Interaction of Surfactant Protein A with Lipopolysaccharide and Regulation of Inflammatory Cytokines in the THP-1 Monocytic Cell Line

Abstract

Pulmonary surfactant protein A (SP-A) is involved in innate immunity in the lung. In this study we investigated the interaction of SP-A with different serotypes of lipopolysaccharide (LPS) on the regulation of inflammatory cytokines in vitro. In the human monocytic cell line, THP-1, combining SP-A with lipid A or rough LPS further enhanced lipid A- or rough LPS-stimulated tumor necrosis factor alpha (TNF-α) mRNA levels, while SP-A-elicited increases in TNF-α mRNA levels were partially neutralized. In contrast, the combination of smooth LPS and SP-A resulted in additive effects on TNF-α mRNA levels. We also demonstrated that there was cross-tolerance between SP-A and LPS in THP-1 cells. Pretreatment of THP-1 cells with LPS modestly inhibited the response of these cells to subsequent challenge with SP-A, with regard to the production of TNF-α, whereas there was no or little effect on the production of interleukin-1β (IL-1β) and IL-8. Conversely, pretreatment of THP-1 cells with SP-A markedly increased the response to subsequent challenge with LPS with regard to the production of IL-1β and IL-8, although the production of TNF-α was modestly decreased. However, a synergistic stimulatory effect was observed when the two agents were added simultaneously to the cells. NF-κB formation was downregulated in SP-A- but not in LPS-induced tolerant cells. These results suggested that SP-A exhibits different interactions with distinct serotypes of LPS. In addition, SP-A is different from LPS with regard to the induction of cross-tolerance, and these actions may be mediated, at least in part, through different mechanisms.

Altered surfactant protein A gene expression and protein metabolism associated with repeat exposure to inhaled endotoxin

“…Similar to pretreatment with LPS, SP-A can induce a state of tolerance to subsequent endotoxin exposures and result in diminished production of TNF-␣ (32). Conversely, in a monocytic cell culture line, SP-A itself was able to induce TNF-␣ expression (32). Our results support the hypothesis that tolerance to repeated doses of endotoxin can develop and that SP-A may play an important role in this process.…”

“…Additionally, SP-A has been reported to induce a state of tolerance to endotoxin. Similar to pretreatment with LPS, SP-A can induce a state of tolerance to subsequent endotoxin exposures and result in diminished production of TNF-␣ (32). Conversely, in a monocytic cell culture line, SP-A itself was able to induce TNF-␣ expression (32).…”

Modulatory effects of ozone on THP-1 cells in response to SP-A stimulation

“…Previous work has shown that SP-A enhances TNF-␣ production via the activation of NF-B signaling cascade (41,73). The present findings indicate that O 3 decreases the ability of SP-A to activate the NF-B cell signaling pathway by reducing the levels of the free NF-B subunit p65 available for translocation to the nucleus and initiation of transcription.…”

“…Although the identity of the macrophage cell surface receptor that mediates SP-A initiation of NF-B activation has not been determined yet, previous work from our lab and others indicates interaction of SP-A with Tlr-2 (54,79) or Tlr-4 (25). Moreover, our published findings have shown that the downstream effect of this activation may involve translocation of cytoplasmic NF-B into the nucleus, NF-B binding to the specific DNA elements, and subsequent upregulation of TNF-␣ expression (41,73,79).…”

“…The proinflammatory effect of SP-A on alveolar macrophages is in part mediated through the activation of the NF-B cell signaling pathway (41,73). This pathway has been previously described to be activated in macrophages via LPS activation of Toll-like receptor (Tlr)-4, which transduces the signal through molecules such as myeloid differentiation protein (MyD88), IL-1 receptor-associated kinase, and I B kinase (IKK) (14,29,85).…”