Yi Xiang

The active contribution of Toll-like receptors to allergic airway inflammation

Epithelia lining the respiratory tract represent a major portal of entry for microorganisms and allergens and are equipped with innate and adaptive immune signaling receptors for host protection. These include Toll-like receptors (TLRs) that recognize microbial components and evoke diverse responses in cells of the respiratory system. TLR stimulation by microorganism-derived molecules activates antigen presenting cells, control T helper (Th) 1, Th2, and Th17 immune cell differentiation, cytokine production by mast cells, and activation of eosinophils. It is clear that TLR are involved in the pathophysiology of allergic airway diseases such as asthma. Dendritic cells (DCs), a kind of antigen presenting cells, which play a key role in the induction of allergic airway inflammation, are privileged targets for pathogen associated molecular patterns (PAMPs). During the allergic responses, engagement of TLRs on DCs determines the Th2 polarization of the T cells. TLR signaling in mast cells increases the release of IL-5, and TLR activation of airway epithelial cells forces the generation of proallergic Th2 type of cytokines. Although these responses aim to protect the host, they may also result in inflammatory tissue damage in the airway. Under certain conditions, stimulation of TLRs, in particular, TLR9, may reduce Th2-dependent allergic inflammation by induction of Th1 responses. Therefore, understanding the complex regulatory roles of TLRs in the pathogenesis of allergic airway inflammation should facilitate the development of preventive and therapeutic measures for asthmatic patients.

New development in studies of formyl-peptide receptors: critical roles in host defense

Formyl‐peptide receptors are a family of 7 transmembrane domain, Gi‐protein‐coupled receptors that possess multiple functions in many pathophysiologic processes because of their expression in a variety of cell types and their capacity to interact with a variety of structurally diverse, chemotactic ligands. Accumulating evidence demonstrates that formyl‐peptide receptors are critical mediators of myeloid cell trafficking in the sequential chemotaxis signal relays in microbial infection, inflammation, and immune responses. Formyl‐peptide receptors are also involved in the development and progression of cancer. In addition, one of the formyl‐peptide receptor family members, Fpr2, is expressed by normal mouse‐colon epithelial cells, mediates cell responses to microbial chemotactic agonists, participates in mucosal development and repair, and protects against inflammation‐associated tumorigenesis. These novel discoveries greatly expanded the current understanding of the role of formyl‐peptide receptors in host defense and as potential molecular targets for the development of therapeutics.

The Formylpeptide Receptor 2 (Fpr2) and Its Endogenous Ligand Cathelin-related Antimicrobial Peptide (CRAMP) Promote Dendritic Cell Maturation

Background: Chemoattractant receptor Fpr2 interacts with host-derived agonist CRAMP and promotes dentritic cell maturation in immune responses. Results: Deficiency in Fpr2 or CRAMP results in impaired maturation of dendritic cells in vitro and in vivo. Conclusion: Fpr2 and its agonist CRAMP play a nonredundant role in DC maturation. Significance: Fpr2 and its agonist CRAMP are potential targets for disease intervention. Mouse formylpeptide receptor 2 (Fpr2) is a homologue of the human G-protein coupled chemoattractant receptor FPR2, which interacts with pathogen and host-derived chemotactic agonists. Our previous studies revealed reduced allergic airway inflammation and immune responses in Fpr2-deficient (Fpr2−/−) mice in association with diminished dendritic cell (DC) recruitment into the airway and draining lymph nodes. These defects prompted us to investigate the potential changes in the differentiation and maturation of DCs caused by Fpr2 deficiency. Bone marrow monocytes from Fpr2−/− mouse mice incubated with GM-CSF and IL-4 in vitro showed normal expression of markers of immature DCs. However, upon stimulation with the TLR4 agonist LPS, Fpr2−/− mouse DCs failed to express normal levels of maturation markers with reduced production of IL-12 and diminished chemotaxis in response to the DC homing chemokine CCL21. Fpr2−/− DCs also failed to induce allogeneic T-cell proliferation in vitro, and their recruitment into the T-cell zones of the spleen was reduced after antigen immunization. The capacity of Fpr2 to sustain normal DC maturation was dependent on its interaction with an endogenous ligand CRAMP expressed by DCs, because neutralization of either Fpr2 or CRAMP inhibited DC maturation in response to LPS. We additionally observed that the presence of exogenous CRAMP in culture increased the sensitivity of WT mouse DCs to LPS stimulation. The importance of CRAMP for DC maturation was further demonstrated by the observations that DCs from CRAMP−/− mice expressed lower levels of costimulatory molecules and MHC II and exhibited poor chemotaxis in response to CCL21 after LPS stimulation. Our observations indicate a nonredundant role for Fpr2 and its agonist CRAMP in DC maturation in immune responses.

The role of chemoattractant receptors in shaping the tumor microenvironment.

