Increased tight junction permeability can result from protein kinase C activation/translocation and act as a tumor promotional event in epithelial cancers

Exposure of LLC-PK1 epithelial cell cultures to phorbol ester tumor promoters causes immediate translocation of protein kinase C-alpha (PKC-alpha) from cytosolic to membrane-associated compartments. With a very similar time course, a dramatic and sustained increase in tight junctional (paracellular) permeability occurs. This increased permeability extends not only to salts and sugars but macromolecules as well. Fortyfold increases of transepithelial fluxes of biologically active EGF and insulin occur. Recovery of tight junction barrier function coincides with proteasomal downregulation of PKC-alpha. The failure to downregulate activated membrane-associated PKC-alpha has correlated with the appearance of multilayered cell growth and persistent leakiness of tight junctions. Accelerated downregulation of PKC-alpha results in only a partial and transient increase in tight junction permeability. Transfection of a dominant/negative PKC-alpha results in a slower increase in tight junction permeability in response to phorbol esters. In a separate study using rat colon, dimethylhydrazine (DMH)-induced colon carcinogenesis has been preceded by linear increases in both the number of aberrant crypts and transepithelial permeability, as a function of weeks of DMH treatment. Adenocarcinomas of both rat and human colon have been found to have uniformly leaky tight junctions. Whereas most human colon hyperplastic and adenomatous polyps contain nonleaky tight junctions, adenomatous polyps with dysplastic changes did possess leaky tight junctions. Our overall hypothesis is that tight junctional leakiness is a late event in epithelial carcinogenesis but will allow for growth factors in luminal fluid compartments to enter the intercellular and interstitial fluid spaces for the first time, binding to receptors that are located on only the basal-lateral cell surface, and causing changes in epithelial cell kinetics. Tight junctional leakiness is therefore a promotional event that would be unique to epithelial cancers.

The transient increase of tight junction permeability induced by bryostatin 1 correlates with rapid downregulation of protein kinase C-alpha

The role of PKC-alpha in altered epithelial barrier permeability following the activation of PKC by TPA (12-O-tetradecanoyl phorbol 13-acetate) and bryostatin 1 in LLC-PK1 cells was investigated in this study. Like TPA, bryostatin 1 binds to and activates PKC but unlike TPA, it is not a tumor promoter. TPA at 10(-7) M induced a sustained 95% decrease in transepithelial electrical resistance (R(t)) across LLC-PK1 epithelial cell sheets, while 10(-7) M bryostatin 1 caused only a 30% decrease in R(t), which spontaneously reversed after 5 h. Simultaneous exposure of cell sheets to 10(-7) M TPA and 10(-7) M bryostatin 1 blunted the increase in epithelial permeability observed with 10(-7) M TPA alone. Co-incubation of cell sheets with bryostatin 1 and MG-132, a proteasomal inhibitor, caused a further decrease in R(t) at the 6-h time point and inhibited the recovery in R(t) seen with bryostatin 1 alone at this time point. TPA caused a rapid translocation of PKC-alpha from the cytosol to the membrane of the cell where it remained elevated. Bryostatin 1 treatment resulted in a slower translocation of PKC-alpha from the cytosol to the membrane and a much more rapid downregulation of PKC-alpha, with disappearance from this compartment after only 6 h. The classical PKC inhibitor Go6976 prevented the decrease in R(t) seen with TPA. Treatment of cells with TPA and bryostatin 1 resulted in a PKC-alpha translocation and downregulation profile which more closely resembled that seen with bryostatin 1 alone. Co-incubation of cells with MG-132 and bryostatin 1 caused a slower downregulation of PKC-alpha from the membrane fraction. Bryostatin 1 treatment of cells expressing a dominant/negative form of PKC-alpha resulted in a slower and less extensive decrease in R(t) compared to the corresponding control cells. For both TPA and bryostatin 1, the level of PKC-alpha in the membrane-associated fraction of the treated cells correlated closely with increased transepithelial permeability. Due to its transient effect on tight junction permeability, bryostatin 1 offers a novel pharmacological tool to investigate junctional physiology.

Plasma concentrations of soluble tumor necrosis factor receptor I and tumor necrosis factor during cardiopulmonary bypass

BACKGROUND

Tumor necrosis factor-alpha (TNF) has been implicated in the development of postoperative morbidity after cardiopulmonary bypass for myocardial revascularization. Despite their postulated roles as modulators of TNF bioavailability, soluble TNF receptors have not been characterized in patients undergoing this procedure and is the focus of this study.

