Apolipoprotein B48, the Structural Component of Chylomicrons, Is Sufficient to Antagonize Staphylococcus aureus Quorum-Sensing

Abstract

Serum lipoproteins (LP) are increasingly being recognized as dual purpose molecules that contribute to both cholesterol homeostasis and host innate defense. In fact, very low LP levels are associated with increased risk of bacterial infection in critically ill patients. In this respect, we reported that apolipoprotein B100 (apoB100), the 4536 amino acid structural protein of very low density lipoprotein (VLDL) produced by the liver, limits Staphylococcus aureus pathogenesis. S. aureus uses quorum-sensing (QS) via the accessory gene regulator (agr) operon and an autoinducing peptide (AIP) to coordinate expression of over 200 virulence genes. ApoB100 prevents agr activation by binding and sequestering secreted AIP. Importantly, human serum LP are produced not only by the liver, but are also produced by enterocytes, in the form of chylomicrons, during uptake of dietary lipids. In contrast to apoB100 in VLDL, human enterocytes use apoB48, the N-terminal 2152 amino acids (48%) of apoB100, as the structural component of chylomicrons. Interestingly, enteral feeding of critically ill patients has been associated with decreased risk of infectious complications, suggesting chylomicrons could contribute to host innate defense in critically ill patients when serum LP production by the liver is limited during the acute phase response. Therefore, we hypothesized that apoB48 would be sufficient to antagonize S. aureus QS. As expected, isolated apoB48-LP bound immobilized AIP and antagonized agr-signaling. ApoB48- and apoB100-LP inhibited agr activation with IC50s of 3.5 and 2.3 nM, respectively, demonstrating a conserved AIP binding site. Importantly, apoB48-LP antagonized QS, limited morbidity and promoted bacterial clearance in a mouse model of S. aureus infection. This work demonstrates that both naturally occurring forms of apolipoprotein B can antagonize S. aureus QS, and may suggest a previously unrecognized role for chylomicrons and enterocytes in host innate defense against S. aureus QS-mediated pathogenesis.

Impact of Environmental Cues on Staphylococcal Quorum Sensing and Biofilm Development

“…The general applicability of AIP sequestration was demonstrated by SPR experiments that showed direct binding of oxLDL to immobilized AIP-II or immobilized AIP-IV. Consistent with the generality of this mechanism, it was recently reported that apolipoprotein B100 in VLDL and apoB48 in chylomicrons also inhibit quorum sensing by AIP sequestration (37). Most recently, it was found that mice with hypolipidemia that reduced ApoB levels in their lungs suffered from increased morbidity and inflammation in an S. aureus pneumonia model (38), highlighting the importance of sequestration of AIP by serum lipoproteins as a mechanism of innate immunity.…”

“…The details of this mechanism have been unveiled through a series of studies (30,31,37). Initially, it was observed that pooled human serum, but not lipoproteindeficient serum, could inhibit quorum sensing by an agr type I reporter strain during in vitro growth (31).…”

Serum Lipoproteins Are Critical for Pulmonary Innate Defense against Staphylococcus aureus Quorum Sensing

“…Since lipoproteins are detectable in lungs shortly after intranasal infection with S. aureus (32) and apoB on LDL binds S. aureus AIP and inhibits agr -activation (23-25), we predicted that decreased circulating lipoprotein levels would result in impaired control of S. aureus QS in the lung. To address this, we compared S. aureus agr -activation in the lungs of mice with reduced apoB levels versus controls.…”

“…This in turn leads to expression of over 200 virulence factors (22), many of which are pre- and post-transcriptionally regulated by a small RNA molecule, called RNAIII, produced by transcription from the agr P3 promoter (20, 21). Importantly, apolipoprotein B (apoB), the sole protein component of LDL lipoprotein particles, and not other serum apoproteins or associated lipids, binds and sequesters AIP, thereby inhibiting agr -signaling and limiting pathogenesis during S. aureus skin infection (23-25). However, the impact of apoB deficiency and hypolipidemia on host innate defense in the lung, and against S. aureus pneumonia in particular, has not been investigated.…”

Ratio of apoB/LDL: a potential clinical index for vascular cognitive impairment

“…ApoB might also contribute to innate defense; studies reported that apoB100 and apoB48 could prevent Staphylococcus aureus (S. aureus) infection by binding and sequestering S. aureus and antagonizing quorum sensing, thereby reducing virulence [35, 36]. …”