Characterization of LYRIC, a novel tight junction protein, and its potential role in hepatocellular carcinogenesis

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The integrity of epithelial layers is dependent upon the presence of a variety of cell contacts, including tight junctions (TJ), which regulate paracellular transport and sequester apical membrane proteins. Within the past few years more than 30 proteins have been identified as part of the TJ complex, and data suggest that this structure plays an important role in regulating cell proliferation and differentiation. We have cloned a novel protein, LYRIC, and shown that it is localized to TJ of polarized epithelial cells. LYRIC is highly conserved between species, but does not appear to be part of a larger protein family, nor does it contain motifs that would suggest function. LYRIC co-localizes with the TJ protein ZO-1 in bile canaliculi of liver, and at the apical margins of luminal epithelial cells in a variety of tissues, including breast, prostate and colon. Unlike ZO-1, LYRIC is not present in endothelial TJ. Using cultured primary hepatocytes, we have found that as dissociated TJ reassemble, LYRIC re-localizes subsequent to ZO-1, suggesting that LYRIC is not required for initial stages of TJ formation. In cultured hepatocellular carcinoma (HCC) cell lines, which lack TJ, LYRIC is strongly expressed, but displays a particulate cytoplasmic distribution. Somewhat surprisingly, LYRIC does not localize to TJ in polarized T84 cells, a human colon carcinoma line commonly used to study TJ assembly and function. These data suggest that presence of TJ is necessary but not sufficient for LYRIC localization. We propose a model in which LYRIC is a marker of TJ maturation, and that complete maturation of these junctions requires polarization in three dimensions, i.e. signals from contact with matrix, as well as cell-cell interactions. We hypothesize that dissociation of LYRIC from TJ is an early event in tumor development, and could contribute to the proliferative potential of tumor cells. Our current efforts are aimed at defining the mechanism for LYRIC localization, determining its relationship to other TJ proteins, and elucidating changes in TJ protein expression that occur during tumor development. This work is supported by NIH RR-P20 RR17695 and DAMD17-02-1-0132.