The role of RAGE in host pathology and crosstalk between RAGE and TLR4 in innate immune signal transduction pathways

Abstract

Although the innate immune receptor protein, Receptor for Advanced Glycation End products (RAGE), has been extensively studied, there has been renewed interest in RAGE for its potential role in sepsis, along with a host of other inflammatory diseases of chronic, noninfectious origin. In contrast to other innate immune receptors, for example, Toll-like receptors (TLRs), that recognize ligands derived from pathogenic organisms that are collectively known as "pathogen-associated molecular patterns" (PAMPs) or host-derived "damage-associated molecular patterns" (DAMPs), RAGE has been shown to recognize a broad collection of DAMPs exclusively. Historically, these DAMPs have been shown to be pro-inflammatory in nature. Early studies indicated that the adaptor molecule, MyD88, might be important for this change. More recent studies have explored further the mechanisms underlying this inflammatory change. Overall, the newer results have shown that there is extensive crosstalk between RAGE and TLRs. The three canonical RAGE ligands, Advanced Glycation End products (AGEs), HMGB1, and S100 proteins, have all been shown to activate both TLRs and RAGE to varying degrees in order to induce inflammation in in vitro models. As with any field that delves deeply into innate signaling, obstacles of reagent purity may be a cause of some of the discrepancies in the literature, and we have found that commercial antibodies that have been widely used exhibit a high degree of nonspecificity. Nonetheless, the weight of published evidence has led us to speculate that RAGE may be physically interacting with TLRs on the cell surface to elicit inflammation via MyD88-dependent signaling.

Keywords: DAMPs; HMGB1; inflammation; toll-like receptors.

Figure 1:. RAGE ligands exhibit crosstalk between RAGE and TLR4 for innate immune signaling.

Hypothetical model of MyD88 activation by RAGE occurs through a TLR4-dependent fashion. We propose that the putative RAGE ligands; AGEs, HMGB1, and S100 proteins can activate macrophages through both TLR4 and RAGE due to a physical interaction at the cell surface to activate MyD88 downstream signaling.

Figure 2:. Some commercial antibodies detect RAGE in lung lysates but not in macrophages, or detect non-specific bands in the approximate MW range of RAGE.

Lung tissue (A) from WT or RAGE^−/− mice were lysed, separated by SDS-PAGE, transferred to PVDF membrane, and probed for RAGE (polyclonal antibody) or GAPDH. (B) Peritoneal macrophages from WT or RAGE^−/− mice were lysed, separated by SDS-PAGE, transferred to PVDF membrane, and probed for RAGE (polyclonal antibody) or GAPDH. Western blots were carried out as described in (B) except membranes were probed with the anti-RAGE monoclonal antibodies A9 (C) or A11(D).