Rapid sequestration of DPP IV/CD26 and other cell surface proteins in an autophagic-like compartment in Caco-2 cells treated with forskolin

Enhanced oral and pulmonary delivery of biomacromolecules via amplified transporter targeting

Ligand-modified nanocarriers can promote oral or inhalative administration of macromolecular drugs across the intestinal or pulmonary mucosa. However, enhancing the unidirectional transport of the nanocarriers through "apical uptake→intracellular transport→basolateral exocytosis" route remains a hot topic and challenge in current research. Forskolin is a naturally occurring diterpenoid compound extracted from the roots of C. forskohlii. In our studies, we found that forskolin could increase the transcellular transport of butyrate-modified nanoparticles by 1.67-fold and 1.20-fold in Caco-2 intestinal epithelial cell models and Calu-3 lung epithelial cell models, respectively. Further mechanistic studies revealed that forskolin, on the one hand, promoted the cellular uptake of butyrate-modified nanoparticles by upregulating the expression of monocarboxylic acid transporter-1 (MCT-1) on the apical membrane. On the other hand, forskolin facilitated the binding of MCT-1 to caveolae, thereby mediating butyrate-modified nanoparticles hijacking caveolae to promote the basolateral exocytosis of butyrate-modified nanoparticles. Studies in normal mice model showed that forskolin could promote the transmucosal absorption of butyrate-modified nanoparticles by >2-fold, regardless of oral or inhalative administration. Using semaglutide as the model drug, both oral and inhalation delivery approaches demonstrated significant hypoglycemic effects in type 2 diabetes mice model, in which inhalative administration was more effective than oral administration. This study optimized the strategies aimed at enhancing the transmucosal absorption of ligand-modified nanocarriers in the intestinal or pulmonary mucosa.

A CRISPR screen in intestinal epithelial cells identifies novel factors for polarity and apical transport

Epithelial polarization and polarized cargo transport are highly coordinated and interdependent processes. In our search for novel regulators of epithelial polarization and protein secretion, we used a genome-wide CRISPR/Cas9 screen and combined it with an assay based on fluorescence-activated cell sorting (FACS) to measure the secretion of the apical brush-border hydrolase dipeptidyl peptidase 4 (DPP4). In this way, we performed the first CRISPR screen to date in human polarized epithelial cells. Using high-resolution microscopy, we detected polarization defects and mislocalization of DPP4 to late endosomes/lysosomes after knockout of TM9SF4, anoctamin 8, and ARHGAP33, confirming the identification of novel factors for epithelial polarization and apical cargo secretion. Thus, we provide a powerful tool suitable for studying polarization and cargo secretion in epithelial cells. In addition, we provide a dataset that serves as a resource for the study of novel mechanisms for epithelial polarization and polarized transport and facilitates the investigation of novel congenital diseases associated with these processes.

Hypoxia Alters the Expression of Dipeptidyl Peptidase 4 and Induces Developmental Remodeling of Human Preadipocytes

Dipeptidyl peptidase 4 (DPP4), a transmembrane protein, has been identified in human adipose tissue and is considered to be associated with obesity-related type 2 diabetes. Since adipose tissue is relatively hypoxic in obese participants, we investigated the expression of DPP4 in human preadipocytes (hPA) and adipocytes in hypoxia, during differentiation and upon insulin stimulation. The results show that DPP4 is abundantly expressed in hPA but very sparsely in adipocytes. During differentiation in vitro, the expression of DPP4 in hPA is reduced on the addition of differentiation medium, indicating that this protein can be hPA marker. Long term hypoxia altered the expression of DPP4 in hPA. In in vitro hypoxic conditions the protease activity of shed DPP4 is reduced; however, in the presence of insulin, the increase in DPP4 expression is potentiated by hypoxia.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

