Occludin is required for apoptosis when claudin–claudin interactions are disrupted

Disruption of tight junctions is often seen during pathogen infection, inflammation, and tumor progression. Mislocalization of the tight junction proteins occludin and claudin in mammary epithelial monolayers leads to apoptosis through the extrinsic pathway. To further investigate the mechanism of this response, a normal mammary epithelial cell line (EpH4) as well as primary mammary epithelial cells were treated with a claudin-disrupting mimic peptide, DFYNP (aspartic acid–phenylalanine–tyrosine–asparagine–proline). Using fluorescent indicators, we found that caspase-3 activation, resulting from treatment with DFYNP, was restricted to EpH4 and primary mammary epithelial cells with mislocalized claudin-4. Mislocalized claudin-4 and occludin were colocalized in non-junctional puncta, and both molecules were found in the death-inducing signaling complex (DISC) where they colocalized with Fas, fas-associated protein with death domain (FADD), active caspase-8 and caspase-3 at distinct apical domains. Importantly, caspase-3 activation was totally repressed in primary mammary epithelial cells from occludin null mice. Thus, the apoptotic response appears to be initiated by the movement of occludin to the DISC suggesting that this molecule has signaling properties that initiate cell death when its tight junction location is disrupted.

Tight junction disruption has an important role in several pathologies including viral and bacterial infection, 1,2 inflammation, 3,4 and tumor progression. 5 Claudin and occludin are major transmembrane proteins of the tight junction that provide tight control over paracellular diffusion within an epithelium. Interestingly, claudin and occludin are often targeted and misplaced by viruses, 1,6 bacteria, 7,8 and inflammatory cytokines. 8,9 Although apoptosis is observed in many diseases that involve disrupted tight junctions, 10-12 the role displaced tight junction proteins may have in the initiation of apoptosis has not been investigated. Importantly, it is not known whether changes in localization of key tight junction proteins, such as claudin and occludin, may be initiating important signaling pathways to preserve the barrier properties of a compromised epithelial monolayer.
Previously, we have shown, using a mimic peptide to a conserved sequence in the second extracellular loop of occludin, that disruption of normal occludin-occludin interactions at the tight junction leads to mislocalization of occludin away from tight junctions and into the cytosol. 13 This mislocalized occludin interacted with components of the death-inducing signaling complex (DISC), such as fasassociated protein with death domain (FADD) and cleaved caspase-8, and induced activation of the extrinsic apoptotic pathway. This finding suggested that occludin not only has a role in barrier formation, but may also be a signaling molecule that induces apoptosis when normal tight junction protein interactions are disrupted. When normal claudin-claudin interactions were disrupted, using similar peptide mimic technology, claudin also mislocalized away from tight junctions into the cytosol. 14 The mislocalization of claudin led to the activation of caspase-8 and -3, similar to what was seen when occludin was disrupted, and suggested that claudin may also be having an important role in cell fate signaling in response to tight junction disruption.
In the present study, the signaling mechanism involved in apoptosis induced by claudin and occludin disruption was further investigated. We asked whether both proteins work through the same pathway or whether one protein is a universal trigger for apoptosis when tight junctions are disrupted. Normal mammary epithelial cells were treated with a claudin mimic peptide, DFYNP (aspartic acid-phenylalanine-tyrosine-asparagine-proline), and changes in localization of claudin and occludin were examined in relation to each other as well as to key apoptotic signaling molecules. The results show that both claudin and occludin move from tight junctions to distinct apical domains where they colocalize with components of the DISC to induce apoptosis. However, it is occludin that is required for induction of apoptosis.

Results
Apoptosis is restricted to epithelial cells with mislocalized claudin. To confirm that disruption of normal claudin localization leads to apoptosis, cells were examined for localization of both claudin-4 and active caspase-3 in the absence and presence of the claudin-disrupting DFYNP mimic peptide. Immunohistochemistry, using antibodies directed to claudin-4 (a claudin subtype that contains DFYNP in the second extracellular loop) and cleaved caspase-3, revealed that claudin-4 is largely restricted to tight junctions and caspase-3 was not activated in untreated EpH4 mammary epithelial cells (Figure 1). After 16 h of treatment with the claudin mimic peptide, claudin-4 localization changed dramatically, with a significant proportion of claudin in the cytosol. Treated cells also showed activated caspase-3.