Chemoattractant receptors are a family of seven transmembrane G protein coupled receptors (GPCRs) initially found to mediate the chemotaxis and activation of immune cells. During the past decades, the functions of these GPCRs have been discovered to not only regulate leukocyte trafficking and promote immune responses, but also play important roles in homeostasis, development, angiogenesis, and tumor progression. Accumulating evidence indicates that chemoattractant GPCRs and their ligands promote the progression of malignant tumors based on their capacity to orchestrate the infiltration of the tumor microenvironment by immune cells, endothelial cells, fibroblasts, and mesenchymal cells. This facilitates the interaction of tumor cells with host cells, tumor cells with tumor cells, and host cells with host cells to provide a basis for the expansion of established tumors and development of distant metastasis. In addition, many malignant tumors of the nonhematopoietic origin express multiple chemoattractant GPCRs that increase the invasiveness and metastasis of tumor cells. Therefore, GPCRs and their ligands constitute targets for the development of novel antitumor therapeutics.

The Antimicrobial Peptide CRAMP Is Essential for Colon Homeostasis by Maintaining Microbiota Balance

Commensal bacteria are critical for physiological functions in the gut, and dysbiosis in the gut may cause diseases. In this article, we report that mice deficient in cathelin-related antimicrobial peptide (CRAMP) were defective in the development of colon mucosa and highly sensitive to dextran sulfate sodium (DSS)-elicited colitis, as well as azoxymethane-mediated carcinogenesis. Pretreatment of CRAMP−/− mice with antibiotics markedly reduced the severity of DSS-induced colitis, suggesting CRAMP as a limiting factor on dysbiosis in the colon. This was supported by observations that wild-type (WT) mice cohoused with CRAMP−/− mice became highly sensitive to DSS-induced colitis, and the composition of fecal microbiota was skewed by CRAMP deficiency. In particular, several bacterial species that are typically found in oral microbiota, such as Mogibacterium neglectum, Desulfovibrio piger, and Desulfomicrobium orale, were increased in feces of CRAMP−/− mice and were transferred to WT mice during cohousing. When littermates of CRAMP+/− parents were examined, the composition of the fecal microbiota of WT pups and heterozygous parents was similar. In contrast, although the difference in fecal microbiota between CRAMP−/− and WT pups was small early on after weaning and single mouse housing, there was an increasing divergence with prolonged single housing. These results indicate that CRAMP is critical in maintaining colon microbiota balance and supports mucosal homeostasis, anti-inflammatory responses, and protection from carcinogenesis.

Deficiency in Fpr2 results in reduced number of Lin-c-Kit+Sca1+ myeloid progenitor cells

The Lin−c-Kit+ Sca-1+ cell population in the bone marrow (BM) serves as the direct precursor for differentiation of myeloid cells. In this study, we report that deficiency in Fpr2, a G protein–coupled chemoattractant receptor in mice, is associated with reduced BM nucleated cells, including CD31+Ly6C+ (granulocytes and monocytes), CD31−/Ly6Cint (granuloid cells), and CD31−/Ly6Chigh (predominantly monocytes) cells. In particular, the number of Lin−c-Kit+Sca-1+ (LKS) cells was reduced in Fpr2−/− mouse BM. This was supported by observations of the reduced incorporation of intraperitoneally injected bromodeoxyuridine by cells in the c-Kit+ population from Fpr2−/− mouse BM. Purified c-Kit+ cells from Fpr2−/− mice showed reduced expansion when cultured in vitro with stem cell factor (SCF). SCF/c-Kit-mediated phosphorylation of P38, STAT1, Akt (Thr-308), and Akt (Ser-473) was also significantly reduced in c-Kit+ cells from Fpr2−/− mice. Furthermore, Fpr2 agonists enhanced SCF-induced proliferation of c-Kit+ cells. Colony-forming unit assays revealed that CFU–granulocyte–macrophage formation of BM cells from Fpr2−/− mice was significantly reduced. After heat-inactivated bacterial stimulation in the airway, the expansion of c-kit+ Sca-1+ cells in BM and recruitment of Ly6G+ cells to the lungs and CD11b+Ly6C+TNFα+ cells to the spleen of Fpr2−/− mice was significantly reduced. These results demonstrate an important role for Fpr2 in the development of myeloid lineage precursors in mouse BM.

Metformin sensitizes lung cancer cells to treatment by the tyrosine kinase inhibitor erlotinib

Lung cancer is one of the deadliest malignant tumors with limited treatment options. Although targeted therapy, using tyrosine-kinase inhibitors such as erlotinib (Erlo), has shown therapeutic benefit, only 15 % patients with mutated epidermal growth factor receptor (EGFR) in lung cancer cells are sensitive. Therefore, additional therapeutic strategy should be developed. In this study, we found that metformin (Met), which is widely used for the treatment of type 2 diabetes (T2D), sensitized lung cancer cells bearing wild-type EGFR to Erlo treatment by enriching cancer cells expressing higher levels of EGFR with persistent phosphorylation. As a consequence, combination of Met and Erlo more efficiently inhibited the growth of lung cancer cells both in vitro and in mice with xenografted tumors. Our results suggest a novel approach to treating lung cancer cases which are originally resistant to Erlo.