DAMPs and PDT-mediated photo-oxidative stress: exploring the unknown

Damage-associated molecular patterns (DAMPs) or cell death associated molecular patterns (CDAMPs) are a subset of endogenous intracellular molecules that are normally hidden within living cells but become either passively released by primary and secondary necrotic cells or actively exposed and secreted by the dying cells. Once released, DAMPs are sensed by the innate immune system and act as activators of antigen-presenting cells (APCs) to stimulate innate and adaptive immunity. Cancer cells dying in response to a subset of conventional anticancer modalities exhibit a particular composition of DAMPs at their cell surface, which has been recently shown to be vital for the stimulation of the host immune system and the control of residual disease. Photodynamic therapy (PDT) for cancer has long been shown to be capable of killing malignant cells and concomitantly stimulate the host immune system, properties that are likely linked to its ability of inducing exposure/release of certain DAMPs. PDT, by evoking oxidative stress at specific subcellular sites through the light activation of organelle-associated photosensitizers, may be unique in incorporating tumour cells destruction and antitumor immune response in one therapeutic paradigm. Here we review the current knowledge about mechanisms and signalling cascades leading to the exposure of DAMPs at the cell surface or promoting their release, the cell death mechanism associated to these processes and its immunological consequences. We also discuss how certain PDT paradigms may yield therapies that optimally stimulate the immune system and lead to the discovery of new DAMPs.

A novel monoclonal antibody against the second extracellular loop of occludin disrupts epithelial cell polarity

The tight junction (TJ) regulates epithelial cell polarity and paracellular permeability. In the present study, to investigate whether the second extracellular loop of occludin affects the localization of carcinoembryonic antigen (CEA) and CD26 expressed on apical membranes, and the fence function of the TJ, the human intestinal epithelial cell line T84 was treated with the monoclonal anti-occludin antibody (MAb) 1H8, corresponding to the second extracellular loop of occludin. In T84 cells treated with MAb 1H8, occludin disappeared, and CEA and CD26 were observed to diffuse from the apical membrane to the basolateral membrane. Furthermore, a decrease in the fence function of TJ was observed without changes in the TJ strands and barrier function. When T84 cells precultured in low calcium (Ca) medium were recultured in normal Ca medium in the presence of MAb 1H8, recruitment of occludin to the apical-most membranes and recovery in distribution of CEA and CD26 were markedly retarded compared with the control. These results suggested that MAb 1H8 against the second extracellular loop of occludin selectively affected formation of the apical/basolateral intramembrane diffusion barrier and that the second extracellular loop of occludin plays a crucial role in the maintenance of epithelial cell polarity by the TJ.

Clearance of dying autophagic cells of different origin by professional and non-professional phagocytes

MCF-7 cells undergo autophagic death upon tamoxifen treatment. Plated on non-adhesive substratum these cells died by anoikis while inducing autophagy as revealed by monodansylcadaverine staining, elevated light-chain-3 expression and electron microscopy. Both de novo and anoikis-derived autophagic dying cells were engulfed by human macrophages and MCF-7 cells. Inhibition of autophagy by 3-methyladenine abolished engulfment of cells dying through de novo autophagy, but not those dying through anoikis. Blocking exposure of phosphatidylserine (PS) on both dying cell types inhibited phagocytosis by MCF-7 but not by macrophages. Gene expression profiling showed that though both types of phagocytes expressed full repertoire of the PS recognition and signaling pathway, macrophages could evolve during engulfment of de novo autophagic cells the potential of calreticulin-mediated processes as well. Our data suggest that cells dying through autophagy and those committing anoikis with autophagy may engage in overlapping but distinct sets of clearance mechanisms in professional and non-professional phagocytes.

Rotavirus impairs the biosynthesis of brush-border-associated dipeptidyl peptidase IV in human enterocyte-like Caco-2/TC7 cells

Rotavirus is the leading cause of severe dehydrating diarrhoea in infants and young children worldwide. This virus infects mature enterocytes in the small intestine, and induces structural and functional damage. In the present study, we have identified a new mechanism by which rotavirus impairs a brush border-associated intestinal protein. We show that infection of enterocyte-like Caco-2/TC7 cells by rhesus monkey rotavirus (RRV) impairs the biosynthesis of dipeptidyl peptidase IV (DPP IV), an important hydrolase in the digestion of dietary proline-rich proteins. We show that the enzyme activity of DPP IV was reduced, and that rearrangements of the protein occurred at the apical domain of the RRV-infected cells. Using pulse-chase experiments and cell surface immunoprecipitation, we have demonstrated that RRV infection did not affect the stability or apical targeting of DPP IV, but did induce a dramatic decrease in its biosynthesis. Using quantitative RT-PCR, we showed that RRV had no effect on the level of expression of DPP IV mRNA, suggesting that the observed decrease in the biosynthesis of the protein is related to an effect of the virus at the translational level.