METHODS

Soluble tumor necrosis factor receptor I (sTNFRI) and TNF were measured by immunoassay in plasma samples collected from 36 patients at events before, during, and after cardiopulmonary bypass.

RESULTS

Plasma concentrations of sTNFRI averaged 1.39 ng/mL at the start of the operation. Preoperative sTNFRI concentrations were found to significantly correlate with a preoperative morbidity assessment score, age, duration of bypass, duration of supplemental oxygen, and length of hospital stay. Plasma sTNFRI increased in all of the patients during the procedure. Plasma concentrations of sTNFRI and TNF did not correlate at any time.

CONCLUSIONS

Preoperative measurement of sTNFRI could potentially serve as a reliable indicator for prophylactic treatment with an anti-TNF therapy. Such a therapeutic approach might help attenuate inflammatory processes thought to underlie postoperative morbidity associated with cardiopulmonary bypass.

Increased tight junctional permeability is associated with the development of colon cancer

Epithelial tissues act as barriers between two fluid compartments, and the epithelial barrier function is provided by the epithelial cells and the tight junctions (TJs) that connect them. We have shown previously that chronic treatment of a cultured epithelial monolayer with phorbol ester tumor promoters induces an increase in transepithelial paracellular permeability and produces tumor-like polyps, suggesting an association between TJ permeability and tumor formation. In this study, we analyzed the association between TJ permeability and formation of tumors in vivo. The permeability of the TJs was assessed in normal human and rat colon epithelia and in colon tumors by measuring the transepithelial electrical resistance, the paracellular flux rate of D-[(14)C]mannitol and the electron microscopic evaluation of the penetration of the electron dense dye ruthenium red across the TJs. By these criteria, the TJs of human colon tumors, including carcinomas and adenomatous polyps, and the TJs of 1,2-dimethylhydrazine (DMH)-induced rat colon tumors were leakier than the TJs of normal colon. Treatment of rats with the carcinogen DMH induced a progressive increase in the number of aberrant colonic crypts, considered the putative pre-neoplastic colonic phenotype while increasing TJ permeability of the colon epithelium prior to the development of tumors. These results showed that increased TJ permeability of the colon epithelium and consequently a decrease in epithelial barrier function precede the development of colon tumors.

Overexpression of protein kinase C-delta increases tight junction permeability in LLC-PK1 epithelia

The Ca2+-independent delta-isoform of protein kinase C (PKC-delta) was overexpressed in LLC-PK1 epithelia and placed under control of a tetracycline-responsive expression system. In the absence of tetracycline, the exogenous PKC-delta is expressed. Western immunoblots show that the overexpressed PKC-delta is found in the cytosolic, membrane-associated, and Triton-insoluble fractions. Overexpression of PKC-delta produced subconfluent and confluent epithelial morphologies similar to that observed on exposure of wild-type cells to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Transepithelial electrical resistance (RT) in cell sheets overexpressing PKC-delta was only 20% of that in cell sheets incubated in the presence of tetracycline, in which the amount of PKC-delta and RT were similar to those in LLC-PK1 parental cell sheets. Overexpression of PKC-delta also elicited a significant increase in transepithelial flux of D-[14C]mannitol and a radiolabeled 2 x 10(6)-molecular-weight dextran, suggesting with the RT decrease that overexpression increased paracellular, tight junctional permeability. Electron microscopy showed that PKC-delta overexpression results in a multilayered cell sheet, the tight junctions of which are almost uniformly permeable to ruthenium red. Freeze-fracture electron microscopy indicates that overexpression of PKC-delta results in a more disorganized arrangement of tight junctional strands. As with LLC-PK1 cell sheets treated with 12-O-tetradecanoylphorbol-13-acetate, the reduced RT, increased D-mannitol flux, and tight junctional leakiness to ruthenium red that are seen with PKC-delta overexpression suggest the involvement of PKC-delta in regulation of tight junctional permeability.

Protein kinase C activation increases transepithelial transport of biologically active insulin

Protein kinase C activation leads to tight junctional leakiness and, consequently, to increased transepithelial (paracellular) solute flux across epithelial barriers. This leakiness is shown here to result in as much as a 20-fold increase in the transepithelial flux of insulin. Using an epithelial/fibroblast coculture model, this transepithelially transported insulin is shown to be biologically active. The 3T3 fibroblasts situated on one side of the epithelial barrier exhibited increased insulin binding and resulting DNA synthesis when the epithelial junctions were made leaky to insulin on the opposite side of the epithelial barrier. The dramatically enhanced permeability of macromolecules across epithelial cell layers undergoing protein kinase C activation may play a significant role in epithelial cancer, immunology, and drug delivery.

