Upregulation and maintenance of gap junctional communication in lens cells

Abstract

The cells of the lens are joined by an extensive network of gap junction intercellular channels consisting of connexins 43, 46, and 50. We have proposed, and experimentally supported, the hypothesis that fibroblast growth factor (FGF) signaling is required for upregulation of gap junction–mediated intercellular coupling (GJIC) at the lens equator. The ability of FGF to increase GJIC in cultured lens cells requires sustained activation of extracellular signal-regulated kinase (ERK). In other cell types, activation of ERK has been shown to block GJIC mediated by connexin43 (Cx43). Why ERK signaling does not block lens cell coupling is not known. Another unresolved issue in lens gap junction regulation is how connexins, synthesized before the loss of biosynthetic organelles in mature lens fiber cells, avoid degradation during formation of the organelle-free zone. We have addressed these questions using serum-free cultures (termed DCDMLs) of primary embryonic chick lens epithelial cells. We show that FGF stimulates ERK in DCDMLs via the canonical Ras/Raf1 pathway, and that the reason that neither basal or growth factor-stimulated GJIC is blocked by activation of ERK is because it is not mediated by Cx43. In fibroblastic cells, the normally rapid rate of degradation of Cx43 after its transport to the plasma membrane is reduced by treatments that either directly (ALLN; epoxomicin) or indirectly (generation of oxidatively un/mis-folded proteins by arsenic compounds) prevent the ubiquitin/proteasome system (UPS) from acting on its normal substrates. We show here that Cx45.6 and Cx56, the chick orthologs of mammalian Cx50 and Cx46, behave similarly in DCDMLs. When organelles lyse during the maturation of fiber cells, they release into the cytosol a large amount of new proteins that have the potential to saturate the capacity, and/or compromise the function, of the UPS. This would serve to spare gap junctions from degradation during formation of the organelle-free zone, thereby preserving GJIC between mature fiber cells despite the lack of de novo connexin synthesis.

Correlations of Differentially Expressed Gap Junction Connexins Cx26, Cx30, Cx32, Cx43 and Cx46 with Breast Cancer Progression and Prognosis

“…Cx46 mRNA expression was also linked with improved survival in ER positive patients which, however, was preserved also in ER negative patients, suggesting a reciprocal regulation of Cx46 compared to Cx43 expression in this subgroup. This is in line with data gained in lens cell cultures showing the simultaneous down-regulation of Cx43 and upregulation of Cx46 expression by the tumor promoter phorbol ester (12-O-tetradecanoylphorbol-13-acetate) [53] or by the activation of the MAPK/ERK pathway [54]. Differential regulation of connexin isotypes are rather common and allows connexins to serve either as conditional tumor suppressors in primary cancer or tumor supporters in advanced, metastatic cases [30], [32], [33], [51].…”

Interleukin‐1 β increases gap junctional communication among synovial fibroblasts via the extracellular‐signal‐regulated kinase pathway

“…First, the ERK pathway has been shown to regulate Cx43 expression and function. ERK activation has been shown to up‐regulate Cx43 expression in several cell types (Cushing et al, 2005; Jia et al, 2008; Boswell et al, 2009). Activation of ERK has been shown to phosphorylate Cx43 at multiple serine residues in the C‐terminal tail and has been shown to regulate Cx43 function, typically inhibiting GJIC (Warn‐Cramer et al, 1998; Lampe and Lau, 2004).…”

Lens Gap Junctions in Growth, Differentiation, and Homeostasis

“…Of more physiological relevance, the coupling conductance is significantly increased by fibroblast growth factor (FGF), and FGF is present in the vitreous humor (110). More recently, it has been shown that FGF requires bone morphogenic protein (BMP) signaling to induce an increase in coupling conductance, that expression of BMP receptors is high in the equatorial DF of the lens, and that BMP is released by the lens through an autocrine system (26). These observations led the authors to suggest the relatively high coupling conductance between equatorial DF is regulated by BMP and FGF.…”