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The human colon adenocarcinoma cell line HT 29 grows in culture without tight junctions (TJ). Tight junction strands of the fascia occludens type can be induced by treatment with proteases and are subsequently degraded during a period of about 3 h. Experiments using a variety of metabolic inhibitors such as 2-deoxyglucose, 2,4-dinitrophenol, and CCCP show that the degradation of TJ is retarded under conditions of ATP depletion. Thus it appears that the removal of TJ from the cell surface is an energy-dependent process. Moreover, DNP can specifically inhibit the degradation of TJ even in the absence of ATP depletion. The possible involvement of a proton gradient in the mechanism of TJ degradation is discussed.

Tight junctions (TJ) of the fascia occludens type can be induced in the human colon adenocarcinoma cell lines HT29 and Caco-2 by treatment with 320 mM cesium sulfate. This process can be completely inhibited by the protease inhibitors leupeptin and antipain. The concentration for 50% inhibition was 32 microM leupeptin and 270 microM antipain, respectively. In the polarized colon carcinoma cell line Caco-2, the spontaneous formation of histotypical TJ and the development of transepithelial electrical resistance do not occur when the cells are cultured in medium containing 400 microM leupeptin. Following the removal of leupeptin, zonula occludens type TJ and electrical resistance develop synchronously during a period of 4 h. Dihydroleupeptin, the alcohol analog of leupeptin, inhibits neither the spontaneous nor the induced assembly of TJ fibrils. Thus, the aldehyde group of leupeptin is essential for activity. These data suggest that the salt-induced as well as the spontaneous formation of TJ involve cellular proteases which are susceptible to protease inhibitors.

The experimental modulation of tight junctions (TJ) was studied in the human adenocarcinoma cell line HT 29 by freeze-fracture electron microscopy. The cell line has virtually no TJ when grown in culture. TJ could be induced by mild treatment with a variety of endopeptidases (trypsin, chymotrypsin, collagenase, elastase, plasmin, thrombin, papain, and pronase). Pronase induced the formation of TJ at low (but not at high) concentrations. All exopeptidases studied were unable to induce the formation of TJ. At 0 degree C the trypsin-induced formation of TJ was greatly slowed down although not entirely inhibited. However, when cells were briefly treated with trypsin at 0 degree C and subsequently transferred to 37 degrees C in the presence of protease inhibitors, TJ were rapidly assembled. Thus an induction phase at low temperature and an assembly phase at high temperature could be experimentally separated. When cells were briefly trypsinized at 0 degrees and subsequently kept at 0 degree C without trypsin for several hours, TJ still formed abundantly upon incubation at 37 degrees C. It appears therefore that the effect produced by the protease is retained for long periods in the cold.

The human colon adenocarcinoma cell line HT 29 grows virtually without tight junctions (TJ) under standard culture conditions. Earlier studies have shown that focal TJ (fasciae occludentes) can be rapidly assembled in this cell line under the influence of various proteases. Here we show that focal TJ can be induced in this cell line by a brief treatment with appropriate salt solutions. Induction by ammonium sulfate in Hanks' buffer reached a maximum value after 15 to 30 min. The amount of TJ increased with the salt concentration and reached a plateau value at a concentration of 160 mM ammonium sulfate. The amount and complexity of TJ induced by ammonium sulfate were similar to those in experiments using trypsin as inducing agent as shown by morphometric analysis. At 0 degrees C, no TJ were formed under the influence of the salt. A comparative study of TJ induction using a variety of inorganic and organic salts gave the following results. All alkali sulfates induced TJ, although with different yield. Both calcium and magnesium chloride were potent inducers. Ammonium and sodium salts encompassing a variety of anions covered a wide range from maximum induction (sulfate, citrate) to almost complete absence of induction (nitrate). Sodium chloride did not induce any TJ. It follows that the induction of TJ is a specific effect of individual ionic components of the solution as opposed to a general effect of osmolarity and ionic strength. The data suggest tentatively that antichaotropic but not chaotropic ions have the potential to trigger the formation of TJ in this experimental system.