Pentobarbital affects transepithelial electrophysiological parameters regulated by protein kinase C in rat distal colon

For rat distal colon, the transepithelial electrical parameters, short circuit current (Iscc) and transepithelial electrical resistance (TER), respectively, measure net transepithelial electrolyte transport activity and the barrier function of the epithelium. Studies with a variety of epithelial cell cultures have shown greater than 90% decreases of TER within minutes of exposure of in vitro cell sheets to phorbol esters. The phorbol ester and protein kinase C (PKC) activator, phorbol dibutyrate (PDBU), was observed to produce an over 100% elevation of Iscc but only a small yet significant 20-30% decrease of TER across rat distal colon. Inhibition of the above effects of PDBU by the PKC inhibitor bisindolylmaleimide (GFX) is further evidence that in rat distal colon, Iscc and TER are under regulatory control by PKC. When animals received anesthesia with intraperitoneal pentobarbital prior to removal of the colon, the effect of PDBU on Iscc was significantly reduced, and the effect of PDBU on TER was almost completely inhibited. This effect of pentobarbital on PKC-mediated transepithelial permeability parameters is consistent with the known ability of anesthetics to alter protein kinase C activity. Exposure of rat colon to pentobarbital produced as much as a 90% inhibition of calcium-dependent PKC activity, whereas calcium-independent activity was stimulated by as much as 35%. Prior anesthetic use may be therefore a complicating factor in observing PKC-mediated effects on epithelial barrier function using epithelial tissue models.

Transepithelial paracellular leakiness induced by chronic phorbol ester exposure correlates with polyp-like foci and redistribution of protein kinase C-alpha

Although exposure of LLC-PK1 epithelial cell sheets to phorbol esters (TPA) causes a near immediate and total decrease of transepithelial electrical resistance (TER), continuation of exposure for 3 to 4 days results in a tachyphylactic response as TER begins to return to control levels. Recovery of TER is maximal by 5 to 6 days, but reaches only 70 to 80% of control level. A reciprocal change in the transepithelial flux of D-mannitol indicates that the TER decrease is indicative of an increase in tight junction permeability. Exposure of cell sheets to TPA for several days also results in the appearance of multilayered polyp-like foci (PLFs) across the otherwise one cell layer thick cell sheets. The pattern of penetration of the electron dense dye, ruthenium red, from the apical surface, across the tight junction and into the lateral intercellular space indicates that the tight junctions of the cell sheet become uniformly leaky after acute exposure to TPA. However, when exposure is continued for several days, only the junctions of cells in the PLFs manifest leakiness. The decrease in TER following acute TPA exposure correlates with the translocation of protein kinase C-alpha (PKC alpha) into a membrane-associated compartment. With exposure of several days, only a trace of PKC alpha is visible by Western immunoblot, and this is in the membrane-associated compartment. Immunofluorescent microscopy indicates that the trace of PKC alpha seen in the Western immunoblots is ascribable distinctly to cells of the PLFs. Monolayer areas between PLFs show no discernible immunofluorescent signal. The data therefore indicate that tight junction barrier function may be restored in certain areas by the down regulation of PKC alpha from the membrane-associated compartment. Failure to down regulate may result in the paracellular leakiness and abnormal cell architecture of the PLFs. Possible implications of this model for in vivo epithelial tumor promotion are discussed.

Different size limitations for increased transepithelial paracellular solute flux across phorbol ester and tumor necrosis factor-treated epithelial cell sheets

