A Tight Rein on Cell Proliferation from the Junction

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Tight junctions between epithelial cells restrict paracellular flux of solutes, isolate the apical from the basolateral cell surface, and may help regulate cell proliferation. Balda et al. investigated the role of the membrane-associated guanylate kinase (MAGUK) ZO-1, a tumor suppressor protein associated with tight junctions, in regulating epithelial cell proliferation through ZO-1-associated nucleic acid binding protein (ZONAB), a transcription factor that binds to the SH3 domain of ZO-1. The authors used Western analysis to show that ZONAB expression was high in proliferating MDCK cells, but low in cells that were not proliferating. Reducing cellular ZONAB content with antisense RNA inhibited [ 3 H]thymidine incorporation. Overexpressing ZO-1, which is found at low levels in proliferating cells and increases with cell density, also inhibited cell proliferation, an effect that required the SH3 domain. Coexpressing ZONAB with ZO-1 counteracted ZO-1's inhibition of proliferation. ZO-1 inhibited cell entry into S phase in response to serum stimulation as assessed by [ 3 H]thymidine and BrdU incorporation; this was counteracted by coexpressing ZONAB. Reducing ZONAB through RNA interference also inhibited entry into the cell cycle. Cells overexpressing ZONAB achieved a higher final density in mature monolayers, whereas cells overexpressing ZO-1 showed a lower final density. The cyclin-dependent kinase 4 (CDK4) was present in ZONAB immunoprecipitates. Both CDK4 and ZONAB were found in nuclear and cytoplasmic pools; overexpressing ZO-1 or its SH3 domain reduced the nuclear content of both ZONAB and CDK4. These data suggest that the tight junction-associated protein ZO-1 regulates cell proliferation at least partly by limiting nuclear accumulation of CDK4. M. S. Balda, M. D. Garrett, K. Matter, The ZO-1-associated Y-box factor ZONAB regulates epithelial cell proliferation and cell density. J. Cell Biol. 160 , 423-432 (2003). [Abstract] [Full Text]