Polarized secretion of plasminogen activators by epithelial cell monolayers

A plasminogen-like protease in thyroid rough microsomes degrades thyroperoxidase and thyroglobulin

Proteasome activity takes place in the cytosolic compartment and acts to degrade several proteins translated and unfolded. In transfected CHO cells expressing thyroid peroxidase (TPO), just-translated TPO undergoes proteasome activity, and then a second proteolytic system degrades more mature forms of TPO. A plasminogen-like (Pl-like) protease is found in microsomal liver membranes and in the thyroid. In the thyroid, this Pl-like protease is localized in the follicular lumen and efficiently degrades thyroglobulin (Tg) in vitro. Here we checked for the presence, in purified endoplasmic reticulum (ER) membranes of transfected CHO and in rough microsomes purified from thyroid tissue, of a second proteolytic system, different from the proteasome, and active against the two major proteins of the thyroid gland, TPO and Tg. We first confirmed that this proteolytic system was able to degrade folded endogenous TPO. We showed also that externally added TPO (folded form) was degraded by opened vesicles of ER in the same system. For thyroid tissue, we showed that added TPO, as well as purified Tg, was degraded by some unknown membrane-associated protease(s) in human and porcine thyroid rough microsomes, whereas BSA and IgG were not. These results indicated that major thyroid glycoproteins are preferential substrates of such protease(s). Immunoblot and zymography experiments identified the unknown membrane-associated protease in rough microsomes from thyroid tissues as being a Pl-like protease. These results highly suggest that this system acts as a nonproteasomal degradation enzyme at the ER level, and we hypothesize that it contributes in regulating the level of major thyroid glycoproteins.

Secretion of proteases in serglycin transfected Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells, which do not normally express the proteoglycan (PG) serglycin, were stably transfected with cDNA for human serglycin fused to a polyhistidine tag (His-tag). Clones with different levels of serglycin mRNA expression were generated. One clone with lower and one with higher serglycin mRNA expression were selected for this study. 35S-labelled serglycin in cell fractions and conditioned media was isolated using HisTrap affinity chromatography. Serglycin could also be detected in conditioned media using western blotting. To investigate the possible importance of serglycin linked to protease secretion, enzyme activities using chromogenic substrates and zymography were measured in cell fractions and serum-free conditioned media of the different clones. Cells were cultured in both the absence and presence of phorbol 12-myristate 13-acetate (PMA). In general, enzyme secretion was strongly enhanced by treatment with PMA. Our analyses revealed that the clone with the highest serglycin mRNA expression, level of HisTrap isolated 35S-labelled serglycin, and amount of serglycin core protein as detected by western blotting, also showed the highest secretion of proteases. Transfection of serglycin into MDCK cells clearly leads to changes in secretion levels of secreted endogenous proteases, and could provide further insight into the biosynthesis and secretion of serglycin and potential partner molecules.

A plasminogen-like protein, present in the apical extracellular environment of thyroid epithelial cells, degrades thyroglobulin in vitro

The prothyroid hormone, thyroglobulin (Tg), is stored at high concentrations in the thyroid follicular lumen as a soluble 19S homo-dimer and as heavier soluble (27S and 37S) and insoluble (Tgm) forms. Follicular degradation of Tg may contribute to maintaining Tg concentrations compatible with follicle integrity. Here, we report on the presence of a plasminogen-like protein in the follicular lumen of normal human thyroids and its synthesis and apical secretion by cultured epithelial thyroid cells. Since all the main luminal forms of Tg are cleaved by this plasminogen-like protein, we suggest that it contributes to Tg degradation in the follicular lumen.

Proliferative effects of apical, but not basal, matrix metalloproteinase-7 activity in polarized MDCK cells

Matrix metalloproteinase-7 (MMP-7) is primarily expressed in glandular epithelium. Therefore, its mechanism of action may be influenced by its regulated vectorial release to either the apical and/or basolateral compartments, where it would act on its various substrates. To gain a better understanding of where MMP-7 is released in polarized epithelium, we have analyzed its pattern of secretion in polarized MDCK cells expressing stably transfected human MMP-7 (MDCK-MMP-7), and HCA-7 and Caco2 human colon cancer cell lines. In all cell lines, latent MMP-7 was secreted to both cellular compartments, but was 1.5- to 3-fold more abundant in the basolateral compartment as compared to the apical. However, studies in the MDCK system demonstrated that MMP-7 activity was 2-fold greater in the apical compartment of MDCK-MMP-7(HIGH)-polarized monolayers, which suggests the apical co-release of an MMP-7 activator. In functional assays, MMP-7 over-expression increased cell saturation density as a result of increased cell proliferation with no effect on apoptosis. Apical MMP-7 activity was shown to be responsible for the proliferative effect, which occurred, as demonstrated by media transfer experiments, through cleavage of an apical substrate and not through the generation of a soluble factor. Taken together, our findings demonstrate the importance of MMP-7 secretion in relation to its mechanism of action when expressed in a polarized epithelium.