By observing increases in the transepithelial paracellular permeability of a range of radiolabeled solutes and electron dense dyes, changes in molecular sieving caused by the cytokine, TNF (tumor necrosis factor), and the phorbol ester, TPA (12-0-tetra-decanoylphorbol-13-acetate), were characterized. Using 14C-labeled mannitol (mw 182), raffinose (mw 504), PEG (polyethylene glycol; mw 4000), and dextran (mw 10,000, 70,000 and 2,000,000), the transepithelial flux rates of these compounds were determined at the peak of the transepithelial electrical resistance (TER) changes caused by these two agents. TNF treatment resulted in increased permeability across LLC-PK1 epithelial cell sheets only to relatively small solutes, with an upper limit of approximately 4,000 mw. The low molecular weight "ceiling" for the TNF-treated epithelium is further evidence against TNF increasing transepithelial permeability by means of inducing nonspecific, microscopic "holes" in the epithelium, for which a "ceiling" would not exist. TPA treatment increases transepithelial paracellular permeability to a much broader range of solutes, extending well beyond 2 million mw. Transmission electron micrographs provide evidence that even the electron-dense dye complex, ruthenium red, can cross tight junctions of TPA-treated cell sheets. However, cationic ferritin cannot cross tight junctions of TPA-treated cell sheets. This shows that there is an upper limit to solutes able to cross TPA-treated cell sheets, but that this upper limit will include most proteins, which would then be able to cross tumor promoter-exposed (protein kinase C-activated) epithelial layers at accelerated rates. The biomedical implications for a high molecular weight cutoff in tumor promoter action in epithelial carcinogenesis, and for a low molecular weight cutoff in cytokine-induced epithelial apoptosis in inflammation, are discussed.

Different size limitations for increased transepithelial paracellular solute flux across phorbol ester and tumor necrosis factor-treated epithelial cell sheets

By observing increases in the transepithelial paracellular permeability of a range of radiolabeled solutes and electron dense dyes, changes in molecular sieving caused by the cytokine, TNF (tumor necrosis factor), and the phorbol ester, TPA (12-0-tetra-decanoylphorbol-13-acetate), were characterized. Using 14C-labeled mannitol (mw 182), raffinose (mw 504), PEG (polyethylene glycol; mw 4000), and dextran (mw 10,000, 70,000 and 2,000,000), the transepithelial flux rates of these compounds were determined at the peak of the transepithelial electrical resistance (TER) changes caused by these two agents. TNF treatment resulted in increased permeability across LLC-PK1 epithelial cell sheets only to relatively small solutes, with an upper limit of approximately 4,000 mw. The low molecular weight "ceiling" for the TNF-treated epithelium is further evidence against TNF increasing transepithelial permeability by means of inducing nonspecific, microscopic "holes" in the epithelium, for which a "ceiling" would not exist. TPA treatment increases transepithelial paracellular permeability to a much broader range of solutes, extending well beyond 2 million mw. Transmission electron micrographs provide evidence that even the electron-dense dye complex, ruthenium red, can cross tight junctions of TPA-treated cell sheets. However, cationic ferritin cannot cross tight junctions of TPA-treated cell sheets. This shows that there is an upper limit to solutes able to cross TPA-treated cell sheets, but that this upper limit will include most proteins, which would then be able to cross tumor promoter-exposed (protein kinase C-activated) epithelial layers at accelerated rates. The biomedical implications for a high molecular weight cutoff in tumor promoter action in epithelial carcinogenesis, and for a low molecular weight cutoff in cytokine-induced epithelial apoptosis in inflammation, are discussed.

Chronic exposure of LLC-PK1 epithelia to the phorbol ester TPA produces polyp-like foci with leaky tight junctions and altered protein kinase C-alpha expression and localization

Acute exposure (up to 4 h) of LLC-PK1 epithelial cell sheets to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) causes a rapid decrease of transepithelial electrical resistance (TER) to less than 15% of initial value. As the TPA exposure period is continued by chronically passaging cells in the presence of TPA, TER begins to recover. By 6 weeks of exposure, TER recovers to almost 50% of its initial value, suggesting that tight junctions (TJs) are recovering barrier function even in the continued presence of TPA. Between 6 and 8 weeks, TER then decreases a second time to approximately 20 to 40% of initial values, and TER values remain at this level for at least 18 weeks of exposure. Transepithelial (paracellular) fluxes of D-mannitol inversely correspond with TER changes. Across chronically treated cell sheets, rates are higher than those across control cell sheets, but lower than those across acutely treated cell sheets. The decrease of TER at 6-8 weeks coincides with the appearance of multilayered, polyp-like foci (PLFs) on the otherwise one cell layer thick epithelium. Electron microscopy shows that the electron-dense dye ruthenium red cannot penetrate across TJs of control cells but passes across all of the TJs of a cell sheet treated acutely with TPA. In chronically treated cultures, ruthenium red penetrates TJs between most cells of PLFs, but not TJs of adjacent morphologically normal epithelium. A clonal subline derived from cells of a PLF (clone PLF-A) is multilayered almost throughout and exhibits ruthenium red penetration across nearly all of its tight junctions, monolayer or multilayer. Acute exposure of control cell sheets to TPA induces activation, translocation, and down-regulation of protein kinase C-alpha (PKC-alpha). In chronically TPA-treated and clone PLF-A cells, total PKC-alpha levels are reduced even further and almost all remaining PKC-alpha is found in the membrane-associated and Triton-insoluble fractions. Immunofluorescence shows that PKC-alpha expression is restricted to the PLFs in chronically TPA-treated cells and is more homogeneously distributed in clone PLF-A cultures. In summary, the data show that chronic treatment of epithelial cells with a tumor promoter induces the formation of abnormal cell architecture (PLFs) associated with increased leakiness of TJs and membrane translocation of PKC-alpha. Recovery of barrier function in portions of chronically TPA-treated cultures does not correlate with up-regulation of PKC-alpha nor translocation back to the cytosolic compartment.

