INTRODUCTION
Alkaline phosphatase (AP) has multiple tissue subtypes such as tissue
non-specific AP (TNAP), intestinal AP (IAP), placenta AP (PLAP) and is
widely distributed in various tissues of the human body, including
cardiac blood vasculature, brain, liver and placenta and intestinal
epithelial brush border (Schultz-Hector, Balz, Böhm, Ikehara, Rieke,
1993; Bell, Scarrow, 1984; Hirschmugl et al., 2018; Bell, Williams,
1979; HAGerstrand, Lindholm, Lindroth, 1976). Currently, the
physiological function of IAP has not been fully elucidated. The
function of alkaline phosphatase was originally thought to be involved
in bone calcification (Millan, Whyte, 2016). In the early 1990s, Hui et
al. used gene transfer technology to induce AP gene expression in
various cell types and found that AP is likely related to pathological
vascular calcification (Hui, Li, Holmyard, Cheng 1997; Hui, Tenenbaum,
1998). But the mystery surrounding the main physiological function of
such a widely distributed membrane-bound enzyme continued to prevail
until an original study by Dutch scientists Poelstra and Meijer first
reported that IAP is capable of inactivation of endotoxin, suggesting
that IAP could inhibit endotoxin-induced inflammation and might have a
potential for treatment lipopolysaccharide (LPS)-related diseases
(Poelstra, Bakker, Klok, Hardonk, Meijer, 1997). By knocking out the
mouse IAP respective gene, Bhan and Sonoko found that animals deficient
in IAP develop type 2 diabetes (Kaliannan, 2013) and hyperlipidemia
(Narisawa, 2013). Clinical studies by Peters and Lukas sponsored by
Dutch biopharmaceutical company AM-Pharma have already validated the use
of recombinant human IAP in providing a novel approach targeting
inflammation in endotoxin-associated sepsis kidney injury and colitis
(Peters, 2016a; Lukas, 2010).
Leukocytes contain 55-70% neutrophils, 20-40% lymphocytes and 3-10%
monocytes. Neutrophil AP activity is increased in pathological sites. A
characteristic response of neutrophils to bacterial infections is the
pronounced increase in AP activity. IAP is thought to be involved in gut
mucosal defense, mainly through the detoxification of LPS. However, the
migration of neutrophils into inflammatory tissues acts like a
double-edged sword. As a primary cell type that removes invasive
microorganisms, neutrophils also contribute to the pathogenesis of human
inflammatory diseases by releasing excess proinflammatory cytokines,
such as TNF-α and IL-6 (Peiseler, Kubes, 2019; Suzuki, 2017; Powell,
Huttenlocher, 2016; Jasper, McIver, Sapey, Walton, 2019;
Kovtun, Messerer, Scharffetter-Kochanek, Huber-Lang, Ignatius 2018;
Williams, Chambers, 2016; Wright, Moots, Bucknall, Edwards, 2010;
Suzuki, 2018; Zhang, 2019; Mortaz, Alipoor, Adcock, Mumby, Koenderman,
2018). TNF-α is a mammalian-secreted protein capable of inducing a wide
variety of effects on many cell types. TNF-α is essential to the
initiation course of inflammatory reactions in the body and is involved
in the triggering and/or amplification of local inflammatory responses
related to apoptotic cell death. Dysregulated continual synthesis of
IL-6 plays a pathological effect on chronic inflammation and
autoimmunity. Currently, it has been unclear if AP has a direct
mechanism of inhibiting leukocytic TNF-α and IL-6 upregulation in
inflammatory human diseases. This study was designed to investigate the
effects of IAP on TNF-α and IL-6 secretion using our cell-based model
comprised of freshly extracted human leukocytes in the presence and
absence of endotoxin LPS.