The role of proteases in fibronectin matrix remodeling in thyroid epithelial cell monolayer cultures

Fischer rat thyroid (FRT) cells organize a matrix of extracellular fibronectin (FN) fibrils, which undergoes extensive remodeling according to cell culture confluence. In non-confluent cells FN forms a fibrillar array associated with the ventral cell surface. However, basal FN is progressively removed in confluent cultures and substituted by non-fibrillar FN deposits at lateral cell domains in regions of cell-cell contacts. FRT cells secrete and expose on the plasma membrane the tissue-type plasminogen activator and, in serum-free cultures, plasminogen induces a rapid loss of FN fibrils. Incubation with plasmin inhibitors greatly reduces this effect. FRT cells also express annexin II, a plasminogen receptor, suggesting that plasmin activity is associated with the pericellular enviroment. This is in agreement with the observation that a great reduction in FN degradation is observed if the cells are pre-incubated with carboxypeptidase B, which prevents plasminogen binding to the cells. A gelatinolytic activity with a molecular weigth equivalent to MMP-2 has been demonstrated by zymography of culture media, and the presence of MMP-2 and MT1-MMP on the cell plasma membrane has been detected by immunofluorescence. These results indicate that in the FN remodeling process, occurring during FRT epithelium maturation, both plasmin-dependent (tPA activated) and plasmin-independent proteolytic activities are involved.

Urothelial function reconsidered: a role in urinary protein secretion

Mammalian bladder epithelium functions as an effective permeability barrier. We demonstrate here that this epithelium can also function as a secretory tissue directly involved in modifying urinary protein composition. Our data indicate that normal bovine urothelium synthesizes, as its major differentiation products, two well-known proteases: tissue-type plasminogen activator and urokinase, as well as a serine protease inhibitor, PP5. Moreover, we demonstrate that the urothelium secretes these proteins in a polarized fashion into the urine via a cAMP- and calcium-regulated pathway. Urinary plasminogen activators of ruminants are therefore urothelium derived rather then kidney derived as in some other species; this heterogeneity may have evolved in response to different physiological or dietary factors. In conjunction with our recent finding that transgenic mouse urothelium can secrete ectopically expressed human growth hormone into the urine, our data establish that normal mammalian urothelium can function not only as a permeability barrier but also as a secretor of urinary proteins that can play physiological or pathological roles in the urinary tract.

Hormonal control of urokinase plasminogen activator secretion by sheep ovarian surface epithelial cells

Secretion of urokinase plasminogen activator (uPA) by ovarian surface epithelium (OSE) adjacent to the preovulatory ovine follicle has been implicated in apical tissue degradation and follicular rupture. In vitro experiments were designed to test the hypothesis that uPA release by OSE is under direct hormonal control. Epithelial cells were isolated from the ovarian surface of sheep using a polytetrafluorethylene scraper designed to dislodge adherent cells from culture flasks. Amidolytic cleavage of a uPA-specific chromogen (carbobenzoxy-L-gamma-glutamyl [alpha-ot-but]-glycyl-arginine-p-nitroanilide monoacetate) was used as a measure of enzymatic bioactivity in OSE-conditioned incubation media. Secretion of uPA by OSE suspensions from proestrous ewes was stimulated by exposure (2 h) to a preovulatory surge-like concentration of LH. OSE cells obtained during the luteal phase or anestrus were not responsive to LH. Baseline rates of uPA secretion and expression of estradiol receptors (in situ immunofluorescence detection) were not affected by reproductive status. Induction of uPA secretion by anestrous OSE was attained after priming (6 h) with estradiol-17beta; responsiveness was attributed to gonadotropin receptor (ligand binding) up-regulation. Monolayers of OSE established on polyethylene membranes secreted uPA predominately in a basal (i.e., toward the substratum) direction. We suggest that OSE in juxtaposition with the (hyperemic) wall of the preovulatory follicle is perfused by surge levels of LH, invoking uPA release into underlying ovarian tissues.