Sodium-independent carrier-mediated inositol transport in cultured renal epithelial (LLC-PK1) cells

In addition to the concentrative, Na(+)-dependent inositol transport system demonstrated in many cell types, carrier-mediated, Na(+)-independent inositol transport is also shown to exist in LLC-PK1 renal epithelia. Inhibition of inositol uptake in Na(+)-free saline by 0.1 mM phloretin, and self-inhibition by net concentrations of inositol exceeding 10 mM, demonstrate the carrier-mediation of the Na(+)-independent uptake and distinguish it from flux through anion channels. The Na(+)-dependent uptake exhibits higher affinity for inositol, as seen by the stronger self-inhibition at lower inositol concentrations in Na+ saline. Kinetic analyses indicate a Km of 178 microM and a Vmax of 2447 pmol/min per microgram DNA for the Na(+)-dependent system, whereas the lower affinity, lower capacity Na(+)-independent system manifests a Km of 5.2 mM and a Vmax of 249 pmol/min per microgram DNA. the Na(+)-independent uptake further differs from the Na(+)-dependent transport by the lack of inhibitory effect of 10 microM glucose, and the greater relative inhibition of phloretin compared to that of phlorizin. Both types of uptake appear to localize predominantly to the basal-lateral cell surface. The Na(+)-independent transport is bidirectional, functioning in efflux as well as influx of inositol.

The protein kinase C inhibitor, bisindolylmaleimide, inhibits the TPA-induced but not the TNF-induced increase in LLC-PK1 transepithelial permeability

The transepithelial paracellular permeability of an epithelium formed by LLC-PK1 cells increases upon activation of protein kinase C (PKC) by the phorbol ester tumor promoter, TPA, or in response to the cytokine tumor necrosis factor-alpha (TNF). Until recently, however, we have not been able to inhibit the permeability effects of TPA or TNF using any of the currently available serine-threonine kinase inhibitors. In this study we report the treatment of epithelial cell sheets with the selective PKC inhibitor bisindolylmaleimide, GF109203X, completely prevents the TPA-induced but not the TNF-alpha induced increase in tight junction permeability. While PKC-alpha still translocates from the cytosol to the membrane of TPA-stimulated epithelial cells overall PKC activity in the membrane fraction is markedly reduced in the presence of GFX.

cAMP modulates transepithelial resistance response of LLC-PK1 renal epithelia to tumor necrosis factor

For "leaky" epithelia the transepithelial resistance (Rt) is an electrophysiological measure of the paracellular pathway within the epithelial barrier. The Rt across a monolayer of LLC-PK1 porcine renal epithelial cells is specifically an inverse measure of paracellular transepithelial permeability and displays a multiphasic and reversible response to the cytokine tumor necrosis factor-alpha (TNF). The Rt response to TNF can be inhibited by the nonhydrolyzable adenosine 3',5'-cyclic monophosphate (cAMP) analogue, dibutyryl-cAMP. In addition, activation of adenylate cyclase (forskolin) or inhibition of phosphodiesterase (3-isobutyl-1-methylxanthine, Ro-20-1724, and pentoxifylline), each of which have been reported to elevate cellular cAMP levels, also inhibited the Rt response to TNF. Incubation of the LLC-PK1 cell sheet with N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, an inhibitor of cAMP-dependent protein kinase (PKA), potentiated the Rt response to TNF. The Rt response to TNF was completely prevented by preincubation of the cultures with cholera toxin, whereas pertussis toxin pretreatment had a slight but significant potentiating effect on the response. Pretreatment with cholera toxin was associated with an approximately 18-fold elevation in cAMP levels in both control and TNF-treated cultures. Measurements of cellular cAMP content at selected intervals after TNF administration showed a significant elevation (P < 0.01) of 140% above time-matched controls at 1 h after the administration of TNF to the cell sheet. The level of cAMP then declined to approximate control level within 2.5 h of TNF administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of tumor necrosis factor on LLC-PK1 transepithelial resistance

