CD36 regulates lipopolysaccharide-induced signaling pathways and mediates the internalization of Escherichia coli in cooperation with TLR4 in goat mammary gland epithelial cells.

Duoyao Cao,Jun Luo,Huifen Xu,Wenjuan Zang,Huaiping Shi,Xiaoqi Jing,Dekun Chen

doi:10.1038/srep23132

Abstract

The scavenger receptor CD36 is involved in pathogen recognition, phagocytosis, and pathogen-induced signaling. This study investigated the relationship between CD36 and TLR4 in modifying lipopolysaccharide (LPS)-induced signaling pathways and mediating Escherichia coli (E. coli) endocytosis in primary goat mammary epithelial cells (pGMECs). The manipulation of CD36 expression significantly influenced TLR4 and nuclear factor kappa B (NF-κB) mRNA expression in pGMECs stimulated with LPS for 12 h. NF-κB and activator protein-1 (AP-1) activity was regulated by the manipulation of CD36 expression in LPS-induced pGMECs. However, CD36-mediated AP-1 activation occurred primarily through c-Jun N-terminal kinase (c-JNK). Adaptor proteins and proinflammatory cytokines were also involved in these signaling pathways and acted by regulating CD36 expression in LPS-stimulated cells. Moreover, CD36 cooperated with TLR4 in TLR4-mediated phagocytosis following E. coli simulation, but this complex was not induced by LPS treatment. Our study is the first to illuminate CD36 as a scavenger receptor in ruminants. Additionally, this study indicates that CD36 plays a vital role in the LPS-induced activation of downstream signaling cascades and mediates E. coli phagocytosis via TLR4 in pGMECs, which offers a novel treatment strategy for mastitis.

Highlights

Escherichia coli (E. coli) is one of the most severe pathogens in epidemiology and is the most common bacteria to cause mastitis in dairy herds
Downstream signal changes revealed that NF-kB-p65, c-Jun N-terminal kinase (c-JNK), p38-mitogen-activated protein kinase (MAPK), and TRAF6 were activated in E. coli-induced mastitis tissue samples compared with normal tissue samples (Fig. 1E)
The results indicated that E. coli-induced mastitis could trigger toll-like receptor 4 (TLR4) and CD36 mRNA expression and activate the downstream signaling pathways in Xinong Saanen dairy goats

Summary

Introduction

Escherichia coli (E. coli) is one of the most severe pathogens in epidemiology and is the most common bacteria to cause mastitis in dairy herds. After the onset of an E. coli infection, TLR4 assembles the TLR4/MD-2/LPS complex on the surface of the cell and initiates a downstream signaling pathway to induce the production of proinflammatory mediators to eliminate the bacteria[5]. Some studies have shown that cells lacking phagocytic abilities will acquire or increase phagocytic functions following transfection with CD3616,17 These reports revealed that CD36 is capable of activating cells alone or in conjunction with other receptors to recognize danger signals, thereby eliminating potential “self ” or “nonself ” threats. CD36, acting as a scavenger receptor, can participate in the bacteria-induced inflammation process and activate downstream signaling pathways. The primary goal of this study was to test the hypothesis that CD36 may be involved in LPS-induced inflammation via the NF-κ B and JNK signaling pathways and accompany TLR4 during TLR4-mediated E. coli endocytosis in pGMECs

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Conclusion