S100A8 and S100A9: New Insights into Their Roles in Malignancy

Recent studies have highlighted key roles played by nonneoplastic host cells of the tumor microenvironment, and by secreted factors from tumor and host cells, in promoting malignancy. In this regard, damage-associated molecular pattern (DAMP) molecules such as S100A8 and S100A9, with well-known functions in inflammation, have been increasingly recognized not only as markers, but also as new candidates with important roles in modulating tumor growth and metastasis. This review focuses on our current understanding of the proand anti-tumorigenic functions of S100A8 and S100A9. Elucidating molecular pathways mediated by these proteins promises to provide potential novel targets for the development of cancer therapeutics and to establish valid biomarkers to identify early stages of tumor progression. Copyright © 2011 S. Karger AG, Basel Received: April 29, 2011 Accepted after revision: June 16, 2011 Published online: September 6, 2011 Journal of Innate Immunity Dr. Geetha Srikrishna Sanford-Burnham Medical Research Institute 10901, North Torrey Pines Road La Jolla, CA 92037 (USA) Tel. +1 858 795 5256, E-Mail gsrikrishna @ sanfordburnham.org © 2011 S. Karger AG, Basel Accessible online at: www.karger.com/jin Srikrishna J Innate Immun 2012;4:31–40 32 ognized as danger signals by the innate immune system, it has now evolved to include those that are actively secreted from stimulated cells to mediate innate immune responses. This review addresses the contributions of DAMP molecules S100A8 and S100A9 to malignancy, and seeks to complement excellent recent review articles on the subject [6–9] , as more novel functions of these proteins in tumors continue to be unraveled. Structure and Expression of S100A8 and S100A9 S100A8 and S100A9 belong to a family of 25 homologous low-molecular-weight intracellular calcium-binding proteins that exhibit tissue and cell-specific expression [10–12] . They are characterized by two distinct EF-hand (helix-loop-helix) calcium-binding domains connected by a hinge region. The N-terminal Ca 2+ binding domain has lower affinity than the canonical C-terminal domain that allows for functionally important second messenger roles dependent on intracellular Ca 2+ levels. Twenty-one of the human S100 genes are clustered at the chromosomal region 1q21, a region that is frequently deleted, translocated or duplicated in tumors, indicating their possible involvement in malignancy [10, 11] . Although S100A8 and S100A9 exist as homodimers similar to many other S100 proteins, they preferentially form functional anti-parallel heterodimers of S100A8/A9, also known as calprotectin, as well as Ca 2+ -induced tetramers [10, 11, 13] . S100A8 and S100A9 are constitutively expressed by myeloid cells, including granulocytes, monocytes, osteoclasts and early-differentiation cells of the myeloid lineage, but not by lymphocytes [12, 14–16] . They comprise 45% of neutrophil cytosolic proteins. Deletion of S100A9 gene leads to the absence of S100A8 at the protein level [17] . Phagocyte-specific expression of S100A8 and S100A9 accounts for many of their known functions in the innate immune system. Expression in myeloid cells is down-regulated during differentiation to macrophages and dendritic cells [14, 18] . In addition, both proteins can be simultaneously induced in monocytes, endothelial cells, keratinocytes or epithelial cells by several mediators, including LPS, TNF , IL1 , IL-1 , IL-10 or IL-22 [reviewed in 6, 9, 16 ]. Factors secreted by primary tumors, such as VEGF-A, TGF and TNF  promote induction of S100A8 and S100A9 in macrophages and endothelial cells of premetastatic lungs [19] . Expression is enhanced by stress response modulators such as norepinephrine and by glucocorticoids [9, 16] . In addition, IL-10, an anti-inflammatory mediator, induces expression of S100A8 and S100A9 in dendritic cells and potentiates TLR ligand-induced expression of the proteins in macrophages [9, 16] . This dual induction of expression by both proand antiinflammatory mediators is cell-type dependent [16] , and suggests that S100A8 and S100A9 can exert opposing roles in inflammation. However, an anti-inflammatory role for the proteins has not been demonstrated in vivo, and therefore molecular mechanisms mediating possible dual roles necessitate further studies. At the gene level, PU.1 and C/EBP and promote transcription of S100A9 in myeloid cells and C/EBPand may contribute to expression of both proteins in keratinocytes [9] . Expression in keratinocytes and in hepatocellular tumor cells is also promoted by activation of NF B, while AP-1 appears to negatively regulate expression in keratinocytes [9, 20] . In addition, STAT-3 promotes transcriptional activation of S100A9 in myeloid progenitors and breast cancer cells [9, 14] . Intracellular and Extracellular S100A8 and S100A9 Like most DAMP molecules, S100A8/A9 lead a double life [21] . In vitro studies indicate that S100A8/A9 contribute to intracellular homeostatic processes that include calcium sensing, activation of NADPH oxidase and arachidonic acid transport [10, 12, 22] . They also play an important role in tubulin-dependent cytoskeletal rearrangements during transendothelial migration of phagocytes [23, 24] . In addition to intracellular and cell-surface expression, S100A8/A9 have also been detected in the extracellular medium, at times in large amounts. It is the extracellular S100A8/A9 proteins that function as danger signals, promoting immune responses and repair mechanisms during inflammation and, as more recent studies show, in tumors. In fact, S100A8/A9 is a well-known marker of inflammatory conditions such as rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, cystic fibrosis and psoriasis, and elevated levels have been associated with diabetes, systemic lupus erythematosus, atherosclerosis, vasculitis, hyperzincemia and other inflammatory disorders [6, 12, 21] . It is now well established that they not only serve as markers, but also mediate both antiand proinflammatory effects [as reviewed in 16 ]. The proteins, especially S100A8, are susceptible to oxidation by peroxide, hypochlorite and nitric oxide [16] . This susceptibility, combined with their expression in neutrophils and activated macrophages, cells that produce abundant reactive oxygen species during inflammation, indicates that S100A8 and S100A9 may protect tissues against oxi-