erbB1 Functions as a Sensor of Airway Epithelial Integrity by Regulation of Protein Phosphatase 2A Activity

Abstract

Two enzymes, protein phosphatase 2A and atypical protein kinase C, are associated with the tight junction and regulate its function. For example, phosphorylation of the tight junction protein occludin is required for its incorporation into the junction. The association of a kinase and phosphatase with the tight junction suggests that a balance between their activities exists and is required for normal tight junction function. This hypothesis predicts that loss of epithelial integrity may disrupt this balance in such a way as to facilitate restoration of epithelial integrity. Our previous data have shown that apically localized growth factors segregate from their basolaterally localized erbB receptors. Loss of epithelial integrity allows ligand access to the basolateral membrane where it immediately binds to and activates erbB receptors. We found that activation of erbB1 leads to phosphorylation of protein phosphatase 2A, inhibiting its activity. Importantly, this phosphorylation event was dependent on factors in the overlying airway surface liquid; washing away this liquid prevented phosphorylation. erbB1-mediated inhibition of phosphatase activity would shift the balance in favor of the kinase such that tight junction proteins would regain their phosphorylation, allowing for their incorporation into the junction complex. This mechanism provides a rapid means of sensing the loss of epithelial integrity and subsequently restoring barrier function.Polarized epithelia line the cavities of the body, forming a physical barrier between the external environment and the body compartments. Essential to the function of an epithelium is its polarity. Cellular polarity is established and maintained by cell-cell adhesion junctions, including the tight junction and the adherens junction. We have previously demonstrated that maintenance of barrier function is required for silencing signaling cascades in differentiated primary cultures of human airway epithelia (1). Airway epithelia secrete growth factors into the airway surface liquid (ASL).2 Among these is heregulin-␣, a ligand for the family of erbB tyrosine kinase receptors that localize to the basolateral membrane in airway epithelia. This arrangement allows for receptor activation the instant epithelial integrity is compromised. In the presence of intact tight junctions, erbB receptors are silent (not phosphorylated). Disruption of cell-cell junctions allows ASL factors immediate access to the basolateral membrane, resulting in erbB receptor activation. Receptor activation triggers signaling cascades involved in injury repair. Following restoration of epithelial integrity, heregulin-␣ regains its restriction to the ASL, and erbB receptors are once again silenced. These data emphasize the importance of epithelial polarity and the cellcell adhesion junctions that maintain it. In the absence of tight junctions, polarity ceases to exist and erbB signaling cascades become activated. Such aberrant signaling can lead to many downstream effects, including cellular proliferation, ...

Modulation of Tight Junction Structure and Function by Kinases and Phosphatases Targeting Occludin

“…In MDCK cells overexpressing the catalytic subunit of PP2A, recruitment of PKC ζ to junctional complexes was impaired during TJ biogenesis [106]. In pulmonary adenocarinoma Calu-3 cells, Vermeer et al [108] showed a similar complex formation between PP2A and occludin, claudin-1 and aPKC λ at the cell junctions and dissociation of PP2A from occludin and claudins when TJs are disintegrated. Disruption of TJs is a prerequisite for apically secreted EGF ligands to activate their basolateral receptors [109].…”

“…Disruption of TJs is a prerequisite for apically secreted EGF ligands to activate their basolateral receptors [109]. Concomitant with an EGF-induced tyrosine phosphorylation of the PP2A catalytic subunit, PP2A is inactivated allowing phosphorylation of aPKC and its translocation to tight junctional complexes thus promoting assembly [106, 108]. Under inflammatory conditions, which were simulated by TNF α /IFN γ treatment, junctional integrity was compromised leading to a drop in TER which was reverted in the presence of ocadaic acid [108].…”

Paracellular Permeability Restricts Airway Epithelial Responses to Selectively Allow Activation by Mediators at the Basolateral Surface

“…Conversely, airway epithelia may provide an important "watchtower" function by amplifying inflammation when the airway mucosal barrier is breached. This could explain the paradoxical observation that selected inflammatory mediators may directly lower airway epithelial barrier function, as has been demonstrated when histamine, protease-activated receptor 2-activating peptide, or both TNF-␣ and IFN-␥ are applied to the basolateral surface (18,25,42,49).…”

A Pilot Study Investigating Clinical Responses and Biological Pathways of Azelastine/Fluticasone in Nonallergic Vasomotor Rhinitis before and after Cold Dry Air Provocation

“…6). These mechanisms include the dysregulation of signaling through ErbB receptors that are constitutively expressed on nasal epithelium [67,68,69], leading to the degradation of tight junction proteins and widening of intercellular spaces, consistent with the pathological features of NAVMR. Additionally, the dysregulation of ciliary protein expression and mucin oversecretion in NAVMR is implied by CDA-induced under-expression of miR-449 with a consequent lack of Notch signaling control [5,70,71] (Fig.…”