Adenosine downregulates DPPIV on HT-29 colon cancer cells by stimulating protein tyrosine phosphatase(s) and reducing ERK1/2 activity via a novel pathway

The multifunctional cell-surface protein dipeptidyl peptidase IV (DPPIV/CD26) is aberrantly expressed in many cancers and plays a key role in tumorigenesis and metastasis. Its diverse cellular roles include modulation of chemokine activity by cleaving dipeptides from the chemokine NH(2)-terminus, perturbation of extracellular nucleoside metabolism by binding the ecto-enzyme adenosine deaminase, and interaction with the extracellular matrix by binding proteins such as collagen and fibronectin. We have recently shown that DPPIV can be downregulated from the cell surface of HT-29 colorectal carcinoma cells by adenosine, which is a metabolite that becomes concentrated in the extracellular fluid of hypoxic solid tumors. Most of the known responses to adenosine are mediated through four different subtypes of G protein-coupled adenosine receptors: A(1), A(2A), A(2B), and A(3). We report here that adenosine downregulation of DPPIV from the surface of HT-29 cells occurs independently of these classic receptor subtypes, and is mediated by a novel cell-surface mechanism that induces an increase in protein tyrosine phosphatase activity. The increase in protein tyrosine phosphatase activity leads to a decrease in the tyrosine phosphorylation of ERK1/2 MAP kinase that in turn links to the decline in DPPIV mRNA and protein. The downregulation of DPPIV occurs independently of changes in the activities of protein kinases A or C, phosphatidylinositol 3-kinase, other serine/threonine phosphatases, or the p38 or JNK MAP kinases. This novel action of adenosine has implications for our ability to manipulate adenosine-dependent events within the solid tumor microenvironment.

Cyclic AMP-dependent protein kinase A negatively modulates adherens junction integrity and differentiation of intestinal epithelial cells

Intestinal epithelial cell differentiation is a complex process in which many different signaling pathways are likely involved. An increase in the intracellular levels of cyclic AMP (cAMP) has been shown to inhibit enterocyte differentiation; however, the mechanisms through which cAMP/PKA signaling modulates differentiation of human intestinal epithelial cells are still not well understood. Herein, we report that: (1) treatment of Caco-2/15 cells with 8Br-cAMP repressed sucrase-isomaltase and villin protein expression and strongly attenuated morphological differentiation of enterocyte-like features in Caco-2/15 such as epithelial cell polarity and brush border formation; (2) treatment of confluent Caco-2/15 cells with 8Br-cAMP led to a strong decrease in F-actin localized at cell-cell contact sites along with a reduced amount of E-cadherin and catenins, but not of ZO-1, at cell-cell interfaces concomitant with a decreased association of these proteins with the actin cytoskeleton; (3) inhibition of PKA by H89 prevented disruption of adherens junctions by extracellular calcium depletion; (4) treatment of Caco-2/15 cells with 8Br-cAMP prevented the recruitment and activation of p85/PI-3K to E-cadherin-mediated cell-cell contacts, an important event in the assembly of adherens junctions and differentiation of these cells; (5) E-cadherin appears to be phosphorylated on serine in vivo in a PKA-dependent mechanism.

CONCLUSION

Our studies show that cAMP/PKA signaling negatively regulates adherens junction integrity as well as morphological and functional differentiation of intestinal epithelial cells.

A cyclic AMP protein kinase A-dependent mechanism by which rotavirus impairs the expression and enzyme activity of brush border-associated sucrase-isomaltase in differentiated intestinal Caco-2 cells

We undertook a study of the mechanism by which rhesus monkey rotavirus (RRV) impairs the expression and enzyme activity of brush border-associated sucrase isomaltase (SI) in cultured, human, fully differentiated, intestinal Caco-2 cells. We provide evidence that the RRV-induced defects in the expression and enzyme activity of SI are not related to the previously observed, RRV-induced, Ca2+ -dependent, disassembly of the F-actin cytoskeleton. This conclusion is based on the facts that: (i) the intracellular Ca2+ blocker, BAPTA/AM, which antagonizes the RRV-induced increase in [Ca2+](i), fails to inhibit the RRV-induced decrease in SI expression and enzyme activity; and (ii) Jasplakinolide (JAS) treatment, known to stabilize actin filaments, had no effect on the RRV-induced decrease in SI expression. Results reported here demonstrate that the RRV-induced impairment in the expression and enzyme activity of brush border-associated SI results from a hitherto unknown mechanism involving PKA signalling. This conclusion is based on the observations that (i) intracellular cAMP was increased in RRV-infected cells and (ii) treatment of RRV-infected cells with PKA blockers resulted in the reappearance of apical SI expression, accompanied by the restoration of the enzyme activity at the brush border. In addition, in RRV-infected cells a twofold increase of phosphorylated form of cytokeratin 18 was observed after immunopurification and Western Blot analysis, which was antagonized by exposing the RRV-infected cells to the PKA blockers.