Occludin is disrupted when claudin is mislocalized. As occludin mislocalization can also lead to apoptosis, 13 we wanted to test whether or not occludin is disrupted in response to claudin mislocalization. Claudin-4 and occludin localization were examined simultaneously in the absence and presence of the claudin mimic peptide. In untreated EpH4 cells, both claudin-4 and occludin are restricted to tight junctions ( Figure 2). After 16 h of treatment with the claudin mimic peptide, disruption of claudin-4 was very clear in defined groups of treated cells where claudin-4 appeared to be less regularly distributed in the tight junctions than in controls. Occludin was also mislocalized into non-junctional puncta in treated EpH4 cells, but there was still a strong presence of occludin at the tight junction. Interestingly, bright puncta of non-junctional occludin colocalized with nonjunctional puncta of claudin-4. This observation suggests occludin is also mislocalized in response to claudin disruption and interacts with claudin in distinct vesicular-like domains.
Claudin and occludin colocalize with components of the DISC. The extrinsic apoptotic pathway is characterized by the activation of a death receptor and formation of the DISC, of which FADD and cleaved caspase-8 are key components, and initiation of the caspase cascade. EpH4 cells treated with the DISC, and subsequent activation of caspase-8, is the activation of effector caspases, such as caspase-3, that carry out the enzymatic cleavage of other cellular proteins. Active caspase-3 was also found within the DISC in response to claudin disruption with the mimic peptide ( Figure 4b). Therefore, in response to claudin disruption, claudin and occludin move out of the tight junction and interact with the key protein components of the intrinsic apoptotic signaling pathway.
Occludin disruption is downstream of claudin mislocalization. The presence of occludin at the DISC when claudin localization is disrupted and caspase activation is induced further suggests that occludin may be having a key role in activation of apoptosis in response to tight junction disruption. To determine whether claudin and occludin act together to induce apoptosis when tight junctions are disrupted or whether occludin is the downstream sensor of tight junction disruption and trigger of apoptosis, we examined the localization of claudin during occludin disruption. Using immunohistochemistry, localization of claudin-4 was imaged after treatment with an occludindisrupting mimic peptide LYHY (lysine-tyrosine-histidinetyrosine). 13 Figure 5 shows that claudin-4 remained at the sites of tight junctions after treatment of the cells with the occludin-disrupting peptide. These data suggest that occludin, in fact, is acting downstream of claudin disruption to activate apoptosis and that occludin is more of a universal 'trigger' of apoptosis when tight junctions are disrupted.
Occludin is required for apoptosis induced by claudin disruption. To further test this hypothesis that occludin is the trigger of apoptosis when tight junctions are disrupted, we examined the ability of the claudin mimic peptide to induce apoptosis in the absence of occludin. Primary mammary epithelial cells were isolated from pregnancy day-15 occludin null-transgenic mice and compared with primary mammary epithelial cells isolated from wild-type mice of the same background (FVB). Western blot analysis (Figure 6a) confirmed that the occludin protein was absent in tissue from the occludin null mouse. Next, we examined claudin-4 localization in the occludin knockout mammary epithelial cells to determine whether claudin-4 localizes to the tight junctions in the absence of occludin. Results show that claudin-4 is, indeed, localized to the tight junctions in the occludin knockout mammary epithelium (Figure 6b). After treatment with the claudin mimic peptide, claudin-4 localization was significantly disrupted (Figure 6c), similar to what we saw with the EpH4 cell line. Next, the ability of the peptide to induce apoptosis in the absence of occludin was examined. Wild-type mammary epithelial cells treated with the claudin mimic peptide showed a significant increase in activated caspase-3 (8.09 ± 0.48% of cells, P ¼ 0.0000169 versus control) versus untreated wild-type control cells (1.39 ± 0.21% of cells, Figure 7). Claudin disruption did not induce apoptosis in occludin null mammary epithelial cells, showing a similar level of caspase-3 activation (1.59±0.16% of cells, P ¼ 0.82013 versus control) as untreated wild-type control cells. Interestingly, although claudin-4 was mislocalized, the change in its localization was unable to elicit the apoptotic response. This observation provides strong evidence that occludin mediates the apoptotic response induced by disruption of tight junction proteins.