Mechanisms of apical protein sorting in polarized thyroid epithelial cells

The process leading to thyroid hormone synthesis is vectorial and depends upon the polarized organization of the thyrocytes into the follicular unit. Thyrocyte membrane proteins are delivered to two distinct domains of the plasma membrane using apical (AP) and basolateral (BL) sorting signals. A recent hypothesis for AP sorting proposes that apically destined proteins cluster with glycosphingolipids (GSLs) and cholesterol, into microdomains (or rafts) of the Golgi membrane from which AP vesicles originate. In MDCK cells the human neurotrophin receptor, p75hNTR, is delivered to the AP surface through a sorting signal, rich in O-glycosylated sugars, identified in its ectodomain. We have investigated whether this signal is functional in the thyroid-derived FRT cell line and whether p75hNTR clusters into lipid rafts to be sorted to the AP membrane. We found that p75hNTR is apically delivered via a direct pathway and does not associate with rafts during its transport to the surface of FRT cells. Therefore, although the same signal could be recognized by different cell types thyroid cells may possess a tissue-specific sorting machinery.

RhoA activity is required for fibronectin assembly and counteracts beta1B integrin inhibitory effect in FRT epithelial cells

FRT thyroid epithelial cells synthesize fibronectin and organize a network of fibronectin fibrils at the basal surface of the cells. Fibronectin fibril formation is enhanced by the overexpression of the ubiquitous beta1A integrin and is inhibited by the expression of the dominant-negative beta1B subunit. We tested the hypotheses that RhoA activity might mediate the integrin-dependent fibronectin fibrillogenesis and might counteract beta1B integrin inhibitory effect. FRT-beta1A cells were transfected with a vector carrying a dominant negative form of RhoA (RhoAN19) or treated with the C3 transferase exoenzyme. Both treatments inhibited fibronectin assembly and caused loss of actin microfilaments and adhesion plaques. On the other hand, FRT-beta1B cells were transfected with the constitutively activated form of RhoA (RhoAV14) or treated with the E. coli cytotoxic necrotizing factor 1, which directly activates RhoA. Either treatment restored microfilament and adhesion plaque assembly and promoted fibronectin fibril organization. A great increase in fibronectin fibril assembly was also obtained by treatment of FRT-beta1B cells with TGF-beta. Our data indicate that RhoA is required to promote fibronectin matrix assembly in FRT cells and that the activation of the signal transduction pathway downstream of RhoA can overcome the inhibitory effect of beta1B integrin.

Beta1B integrin interferes with matrix assembly but not with confluent monolayer polarity, and alters some morphogenetic properties of FRT epithelial cells

Beta1B is a beta1 integrin splice variant that differs from the ubiquitous beta1A in the terminal portion of the cytosolic tail. The expression of this variant in CHO cells results in reduced fibroblast adhesion and motility (Balzac, E et al., J. Cell Biol. 127, 557-565 (1994)). We have evaluated the phenotypic changes induced by the expression of beta1B in the FRT epithelial cell line. Stable transfectants of FRT cells expressing beta1B or beta1A human integrins were obtained. The transfected integrins associated with the endogenous alpha subunits and were delivered to the plasma membrane. Beta1B expressing cells attached less efficiently and spread less on fibronectin, laminin or type IV collagen coated dishes. A great reduction of fibronectin fibrils associated to the basal membrane of non-confluent beta1B transfected cells was observed. This was paralleled by the disappearance of microfilament bundles and loss of basally located focal adhesions. On the contrary, upon beta1A transfection, a higher amount of fibronectin fibrils, together with microfilament bundles and focal adhesions, was observed. Expression of beta1B did not significantly modify the ability to manifest the polarized phenotype when cells were grown to confluence on filters in two-chamber-systems. Beta1B-transfected cells showed reduced motile properties when embedded as aggregates in type I collagen gels. Moreover, formation of polarized cysts in suspension culture was impaired. The results show that beta1B, by interfering with focal adhesion organization, microfilament and fibronectin assembly, cell spreading and migration, affects some morphogenetic properties of FRT epithelial cells.

