Mucin expression in CF via TLRs, EGFR signaling pathways

Mucin expression in CF via TLRs, EGFR signaling pathways

Cystic Fibrosis (CF) is a potentially lethal genetic disease that typically results in the development of bronchial inflammation, bronchiectasis, the progressive loss of lung function and ultimately death [1].

CF was initially called "mucoviscidosis" because of copious amounts of "mucoproteins" in the respiratory and gastrointestinal tracts of CF patients [2].

CF is a recessive genetic disease caused by mutations in the CFTR gene, which encodes the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), a chloride channel. Expression of mutant CFTR in CF respiratory cells results in defective chloride secretion and elevated sodium absorption, resulting in altered salt concentrations in airway secretions. Alterations in mucus volume may impact mucus hydration, and thus the rheology of CF airway mucus to increase susceptibility to infection in CF airways. Lack of functional CFTR in lung cells could engender a hyperinflammatory state that alters homeostasis in CF airways. Inflammatory mediators in the airways of CF can increase expression of mucin genes, contributing to recurring cycles of infection followed by increased expression of mucins that culminates in airway obstruction with mucus [2].

Pseudomonas aeruginosa is the predominant pathogen of CF chronic lung infection [3]. Reduced secretion of chloride and fluid hydration, as well as excessive secretion of mucins, produce a biological matrix that facilitates growth of P. aeruginosa in biofilm [1].

Secretory mucus/gel-forming mucins (Mucin 2, Mucin 5AC, and Mucin 5B) are secreted by airway mucus-secreting cells. The mucins are subject to regulation by CF inflammatory stimuli. Mucin 5AC and Mucin 5B have been identified as major gel-forming macromolecules; whereas Mucin 2 contributes only to a lesser extend to the matrix [2].

In normal human airways, Mucin 5AC is mainly expressed in surface goblet epithelial cells, whereas Mucin 5B is predominantly expressed in mucous cells of submucosal glands and Mucin 2 is weakly expressed in both cell types [2], [4], [5], [6], [7]. However, Mucin 5B gene products in diseased airways (e.g. in CF or asthma) are also found in the surface epithelium, rather than just being limited to the submucosal glands [6], [8], [9], [10]. Expression of Mucin 5B might be a result of goblet cell hyperplasia and mucus hypersecretion associated with various airway diseases [6], [9], [11].

A wide variety of stimuli present in the airways of patients with CF (e.g. Pseudomonas aeruginosa components and proinflammatory cytokines) are known to cause mucin overproduction.

P. aeruginosa components transcriptionally upregulate Mucin 2 gene expression [12], [13]. P. aeruginosa products have also been reported to upregulate Mucin 5AC expression [14]. The ultimate step leading to Mucin 2 or Mucin 5AC gene upregulation is the activation of several transcription factors including Nuclear Factor kappa-B (NF-kB), Activator protein 1 (AP-1) that is mainly composed of c-Jun and c-Fos (c-Jun/c-Fos heterodimer), and Sp1 transcription factor (SP1) [15].

Flagellin (P. aeruginosa) and P. aeruginosa component pilin (PilA (P. aeruginosa)) are recognized by the surface receptors, asialo-GM1 ganglioside (asialo-ganglioside GA1) and Toll-like receptors (TLRs) [3], [16], [17].

Flagellin (P. aeruginosa) is recognized by TLR5 [3]. Flagellin (P. aeruginosa) [13] and PilA (P. aeruginosa) [18] bind bacteria to the host cell glycolipid receptor, asialo-ganglioside GA1. The TLR2-asialo-ganglioside GA1 complex in response to P. aeruginosa induces the activation of NF-kB, initiates the proinflammatory signaling and stimulates transcription of Mucin 2 [13].

TLRs activate the canonical NF-kB pathway: Myeloid differentiation primary response gene 88 (MyD88)/ Interleukin-1 receptor-associated kinases 4, 1 and 2 (IRAK4 and IRAK1/2)/ TNF Receptor-associated factor 6 (TRAF6)/ Mitogen-activated protein kinase kinase kinase 7 interacting proteins 1 and 2 (TAB1 and TAB2)/ Mitogen-activated protein kinase kinase kinase 7 (TAK1)/ Mitogen-activated protein kinase kinase kinase 14 (NIK)/ I-kB kinase complex (IKK(cat))/ Nuclear factor kappa-B inhibitor (I-kB)/ NF-kB [19], [20], [21]. TLR2 (or TLR4) signaling also requires an additional adaptor Toll-Interleukin 1 receptor domain containing adaptor protein (TIRAP) [22], [23].

P. aeruginosa has also been shown to activate another pathway: Tyrosine Kinase (c-Src)/ Harvey Rat Sarcoma Viral Oncogene Homolog (H-Ras)/ Murine Leukemia Viral Oncogene Homolog 1 (c-Raf-1)/ Mitogen-Activated Protein Kinase Kinase 1 and 2 (MEK1/2)/ Mitogen-Activated Protein Kinase 1 and 3 (ERK1/2)/ Ribosomal Protein S6 Kinase Alpha-1 (p90RSK1), which in turn leads to the activation of NF-kB and triggers Mucin 2 transcription [12], [24].

Epithelial responses to CF bacterial ligands mediated by TLRs also result in the NF-kB-induced transcription of Interleukin 6 (IL-6) involved in the expression of mucin genes [3].

Overproduction of mucin in the airways of patients with CF is also known to cause by Epidermal Growth Factor Receptor (EGFR) activation [25]. A prominent EGFR ligand, Transforming Growth Factor, Alpha (TGF-alpha), is markedly increased in the epithelium of patients with CF [26]. EGFR activates ERK1/2 cascade via SHC Transforming Protein 1 (Shc)/ Growth Factor Receptor-Bound Protein 2 (GRB2)/ Son of Sevenless Homolog 1 and 2 (SOS)/ H-Ras/ c-Raf-1/ MEK1/2 pathway [27], leading to the activation of c-Jun/c-Fos and SP1 transcription factors that can trigger Mucin 5AC and Mucin 2 transcription [10], [28], [29].

Unlike in the case of Mucin 2 and Mucin 5AC, little is known about the mechanisms of Mucin 5B expression. In human bronchial epithelial cell cultures, Mucin 5B expression is activated via an EGFR/ERK-independent Protein Kinase delta (PKC-delta), H-Ras, Mitogen-Activated Protein Kinase Kinase Kinase 1 (MEKK1)-mediated, c-Jun N-terminal kinase (JNK)/ Mitogen-Activated Protein Kinase 14 (p38alpha), SP1-dependent signaling pathway [10].

EGFR signaling could increase Mucin 5AC secretion in the airway CF epithelium, whereas Mucin 5B production is more prominent in the lumen of patients with CF [29].

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