Pathophysiological conditions such as fibrosis, inflammation, and tumor progression are associated with modification of the extracellular matrix (ECM). These modifications create ligands that differentially interact with cells to promote responses that drive pathological processes. Within the tumor stroma, fibroblasts are activated and increase the expression of type I collagen. In addition, activated fibroblasts specifically express fibroblast activation protein-α (FAP), a post-prolyl peptidase. Although FAP reportedly cleaves type I collagen and contributes to tumor progression, the specific pathophysiologic role of FAP is not clear. In this study, the possibility that FAP-mediated cleavage of type I collagen modulates macrophage interaction with collagen was examined using macrophage adhesion assays. Our results demonstrate that FAP selectively cleaves type I collagen resulting in increased macrophage adhesion. Increased macrophage adhesion to FAP-cleaved collagen was not affected by inhibiting integrin-mediated interactions, but was abolished in macrophages lacking the class A scavenger receptor (SR-A/CD204). Further, SR-A expressing macrophages localize with activated fibroblasts in breast tumors of MMTV-PyMT mice. Together, these results demonstrate that FAP-cleaved collagen is a substrate for SR-A-dependent macrophage adhesion, and suggest that by modifying the ECM, FAP plays a novel role in mediating communication between activated fibroblasts and macrophages.

SR-A mediated macrophage adhesion to modified ECM proteins in a process that involves physical attachment of SR-A to modified ECM and activation of Lyn-PI3K and PLA2-12/15-lipoxygenase signaling pathways. Structurally, SR-A-mediated cell adhesion requires a 6-aa membrane-proximal cytoplasmic motif. However, the mechanism that couples SR-A-mediated adhesion to activation of these distinct signaling pathways is not known. For other adhesion receptors, including integrins, localization in cholesterol-rich LRs is an important mechanism for coupling the receptor with the activation of specific signaling pathways. We hypothesized that SR-A-mediated macrophage adhesion might also involve LRs. Our results demonstrate that SR-A is enriched in LRs in HEK cells that heterologously express SR-A and in macrophages that endogenously expressed the receptor. We further show that a truncated SR-A construct (SR-A(Δ1-49)), which mediates cell adhesion but not ligand internalization, is also enriched in LRs, suggesting an association between LRs and SR-A-dependent cell adhesion. To examine this association more directly, we used the cholesterol chelator MβCD to deplete cholesterol and disrupt LR function. We found that cholesterol depletion significantly decreased SR-A-mediated macrophage adhesion. We further show that decreased SR-A-dependent macrophage adhesion following cholesterol depletion results from the inhibition of PLA2 but not PI3K activation. Overall, our results demonstrate an important role for LRs in selectively coupling SR-A with PLA2 activation during macrophage adhesion.

Inflammation is associated with modification of the extracellular environment, changes in cytokine expression, and the accumulation of immune cells. Such modifications create ligands that support SR-A-mediated macrophage adhesion and retention. This may be particularly important in settings, such as atherosclerosis and diabetes, as modified lipoproteins and gluc-collagen are ligands for SR-A. SR-A-mediated adhesion requires the PLA-dependent generation of AA and its metabolism by 12/15 LOX. In contrast, the inhibition of the COX-dependent conversion of AA to PG had no effect on SR-A-mediated adhesion. In this study, macrophages were isolated from SR-A and SR-A mice and plated on gluc-collagen to test the hypothesis that COX-derived PGs are produced during SR-A-mediated adhesion and regulate macrophage function. SR-A-mediated binding to gluc-collagen induced a rapid but transient increase in PG production, which required the activation of PLA and Src kinase but not PI3K. SR-A macrophages cultured on gluc-collagen for 24 h secreted a similar amount of TNF- and 2.5-fold more IL-10 than SR-A macrophages. The inhibition of COX substantially increased TNF- production but reduced IL-10 levels in SR-A macrophages. These effects of COX inhibition were reversed by exogenous PGE and mimicked by specific antagonism of the EP4 receptor. Thus, in addition to the enhancement of macrophage adhesion, SR-A binding to gluc-collagen stimulates PG production, which in turn, differentially regulates the expression of inflammatory cytokines.

Neurofibromatosis is a condition that can occur in a number of forms, the commonest of which are types 1 and 2. As a group, they fall under the phacomatoses family of conditions, otherwise known as neurocutaneous syndromes, owing to the fact that they concurrently have disorders of the nervous system and the integument, which organs share a common ectodermal origin. Other examples include Schwannomatosis and Von Hippel Lindau syndromes. We describe a case of a young girl who presented with features of NF2 and was discovered to have a cerebellar hemangioblastoma at the same time. We are not aware of this association being described in the literature.

SR‐A is a membrane receptor that mediates cell adhesion and ligand internalization. Previous studies suggest that SR‐A‐mediated macrophage adhesion, but not ligand internalization, activates signaling pathways that promote inflammation. We hypothesized that SR‐A function is regulated by localization of SR‐A in distinct plasma membrane domains; specifically, that SR‐A‐ mediated cell adhesion requires localization in cholesterol‐rich lipid rafts. To test this hypothesis, HEK cells were transfected to express either wild‐type (SR‐Aw/t) or a truncated receptor (SR‐AΔ1‐ 49), which lacks all but the membrane‐proximal 6 cytoplasmic amino acids and mediates only adhesion. A detergent‐free cell fractionation protocol was used to show that SR‐A and SR‐AΔ1–49 localized to a similar extent in lipid rafts. To determine if lipid raft localization was required for cell adhesion, cells were treated with methyl‐beta‐cyclodextrin (MβCD), which depletes cell cholesterol and disrupts lipid rafts. MβCD treatment inhibited the adhesion of cells expressing SR‐Aw/t, but did not affect adhesion mediated by SR‐AΔ1–49. Together, these results demonstrate that SR‐A localizes in lipid rafts, that this localization is required for cell adhesion, and that the membrane proximal amino acids are sufficient to localize the receptor in lipid rafts but not sufficient for functional regulation. Support: NIH‐HLRO1‐076682 and AHA Predoctoral fellowship AHA‐11PRE‐7950020.