are cell surface receptors crucial to macrophage apoptosis and foam cell formation. LOX‐1, CD‐36, and SR‐A I/II are the main macrophage and endothelial receptors responsible for internalization of oxidized LDL. Incubation with CdCl2 caused a four‐fold increase in LDL absorption by RAW 264.7 (murine) macrophages. To study mechanisms of this increase, macrophages were treated with 2 mg/ml of LDL only, 5nM to 200 nM CdCl2 and then both treatments combined, all for a 48 hour period. Cell lysates were then subjected to a phospho‐kinase array, which simultaneously analyzes the rate of phosphorylation for 43 kinases. When exposed to 10 nM CdCl2, Signal Transducer and Activator of Transcription 5 (STAT5) had a 5 fold increase in phosphorylation compared to control (macrophage cells only), which was the highest in the array. 30 kinases had a significant increase in phosphorylation, while 13 kinases showed a significant decrease in phosphorylation, the greatest decrease being p53 which was knocked down to half. Interestingly, the cadmium induced changes in phosphorylation were attenuated by LDL co‐incubation. In vitro studies were performed to assess levels of LOX‐1, CD‐36 and SR‐A I/II. Macrophage cells were exposed to 200 nM CdCl2, STAT5 inhibitor AC‐4‐130, and 2 mg/ml of LDL for 48 hrs. Lysates were assessed for scavenger receptor expression by ELISA assay. Both the 200 nM CdCl2 and STAT5 inhibitor groups showed significantly increased expression of all 3 scavenger receptors, and the 200 nM CdCl2 STAT5 inhibitor combination resulted in an even higher fold expression. Overall, cadmium plays a role in the progression of atherosclerosis which could in part be via the phosphorylation cascade. STAT5 inhibits the transcription of CD36, hence the increased expression of CD36 and other scavenger receptors noted in the presence of a STAT5 inhibitor. However, cadmium increases the phosphorylation and subsequent function of STAT5 while also increasing the expression of CD36, LOX‐1 and SR‐A I/II. This implies a non‐STAT5 mediated mechanism. It would be interesting to examine the function of the other cadmium‐modulated highly phosphorylated or dephosphorylated kinases in scavenger receptor expression.