Substrate-bound fibronectin enhances scavenger receptor activity of macrophages by calcium signaling

We have previously found that ability of mouse macrophages to bind and take up oxidized low-density lipoprotein (oxLDL) through scavenger receptors is significantly enhanced when the cells are plated on fibronectin (FN)-coate d culture substrates. Here, the mechanisms of the enhancement of the scaven ger receptor activity by the substrate-bound FN was investigated using thio glycollate-induced mouse peritoneal macrophages. A Ca2+ channel blocker dil tiazem and a calmodulin inhibitor W-7 reduced the scavenger receptor activi ty of the macrophages plated on FN-coated substrate to the level of the cel ls plated on uncoated substrate, as assessed by oxLDL binding, while the sc avenger receptor activity of the macrophages on uncoated substrate was litt le affected. Similarly, FN-induced enhancement of the scavenger receptor ac tivity assessed by oxLDL uptake was selectively inhibited by Ca2+ channel b lockers (diltiazem, nifedipine, verapamil) and calmodulin inhibitors (W-7, trifluoperazine), Intracellular free Ca2+ level of the macrophages was incr eased, depending on extracellular Ca2+, when plated on FN-coated substrate, This increase in the Ca2+ level was inhibited by diltiazem and RGD-contain ing peptides present in cell adhesive region of FN. Like the substrate-boun d FN, Ca2+ ionophore A23187 enhanced the scavenger receptor activity of bin ding and taking up of oxLDL. These results indicate that substrate-bound FN enhances scavenger receptor activity of macrophages by increasing channel- dependent Ca2+ influx. A microtubule disrupter, colchicine, and an actin fi lament disrupter, cytochalasin B, inhibited the FN-induced enhancement of t he scavenger receptor activity, suggesting that these cytoskeletal structur es are required for transmission of the adhesion signal of FN. The number o f the scavenger receptors was found to increase by 1.4-fold upon adhesion s ignal of FN. We suggest that substrate-bound FN increases the number of the macrophage scavenger receptors as a result of induction of Ca2+ influx and causes increased accumulation of oxLDL within the cells, rendering the cel ls more susceptible to conversion into foam cells. (C) 2001 Academic Press.