The cytoplasmic/transmembrane domain of dipeptidyl peptidase IV, a type II glycoprotein, contains an apical targeting signal that does not specifically interact with lipid rafts

We investigated the signals involved in the apical targeting of dipeptidyl peptidase IV (DPP IV/CD26), an archetypal type II transmembrane glycoprotein. A secretory construct, corresponding to the DPP IV ectodomain, was first stably expressed in both the enterocytic-like cell line Caco-2 and the epithelial kidney MDCK cells. Most of the secretory form of the protein was delivered apically in MDCK cells, whereas secretion was 60% basolateral in Caco-2 cells, indicating that DPP IV ectodomain targeting is cell-type-dependent. A chimera (CTM-GFP) containing only the cytoplasmic and transmembrane domains of mouse DPP IV plus the green fluorescent protein was then studied. In both cell lines, this chimera was preferentially expressed at the apical membrane. By contrast, a secretory form of GFP was randomly secreted, indicating that GFP by itself does not contain cryptic targeting information. Comparison of the sequence of the transmembrane domain of DPP IV and several other apically targeted proteins does not show any consensus, suggesting that the apical targeting signal may be conformational. Neither the DPP IV nor the CTM-GFP chimera was enriched in lipid rafts. Together these results indicate that, besides the well-known raft-dependent apical targeting pathway, the fate of the CTM domain of DPP IV may reveal a new raft-independent apical pathway.

Apical secretion and sialylation of soluble dipeptidyl peptidase IV are two related events

The role of glycans in the apical targeting of proteins in epithelial cells remains a debated question. We have expressed the mouse soluble dipeptidyl peptidase IV (DPP IV ectodomain) in kidney (MDCK) and in intestinal (Caco-2) epithelial cell lines, as a model to study the role of glycosylation in apical targeting. The mouse DPP IV ectodomain was secreted mainly into the apical medium by MDCK cells. Exposure of MDCK cells to GalNac-alpha-O-benzyl, a drug previously described as an inhibitor of mucin O-glycosylation, produced a protein with a lower molecular weight. In addition this treatment resulted in a decreased apical secretion and an increased basolateral secretion of mouse DPP IV ectodomain. When expressed in Caco-2 cells, the mouse DPP IV ectodomain was secreted mainly into the basolateral medium. However, BGN was still able to decrease the amount of apically secreted protein and to increase its basolateral secretion. Neuraminidase digestion showed that the most striking effect of BGN was a blockade of DPP IV sialylation in both MDCK and Caco-2 cells. These results indicate that a specific glycosylation step, namely, sialylation, plays a key role in the control of the apical targeting of a secreted DPP IV both in MDCK and Caco-2 cells.

A 60 kDa plasma membrane protein changes its localization to autophagosome and autolysosome membranes during induction of autophagy in rat hepatoma cell line, H-4-II-E cells

We previously reported the preparation and characterization of an antibody against membrane fraction of autolysosomes from rat liver (J. Histochem. Cytochem. 38, 1571-1581, 1990). Immunoblot analyses of total membrane fraction of a rat hepatoma cell line, H-4-II-E cells by this antibody suggested that H-4-II-E cells expressed several autolysosomal proteins, including a protein with apparent molecular weight of 60 kDa. It was suggested that this 60 kDa protein was a peripheral membrane protein, because it was eluted from the membrane by sodium carbonate treatment. We prepared an antibody against this 60 kDa protein by affinity purification method, and examined its behavior during induction of autophagy. Autophagy was induced by transferring the cells from Dulbecco's modified Eagle medium (DMEM) containing 12% fetal calf serum into Hanks' balance salt solution. In DMEM, the 60 kDa protein showed diffused immunofluorescence pattern, and immunoelectron microscopy suggested that this protein was located on the extracellular side of the plasma membrane. After inducing autophagy, the immunofluorescence configuration of the 60 kDa protein changed from the diffused pattern to a granulous one. Immunoelectron microscopy showed that the 60 kDa protein was localized on the luminal side of the limiting membrane of autolysosomes and endosomes. In the presence of bafilomycin A1 which prevents fusion between autophagosomes and lysosomes, the 60 kDa protein was localized on the limiting membrane of the autophagosomes and endosomes. These results suggest that the 60 kDa protein is transported from the plasma membrane to the autophagosome membrane through the endosomes.