Renal epithelial LLC-PK1 cell sheets incubated with tumor necrosis factor (TNF) undergo an acute, spontaneous, and rapidly reversible decrease in transepithelial resistance (TER). (Mullin et al., 1992). However, 24 to 72 h following TNF exposure, TER across the cell sheet increases 2-fold. This later effect of TNF is also reversible, albeit slowly. The TER of TNF-treated cell sheets then declines toward initial levels between 72 and 144 h following exposure to the cytokine. Whereas the long-term increase in TER following TNF exposure is not associated with a decreased transepithelial 14C-mannitol flux (size selectivity), the charge (anionic) selectivity of the LLC-PK1 tight junction is decreased. Basal-lateral (ouabain and bumetanide-insensitive) Rb+ and apical Na+-dependent alpha-methylglucoside (AMG) uptake into the cell are both reduced in cultures exposed to TNF 24 h earlier. Correspondingly, this long-term effect on TER is accompanied by a 30% decrease in short circuit current (iscc). Along with an observed increase in basal-lateral methylamino-isobutyric acid (MeAIB) influx into the cells, an increased incorporation of [3H]-thymidine into DNA indicates increased cell cycling after exposure to TNF. While the increase in cell cycling is not sustained for the duration of the elevation in TER, it does appear to initiate a sequence of events that lead to the sustained increase in TER. A decrease in the lateral intercellular space, observed between these epithelial cells after long-term TNF exposure, may be a mechanism for the elevated TER following from the mitogenesis and/or transport changes. This overall long-term tightening of an epithelium in response to TNF may function, in part, as a compensatory action of the epithelium to reestablish its effectiveness as a physiological barrier, following the acute effect of TNF.

Effects of epidermal growth factor versus phorbol ester on kidney epithelial (LLC-PK1) tight junction permeability and cell division

In polarized epithelia, the tight junctions (TJs) constitute a barrier that controls the paracellular flux of solutes and water. In the renal LLC-PK1 cells, the TJ permeability can be correlated directly with the unidirectional transepithelial flux of solutes, such as D-mannitol, which have negligible affinity for cell membrane transport systems, and inversely to the transepithelial electrical resistance (TER). This study investigates TJ permeability and cell proliferation in LLC-PK1 cells treated with the phorbol ester, tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) or with epidermal growth factor (EGF), a mitogen without secondary carcinogenic effects. Both TPA and EGF induced mitogenesis in LLC-PK1 cells. The TJs in TPA-treated cells were leaky, as indicated by decreased TER, increased D-mannitol flux, and TJ penetration by ruthenium red. In contrast, EGF treatment did not result in a decrease in TER, only slightly increased the D-mannitol flux, and did not result in ruthenium red penetration of the TJs. This inability of ruthenium red to penetrate TJs between EGF-treated epithelial cells was true even for cells in mitosis. The data therefore indicate that mitogenesis per se does not increase TJ permeability, suggesting that the TJ leakiness observed during tumor promotion with phorbol esters does not arise from cell proliferation and is perhaps associated distinctly with or causal to the transformation of those cells.

Modulation of tumor necrosis factor-induced increase in renal (LLC-PK1) transepithelial permeability

Tumor necrosis factor-alpha (TNF) causes a spontaneously reversible increase in tight junction permeability. TNF was the only cytokine tested that produced this effect. The effect on transepithelial permeability proceeds in four distinct phases: 1) a 60- to 90-min delay from time of application of TNF, 2) a rapid decrease in transepithelial resistance, 3) a recovery of transepithelial resistance to control level within 1 h, and 4) a further increase of transepithelial resistance above control levels. The recovery of transepithelial resistance occurs with or without TNF in the culture medium. Different protein kinase inhibitors affected different phases of this overall process. The tyrosine kinase inhibitor genistein significantly blocked the TNF effect. Neither transcription nor protein synthesis was required for transepithelial permeability to increase, but were required for the recovery. After the tight junctions have opened at 2 h in response to TNF, a second application of TNF will not produce the effect again for at least 12 h. The tight junctions will, however, open in response to phorbol esters during this time frame. Electron microscopy studies using apically applied ruthenium red suggest that TNF action results in < 10% of the junctions having increased permeability at any given time during the resistance decrease. The role of epithelial barrier permeability changes in TNF action in vivo is discussed.