Replicative senescence is another condition that can

Replicative senescence is another condition that can be preceded and propagated by a very similar cascade of proinflammatory signals, which is called a senescence-associated proinflammatory phenotype (SASP) (Velarde et al., 2013). Since this SASP can “infect” neighboring guanabenz a condition can be spread that is capable of impairing fundamental processes of regeneration and lineage commitment. Serum amyloid A proteins 1 and 2 (SAA1 and 2) are the major acute-phase proteins systemically secreted by the liver in response to inflammation, trauma or infection but local expression was also detected in various other tissues such as the bone, cartilage and vessels, as well as in inflamed tissues and serum in osteoarthritis and atherosclerosis (Kovacevic et al., 2008; Vallon et al., 2001; de Seny et al., 2013; Meek et al., 1994; Eklund et al., 2012). SAA proteins are encoded by the SAA1 and SAA2 genes, which seem to be coordinately expressed (Thorn et al., 2004). Their transcription is induced by glucocorticoids and cytokines like TNFα, IL1α, IL1β and IL6 (Kovacevic et al., 2008; Vallon et al., 2001; Thorn et al., 2004; Hagihara et al., 2004). In turn autocrine SAA was described to amplify the proinflammatory environment by again stimulating cytokine expression, e.g. IL6, IL8, and CXCL1, and to induce the expression of metalloproteinases at the tissue level (Vallon et al., 2001; de Seny et al., 2013). The signal transduction pathways SAA proteins can address are transmitted via TLR2 and 4 receptors and via the formyl peptide receptor FPRL1 in various cell systems (Cheng et al., 2008; Sandri et al., 2008; Dong et al., 2011; Su et al., 1999). We show here that SAA is locally expressed in hMSC, its expression is enhanced during both in vitro aging and normal osteogenic differentiation and that by binding to TLR4 it further contributes to a self-sustaining pro-inflammatory amplification loop that consists of TLR4 activation via p38 kinase, NFκB activation by p65 phosphorylation and downstream stimulation of pro-inflammatory cytokines. This micro-milieu, when transient, can either support wnt/frz-related osteogenic differentiation (Baron and Kneissel, 2013) or induce cellular aging and senescence because it resembles a SASP phenotype. Hence SAA via TLR4 activation may both be an enhancer of osteogenic differentiation and a trigger of inflammation-associated calcification.
Materials and methods
Results
Discussion Toll like receptors like TLR2 and 4 are important mediators of inflammation, differentiation and aging in bone marrow derived mesenchymal stem cells (hMSCs) (Delarosa et al., 2012). Chronic inflammation has been associated with cellular and organismal aging and the term “inflammaging” has been coined for this phenomenon (Franceschi et al., 2007). Micro-RNAs have recently been discussed to dysregulate TLR activation and the consecutive propagation of a senescence associated secretory phenotype (SASP), which comprises stimulation of proinflammatory cytokine secretion such as IL1β, IL8 and of a distinct pattern of metalloproteinases (Brown et al., 2011; Olivieri et al., 2013). Serum amyloid protein is another stimulator of inflammation that is secreted by the liver in large amounts in response to injury and infection. However it can also be locally produced in diseases such as rheumatoid arthritis, osteoarthritis and atherosclerosis (Kovacevic et al., 2008; Vallon et al., 2001; de Seny et al., 2013; Meek et al., 1994; Eklund et al., 2012). TLR2 and TLR4 have been shown to be functional receptors for SAA (Cheng et al., 2008; Sandri et al., 2008). Here we show that SAA is increasingly produced by hMSC during in vitro aging. In this situation SAA expression is associated with the development of a SASP and the rising expression of e.g. IL1β, IL8 and OPG in aging hMSCs (Figs. 1 and 3 and reported in Ren et al. (2013)). In order to establish a causal relationship between those two phenomena we stimulated early passage hMSC with rhSAA1 and we could induce cytokine production after 24h resembling a SASP phenotype, even in cells that so far have not yet been presenescent. Moreover, rhSAA1 stimulated the endogenous production of SAA1 and 2 thus propagating an autocrine feedback loop, which is capable of sustaining and amplifying a proinflammatory microenvironment. We conclude therefrom that both endogenous and exogenous SAA can stimulate the SASP in natural defense and in disease in hMSC and can propagate cellular aging and replicative senescence. Our results in this model of in vitro aging of hMSC closely resemble the concept established by Rodier and Campisi, which describes the development of an increasingly complex and self-sustaining secretory phenotype that ceases at the very moment of established cellular aging and may have implications for tumor development and other age-associated diseases (Rodier and Campisi, 2011). Endogenous and exogenous SAA may be a novel stimulator and orchestrator of this process, which is a self-sustaining one in several aspects like amplification of the proinflammatory cascade through several players, but also through paracrine spreading of the SASP and of presenescence. One important self-sustaining component is for example inflammasome activation, which exerts IL1α and β production, where the former further orchestrates inflammasome activation. Not only is this process self-sustaining but it is also “infectious” as it has been demonstrated by Acosta et al. in a recent paper, where they coined the term “paracrine senescence” (Acosta et al., 2013). We propose that SAA can be added to the players, which can initiate and amplify autocrine and paracrine senescence.