Transcriptional regulation of cytokine function in airway smooth muscle cells

Abstract

The immuno-modulatory properties of airway smooth muscle have become of increasing importance in our understanding of the mechanisms underlying chronic inflammation and structural remodeling of the airway wall in asthma and chronic obstructive pulmonary disease (COPD). ASM cells respond to many cytokines, growth factors and lipid mediators to produce a wide array of immuno-modulatory molecules which may in turn orchestrate and perpetuate the disease process in asthma and COPD. Despite numerous studies of the cellular effects of cytokines on cultured ASM, few have identified intracellular signaling pathways by which cytokines modulate or induce these cellular responses. In this review we provide an overview of the transcriptional mechanisms as well as intracellular signaling pathways regulating cytokine functions in ASM cells. The recent discovery of toll-like receptors in ASM cells represents a significant development in our understanding of the immuno-modulatory capabilities of ASM cells. Thus, we also review emerging evidence of the inflammatory response to toll-like receptor activation in ASM cells.

Figure 1. Schematic overview of MAPK pathways regulating airway smooth muscle functions

A variety of external stimuli activate immune cells or airway epithelial cells to release a variety of biological mediators. These mediators transduce their effects through ERK, p38 or JNK signaling cascades leading to expression of genes that modulate airway smooth muscle contractile, proliferative and secretory responses.

Figure 2. NF-κB signal transduction pathways

In resting cells, the majority of NF-κB is bound to I-κB inhibitory protein, often IκBκ, which masks the nuclear localisation sequence (NLS) and holds the complex in the cytoplasm. In the ‘conical’ or ‘classical’ NF-κB activation pathway, ligand binding to a cell surface receptor (e.g. tumor necrosis factor-receptor (TNFR) or Toll-like receptor) recruits adaptors (e.g., TRAFs and RIP) leading to the recruitment of an IKK complex directly onto the cytoplasmic adaptors, activating the IKK complex. IKK then phosphorylates IκB at two serine residues, which leads to its ubiquitination and degradation by the proteasome. NF-κB then enters the nucleus to turn on target genes. TNFR activation can also lead to the phosphorylation of p65 at Ser 276 and 536, and recruitment of cofactors such as p/CAF (via PKCβ) to heighten transcription. TCR engagement leads to recruitment and activation of receptor-associated tyrosine kinases of the Src and Syk families. The latter phosphorylate phospholipase C and phosphatidylinositol 3-kinase (PI3K). Phosphorylation of phosphoinositides by PI3K leads to membrane recruitment and activation of PDK1, which may directly phosphorylate and activate PKCθ to control further recruitment of CARMA1 into the signaling complex. Assembly of these molecules into lipid rafts and PKCθ-dependent phosphorylation of CARMA1 initiate recruitment of BCL10 and MALT1 and possibly TRAF6 and TAK1, leading to IKK activation. The general model shown here for TCR signaling can also be applied to BCR signaling, although a role of PDK1 in this pathway needs to be demonstrated and instead of PKCθ, it involves PKCβ. The non-canonical or non-classical pathway differs from the canonical pathway in that only certain receptor signals (e.g., Lymphotoxin B (LTb), B-cell activating factor (BAFF), CD40) activate this pathway and because it proceeds through an IKK complex that contains two IKKα subunits (but not NEMO). In the noncanonical pathway, receptor binding leads to activation of the NF-κB-inducing kinase NIK, which phosphorylates and activates an IKKα complex, which in turn phosphorylates two serine residues adjacent to the ankyrin repeat C-terminal IκB domain of p100, leading to its partial proteolysis and liberation of the p52/RelB complex. This complex then enters the nucleus to turn on target genes. Figure adapted from Edwards et al, 2008 [66].

Figure 3. Schematic overview of the mechanism underlying TNF-α and IFN-γ synergism

IFN-γ and TNF-α synergistically modulate the expression of different inflammatory genes such ICAM-1, RANTES, IL-8 and CD38. Their cooperativity may be explained at the receptor level by the IFNγ-induced up-regulation of TNF-α receptors or vice-versa. Alternatively, both cytokines may collaborate at the gene level by increasing promoter activation through a synergistic interaction between transcription factors activated by IFN-γ (STATs, IRF-1) and TNF-α (NF-κB). Another mechanism underlying such cooperation could be the induction of defined genes by TNF-α via activation of the autocrine action of IFN-β.