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)-coated culture substrates. Here, the mechanisms of the enhancement of the scavenger receptor activity by the substrate-bound FN was investigated using thioglycollate-induced mouse peritoneal macrophages. A Ca2+ channel blocker diltiazem and a calmodulin inhibitor W-7 reduced the scavenger receptor activity of the macrophages plated on FN-coated substrate to the level of the cells plated on uncoated substrate, as assessed by oxLDL binding, while the scavenger receptor activity of the macrophages on uncoated substrate was little affected. Similarly, FN-induced enhancement of the scavenger receptor activity assessed by oxLDL uptake was selectively inhibited by Ca2+ channel blockers (diltiazem, nifedipine, verapamil) and calmodulin inhibitors (W-7, trifluoperazine). Intracellular free Ca2+ level of the macrophages was increased, depending on extracellular Ca2+, when plated on FN-coated substrate. This increase in the Ca2+ level was inhibited by diltiazem and RGD-containing peptides present in cell adhesive region of FN. Like the substrate-bound FN, Ca2+ ionophore A23187 enhanced the scavenger receptor activity of binding 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 disruptor, colchicine, and an actin filament disruptor, cytochalasin B, inhibited the FN-induced enhancement of the scavenger receptor activity, suggesting that these cytoskeletal structures are required for transmission of the adhesion signal of FN. The number of the scavenger receptors was found to increase by 1.4-fold upon adhesion signal 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 cells more susceptible to conversion into foam cells.