Discussion
Tight junction disruption is usually thought of as a downstream consequence of caspase cleavage during the apoptotic process. Results from the present study, however, suggest that tight junction disruption can be an early event that initiates caspase activation and cell death. Previously, we have shown that disruption of the normal tight junctional localization of occludin 13 and claudin 14 with mimic peptides can induce apoptosis in normal epithelium. Immunoprecipitation along with immunohistochemistry confirmed that occludin physically interacts with the DISC via the adaptor molecule FADD. 13 In the present study, we have demonstrated that both occludin and claudin can interact with components of the DISC, but that occludin is required for the induction of apoptosis. Therefore, it appears that occludin is having an important role in sensing tight junction disruption and initiating apoptosis, a highly regulated form of cell death. As apoptotic cells can be extruded The implication that occludin may have a role outside of tight junction barrier formation can be inferred from the first experiments with occludin knockout mice: normally functioning tight junctions can form in the absence of occludin. 15 The hypothesis that occludin has an important role in cell death signaling is supported by the results of the current study that show apoptosis is inhibited when occludin is absent. A hallmark characteristic of tumor cells is that they are resistant to apoptosis. That occludin can have a key role in apoptosis is supported by the reported loss of occludin expression in endometrial 16 and breast 17 carcinomas. Interestingly, Osanai et al. 18 have shown that forced expression of occludin in a cervical cancer cell line (HeLa) that does not express occludin increased sensitivity of these cells to oxidative stress-induced apoptosis, further implicating occludin in apoptotic signaling.
The mechanism by which tight junction disruption leads to the interaction of occludin and claudin with the extrinsic apoptotic signaling pathway is not fully understood. Previously, we have shown that occludin can bind to FADD when tight junctions are disrupted. 13 It is possible that displaced occludin is able to interact with FADD by movement within or between plasma membrane domains. Tight junction proteins, including occludin, have been found in cholesterol-rich lipid raft-like microdomains in the plasma membrane. 19,20 The Fas death receptor has also been found in lipid raft domains, being recruited to lipid rafts upon stimulation, and further recruiting FADD and procaspase-8 for assembly of the death inducing signaling complex. 21,22 Whether or not occludin is having a role in recruiting FADD downstream of Fas stimulation is unknown. However, Algeciras-Schimnich et al. 21 have shown that FADD and procaspase-8 recruitment to the DISC and activation of caspase-8 is required for clustering of FAS at the plasma membrane and internalization of the DISC. Therefore, it is possible the occludin interaction with FADD may be the initiating factor for DISC assembly, followed by clustering of Fas and internalization of the entire complex. It is also possible that the complex is responsible for the distinct vesicle-like apical domains seen in the z-stack images in  Tight junction disruption, including occludin and claudin mislocalization, has been observed in Crohn's disease, 12,23 hepatitis C virus infection, 1,10 Helicobacter pylori infection, 8,11,24 and many epithelial-derived tumors. 5 Each of these disorders is associated with either upregulation of apoptosis, as seen in Crohn's disease, hepatitis C virus, and H. pylori infection, 12 or inhibition of apoptosis, as seen in breast cancer. 17 It is a possibility that the signaling pathway identified in the current study may be contributing to the pathology of these diseases, either being upregulated to create a leaky epithelium or disrupted to create resistance to regulated cell death. Understanding the interaction of tight junction proteins with apoptotic signaling molecules will not only help to elucidate the pathology of these diseases but may provide potential therapeutic targets for their treatment.
Statistics. Data are presented as means ± S.E.M. An unpaired Student's t-test was used for statistical comparison between control and treatment groups. A P-value of o 0.05 was considered significant.