S100 proteins are a group of small (molecular weight 10-12 kDa), dimeric Ca2+-binding proteins that share a similar sequence and structure. They are found in a wide variety of tissues with cell- and tissue-specific expression patterns. Due to their broad range of extracellular and intracellular interaction partners, S100 proteins are involved in a plethora of extra- and intracellular functions, including regulation of inflammation, cell differentiation and proliferation, migration, tissue repair, apoptosis, and protein phosphorylation.
Extracellular S100 proteins are secreted actively or passively in response to cell stress and inflammation e.g., by inflammatory cells and other granulocytes, macrophages, epithelial cells, or cancer cells. Upon release, they act at damage associated patterns (DAMPs) and interact with a range of pattern recognition receptors such as toll-like receptor 4 (TLR4) and receptor for advanced glycation endproducts (RAGE), thus leading to the activation of proinflammatory signaling pathways such as the NF-kB and p38 pathways. S100 proteins also trigger immune cell migration, invasion, and differentiation.
Because of their association with various pathologies and the ease to identify them in body fluids, several S100 proteins can serve as biomarkers for certain diseases. These include e.g. Alzheimer’s disease (S100A7), inflammatory arthritis (S100A8/A9 heterodimers), or diabetes (S100A12). Many S100 proteins play a role in cancer prognosis or and some of them are suggested as biomarkers to certain types of cancer. Systemic upregulation of S100A8/A9 has also been shown to be a feature of COVID-19.