Regulation of the expression of aminopeptidase A, aminopeptidase N/CD13 and dipeptidylpeptidase IV/CD26 in renal carcinoma cells and renal tubular epithelial cells by cytokines and cAMP-increasing mediators

Aminopeptidase (AP) A is a transmembrane type II molecule widely distributed in mammalian tissues. Since APA expression may be absent in renal cell carcinoma (RCC), it is possible that there is an altered regulation or other defect of APA upon malignant transformation of proximal tubular cells. However, investigations into the regulation of APA on tumour cells are rare. We report, for the first time, that both transforming growth factor-beta 1 (TGF-beta1) and tumour necrosis factor-alpha (TNF-alpha) down-regulate APA mRNA as well as protein expression in renal tubular epithelial cells and RCC cells in culture. In addition to this, both cytokines decrease dipeptidylpeptidase (DP) IV/CD26 mRNA, but not APN/CD13 mRNA expression. Otherwise, IL-4 and IL-13 increase CD13 as well as CD26 expression, but do not alter APA expression. Interferon-alpha (IFN-alpha), IFN-beta and IFN-gamma increase mRNA expression of all the three membrane ectopeptidases, whereas IL-1, IL-6, IL-7, IL-12 and granulocyte-macrophage colony-stimulating factor (GM-CSF) have been found to be without any significant effect. Treatment of cultured cells with cAMP-increasing agents, such as 8-bromo-cAMP or A23187, results in an increase in APA and DPIV/CD26, but no change in APN/CD13 mRNA expression or even a decrease in it. Furthermore, AP inhibitors can influence APA mRNA expression, since bestatin causes an increase in APA expression in a time- and dose-dependent manner, whereas bestatin does not change CD13 or CD26 expression. No difference could be found with respect to the modulation by different mediators between RCC cells and renal epithelial cells, though permanent tumour cell lines such as Caki-1 and Caki-2 may have lost some of the normally expressed peptidases.

PKC-dependent long-term effect of PMA on protein cell surface expression in Caco-2 cells

Several recent data indicate that protein traffic is under the control of different phosphorylation pathways. In previous works, we have shown that cell surface expression of apical hydrolases and of a basolateral protein, "525" antigen, was impaired in Caco-2 cells treated with forskolin, a potent PKA activator (L. Baricault et al., 1995, J. Cell Sci., 108, 2109-2121). Surprisingly, in these experiments forskolin did not seem to act through PKA activation. These cAMP-independent effects of FK may rely on cross-talk between intracellular phosphorylation pathways as described recently for PKA and PKC pathways. Therefore, we tested the hypothesis that PKC activation may induce effects comparable to those of FK on three brush border hydrolases as well as on 525 antigen cell surface expression in Caco-2 cells. Using enzymatic activity measurements and pulse-chase experiments combined with cell surface biotinylation assays, we show that long-term treatment with phorbol 12-myristate 13-acetate (PMA) impairs the overall expression of neither brush border hydrolases nor that of the 525 antigen but decreases total cell surface expression of these proteins. The apical and basolateral delivery pathways are equally affected. Using confocal laser scanning microscopy we show that the DPP IV and the 525 antigen that were not recovered from the cell surface were sequestrated in Lamp-1-positive lysosomal-related vesicles. PMA stimulates PKC translocation even after a 3-week treatment and induces PKC epsilon redistribution to a vesicular- and membrane-associated compartment also labeled with cytokeratins. These results demonstrate that PMA-dependent PKC activation strongly impairs protein cell surface targeting. They also suggest that these PKC-dependent effects which are similar to those previously obtained with FK are relevant to the described cross-talk between PKA- and PKC-dependent phosphorylation pathways.

Cellular mechanisms of adaptation of grafts to antibody

New, more effective, strategies of immunosuppression, including those recently designed to induce durable T cell tolerance (by grafting allogeneic or xenogeneic haematopoietic cells into T lymphocyte-depleted recipients), leave humoral rejection as the main barrier to transplantation of vascularized organs between different species. Recent experimental work indicates that hyperacute rejection can be prevented by manipulations of antibodies and complement. In this paper, we review the mechanisms governing the interaction of antibodies with cell surface antigens in vitro and in vivo, and their cellular consequences. Evidence is presented that, in appropriate conditions, antibodies can protect by effecting modification of graft antigenicity (adaptation or accommodation).