Numerous intestinal diseases are characterized by immune cell activation and compromised epithelial barrier function. We have shown that cytokine treatment of epithelial monolayers increases myosin II regulatory light chain (MLC) phosphorylation and decreases barrier function and that these are both reversed by MLC kinase (MLCK) inhibition. The aim of this study was to determine the mechanisms by which interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha regulate MLC phosphorylation and disrupt epithelial barrier function. We developed a model in which both cytokines were required for barrier dysfunction. Barrier dysfunction was also induced by TNF-alpha addition to IFN-gamma-primed, but not control, Caco-2 monolayers. TNF-alpha treatment of IFN-gamma-primed monolayers caused increases in both MLCK expression and MLC phosphorylation, suggesting that MLCK is a TNF-alpha-inducible protein. These effects of TNF-alpha were not mediated by nuclear factor-kappaB. However, at doses below those needed for nuclear factor-kappaB inhibition, sulfasalazine was able to prevent TNF-alpha-induced barrier dysfunction, MLCK up-regulation, and MLC phosphorylation. Low-dose sulfasalazine also prevented morphologically evident tight junction disruption induced by TNF-alpha. These data show that IFN-gamma can prime intestinal epithelial monolayers to respond to TNF-alpha by disrupting tight junction morphology and barrier function via MLCK up-regulation and MLC phosphorylation. These TNF-alpha-induced events can be prevented by the clinically relevant drug sulfasalazine.

Epithelial tight junctions form a barrier against passive paracellular flux. This barrier is regulated by complex physiologic and pathophysiologic signals that acutely fine-tune tight junction permeability. Although actomyosin contraction and myosin light chain phosphorylation are clearly involved in some forms of tight junction regulation, the contributions of other signaling events and the role of myosin light chain phosphorylation in this response are poorly understood. Here we ask if activation of myosin light chain kinase alone is sufficient to induce downstream tight junction regulation. We use a confluent polarized intestinal epithelial cell model system in which constitutively active myosin light chain kinase, tMLCK, is expressed using an inducible promoter. tMLCK expression increases myosin light chain phosphorylation, reorganizes perijunctional F-actin, and increases tight junction permeability. TJ proteins ZO-1 and occludin are markedly redistributed, morphologically and biochemically, but effects on claudin-1 and claudin-2 are limited. tMLCK inhibition prevents changes in barrier function and tight junction organization induced by tMLCK expression, suggesting that these events both require myosin light chain phosphorylation. We conclude that myosin light chain phosphorylation alone is sufficient to induce tight junction regulation and provide new insights into the molecular mechanisms that mediate this regulation.

Activation of Na؉ -nutrient cotransport leads to increased tight junction permeability in intestinal absorptive (villus) enterocytes. This regulation requires myosin II regulatory light chain (MLC) phosphorylation mediated by MLC kinase (MLCK). We examined the spatiotemporal segregation of MLCK isoform function and expression along the crypt-villus axis and found that long MLCK, which is expressed as two alternatively spliced isoforms, accounts for 97 ؎ 4% of MLC kinase activity in interphase intestinal epithelial cells. Expression of the MLCK1 isoform is limited to well differentiated enterocytes, both in vitro and in vivo, and this expression correlates closely with development of Na ؉ -nutrient cotransport-dependent tight junction regulation. Consistent with this role, MLCK1 is localized to the perijunctional actomyosin ring. Furthermore, specific knockdown of MLCK1 using siRNA reduced tight junction permeability in monolayers with active Na ؉ -glucose cotransport, confirming a functional role for MLCK1. These results demonstrate unique physiologically relevant patterns of expression and subcellular localization for long MLCK isoforms and show that MLCK1 is the isoform responsible for tight junction regulation in absorptive enterocytes.