Regulation of E-cadherin junctions by the c-Jun N-terminal kinase in oral keratinocytes

Abstract

Maintaining cell-cell adhesions of keratinocytes is crucial in tightly packing the cells in the epithelium and protecting tissues underlying epithelium against harmful agents such as microorganisms or toxins. E-cadherin junctions (ECJs) have a major role in keeping the integrity of adhesion complex of oral keratinocytes. Therefore understanding of the molecular mechanisms of their formation is critical in both normal and transformed oral keratinocytes. First, HOK-16B cells, a normal human gingival epithelial cell line, were used to identify the molecules involved in the regulation of the formation of intercellular E-cadherin junctions between human gingival epithelial cells. Activation of c-Jun N-terminal kinase (JNK) disrupted the intercellular junctions through the dissociation of E-cadherin. The role of JNK in the formation of these E-cadherin junctions was further confirmed by demonstrating that JNK inhibition induced the formation of intercellular E-cadherin junctions. The upstream signaling of JNK was also examined. Activation of the small GTPase RhoA disrupted the formation of E-cadherin junctions between HOK-16B cells, which was accompanied by JNK activation. Disruption of these intercellular junctions upon RhoA activation was prevented when JNK activity was inhibited. In contrast, RhoA inactivation led to HOK-16B cell aggregation and the formation of intercellular junctions, even under conditions in which the cellular junctions were naturally disrupted by growth on a strongly adhesive surface. Then, the JNK-dependent regulation of E-cadherin junction was confirmed in gingival epithelium including junctional epithelium (JE) of which intercellular junction is poorly formed. Expectedly, JE of mouse molars had high JNK activity associated with low E-cadherin expression, which was reversed in the other gingival epithelia, including the sulcular epithelium. Next, the molecular mechanism by which squamous cell carcinoma cells grow scattered at the early phase of transformation while maintaining the epithelial phenotype was studied because cell scattering of epithelial carcinoma cancer cells is one of the critical event in tumorigenesis. Cells losing epithelial cohesion detach from aggregated epithelial cell masses and may migrate to fatal organs through metastasis. YD-10B cells which are established from human oral squamous cell carcinoma was studied because the cells grow scattered without the formation of ECJs under routine culture conditions despite the high expression of functional E-cadherin. The functionality of their E-cadherin was demonstrated in that YD-10B cells formed ECJs, transiently or persistently, when they were cultured on substrates coated with a low amount of fibronectin or to confluence. The phosphorylation of JNK was up-regulated in YD-10B cells compared with that in human normal oral keratinocyte cells or human squamous cell carcinoma cells, which grew aggregated along with well-organized ECJs. The suppression of JNK activity induced the aggregated growth of YD-10B cells concomitant with the development of ECJs. These results indicate that the decreased formation of intercellular E-cadherin junctions between human gingival keratinocytes may be coupled to high JNK activity and the inherently up-regulated JNK activity induces the scattered growth of the oral squamous cell carcinoma cells through disassembly of ECJ despite the expression of functional E-cadherin, a hallmark of the epithelial phenotype.