Non-polarized secretion of mouse interferon-beta from gene-transferred human intestinal Caco-2 cells

PURPOSE

The intestinal epithelium is considered to be a feasible target for somatic gene therapy. To this end, Caco-2 cells derived from human colon carcinoma were transfected with a mouse interferon-beta (IFN-beta) expression vector and several stable sublines were established; this hetero-specific cytokine allows unexpected cellular effects to be avoided. Using the highest mouse IFN-beta-producing sublines, the mode of IFN secretion was examined.

METHODS

The secretion polarity of mouse IFN-beta in its gene-transduced Caco-2 sublines was studied in a bicameral culture system in which the chambers were separated by microporous filters.

RESULTS

Mouse IFN-beta was secreted to the same extent from both apical and basolateral surfaces of the transduced cells regardless of cell aging.

CONCLUSIONS

These results suggest that in the intestinal epithelium exogenous gene products such as IFNs can be delivered to both the luminal and blood sides in vivo. Thus, the intestinal epithelium may be suitable for systemic and local delivery of therapeutic proteins by gene transfer.

Polarity of constitutive and regulated von Willebrand factor secretion by transfected MDCK-II cells

Von Willebrand factor (vWF), synthesized by endothelial cells, is both rapidly secreted by the constitutive pathway and stored in Weibel-Palade bodies. Secretion from these organelles occurs upon activation of the protein kinase C signal transduction pathway and yields highly multimerized vWF. Highly multimerized vWF acts as a more effective adhesive ligand than the lower molecular weight forms that are constitutively secreted. We employed the extensively characterized polar Madin-Darby Canine Kidney II (MDCK-II) epithelial cell line, stably transfected with full-length vWF cDNA or deletion mutants thereof, to gain insight in the polarity of vWF secretion by either one of the two pathways. Immunofluorescence analysis and metabolic labeling experiments revealed that multimeric "wild-type" vWF is stored in MDCK-II cells and released upon stimulation with phorbol esters. Furthermore, we show that 62.0 +/- 3.8% of constitutively secreted and 83.2 +/- 6.6% of the regulated secreted wild-type vWF is encountered at the apical side of the cell. The polarity of the constitutive secretion of deletion mutant vWFdelD'D3 is similar to that of constitutively secreted wild-type vWF, whereas deletion mutant vWFdelD1D2 displays no polar secretion (50.1 +/- 5.7% apical).

Direct targeting of neutral endopeptidase (EC 3.4.24.11) to the apical cell surface of transfected LLC-PK1 cells and unpolarized secretion of its soluble form

LLC-PK1 cells were transfected with a cDNA encoding rabbit neutral endopeptidase (NEP; EC 3.4.24.11), an abundant enzyme of the kidney proximal brush border. Clones of cells expressing high levels of the protein were isolated. Selective biotinylation and radioimmunolabelling were used to determine that 85-95% of NEP was localized in the apical domain of filter-grown LLC-PK1 cells. Pulse-chase and selective biotinylation studies revealed that the majority (85%) of newly made NEP was directly targeted to the apical membrane. However, a soluble form of NEP was found to be secreted in approximately equal amounts from both sides of the monolayer when expressed in LLC-PK1 cells. Transfected pro-opiomelanocortin, a pituitary hormone precursor, was secreted almost exclusively into the basolateral medium, suggesting that the bulk flow is to the basolateral membrane. This behaviour contrasts with that observed in MDCK cells, where both the transmembrane and secreted forms of NEP are directly targeted to the apical membrane and where the secretion of pro-opiomelanocortin is unpolarized.

Uteroglobin, an apically secreted protein of the uterine epithelium, is secreted non-polarized form MDCK cells and mainly basolaterally from Caco-2 cells

A complete cDNA encoding rabbit uteroglobin was constructed and expressed in MDCK and Caco-2 cells. The MDCK cells secrete uteroglobin in approximately equal amounts to the apical and the basolateral side, whereas the Caco-2 cells secrete uteroglobin mainly to the basolateral side. Both MDCK and Caco-2 cells thus secrete uteroglobin in a non-sorted manner. It has, however, previously been shown that uteroglobin is secreted exclusively at the apical membrane in primary cell culture of endometrial epithelial cells [S.K. Mani et al. (1991) Endocrinology 128, 1563-1573]. This suggests that either the endometrial epithelium has an apical default pathway or recognises a sorting signal not recognised by MDCK cells and Caco-2 cells. Our data thus show that a soluble molecule can be secreted at the apical, the basolateral or both membranes depending on the cell type.