Transcriptional modulation of genes encoding structural characteristics of differentiating enterocytes during development of a polarized epithelium in vitro

Cross-talk between adipose and gastric leptins for the control of food intake and energy metabolism

The understanding of the regulation of food intake has become increasingly complex. More than 20 hormones, both orexigenic and anorexigenic, have been identified. After crossing the blood-brain barrier, they reach their main site of action located in several hypothalamic areas and interact to balance satiety and hunger. One of the most significant advances in this matter has been the discovery of leptin. This hormone plays fundamental roles in the control of appetite and in regulating energy expenditure. In accordance with the lipostatic theory stated by Kennedy in 1953, leptin was originally discovered in white adipose tissue. Its expression by other tissues was later established. Among them, the gastric mucosa has been shown to secrete large amounts of leptin. Both the adipose and the gastric tissues share similar characteristics in the synthesis and storage of leptin in granules, in the formation of a complex with the soluble receptor and a secretion modulated by hormones and energy substrates. However while adipose tissue secretes leptin in a slow constitutive endocrine way, the gastric mucosa releases leptin in a rapid regulated exocrine fashion into the gastric juice. Exocrine-secreted leptin survives the extreme hydrolytic conditions of the gastric juice and reach the duodenal lumen in an intact active form. Scrutiny into transport mechanisms revealed that a significant amount of the exocrine leptin crosses the intestinal wall by active transcytosis. Leptin receptors, expressed on the luminal and basal membrane of intestinal epithelial cells, are involved in the control of nutrient absorption by enterocytes, mucus secretion by goblet cells and motility, among other processes, and this control is indeed different depending upon luminal or basal stimulus. Gastric leptin after transcytosis reaches the central nervous system, to control food intake. Studies using the Caco-2, the human intestinal cell line, in vitro allowed analysis of the mechanisms of leptin actions on the intestinal mucosa, identification of the mechanisms of leptin transcytosis and understanding the modulation of leptin receptors by nutrients and hormones. Exocrine-secreted gastric leptin thus participates in a physiological axis independent in terms of time and regulation from that of adipose tissue to rapidly control food intake and nutrient absorption. Adipocytes and gastric epithelial cells are two cell types the metabolism of which is closely linked to food intake and energy storage. The coordinated secretion of adipose and gastric leptins ensures proper management of food processing and energy storage.

A flippase-independent function of ATP8B1, the protein affected in familial intrahepatic cholestasis type 1, is required for apical protein expression and microvillus formation in polarized epithelial cells

Mutations in ATP8B1 cause familial intrahepatic cholestasis type 1, a spectrum of disorders characterized by intrahepatic cholestasis, reduced growth, deafness, and diarrhea. ATP8B1 belongs to the P(4) P-type adenosine triphosphatase (ATPase) family of putative aminophospholipid translocases, and loss of aminophospholipid asymmetry in the canalicular membranes of ATP8B1-deficient liver cells has been proposed as the primary cause of impaired bile salt excretion. To explore the origin of the hepatic and extrahepatic symptoms associated with ATP8B1 deficiency, we investigated the impact of ATP8B1 depletion on the domain-specific aminophospholipid translocase activities and polarized organization of polarized epithelial Caco-2 cells. Caco-2 cells were stably transfected with short hairpin RNA constructs to block ATP8B1 expression. Aminophospholipid translocase activity was assessed using spin-labeled phospholipids. The polarized organization of these cells was determined by pulse-chase analysis, cell-fractionation, immunocytochemistry, and transmission electron microscopy. ATP8B1 was abundantly expressed in the apical membrane of Caco-2 cells, and its expression was markedly induced during differentiation and polarization. Blocking ATP8B1 expression by RNA interference (RNAi) affected neither aminophospholipid transport nor the asymmetrical distribution of aminophospholipids across the apical bilayer. Nonetheless, ATP8B1-depleted Caco-2 cells displayed profound perturbations in apical membrane organization, including a disorganized apical actin cytoskeleton, a loss in microvilli, and a posttranscriptional defect in apical protein expression.

CONCLUSION

Our findings point to a critical role of ATP8B1 in apical membrane organization that is unrelated to its presumed aminophospholipid translocase activity, yet potentially relevant for the development of cholestasis and the manifestation of extrahepatic features associated with ATP8B1 deficiency.

A dynamic ratio of the alpha+ and alpha- isoforms of the tight junction protein ZO-1 is characteristic of Caco-2 cells and correlates with their degree of differentiation

ZO-1 is a peripheral protein that plays a central role in the macromolecular assembly of tight junctions by interacting with integral proteins (occludin, claudins, JAMs) of the membrane of adjoining cells, with the actin cytoskeleton, and with nuclear factors. Human ZO-1 is expressed in all epithelia and some specialized endothelia as variable amounts of two related isoforms, which originate from the alternatively spliced mRNA transcripts alpha(+) and alpha(-) and whose specific differential role is still unknown. Moreover, little is known about the timing of expression of ZO-1 isoforms at the protein and mRNA level. This study shows that during growth of freshly plated Caco-2 cells, the alpha(+)/alpha(-) ratio increased as a result of simultaneous increase of alpha(+) and decrease of alpha(-). Differences in the isoform ratio also correlated with differences in epithelium differentiation. This was determined by aminopeptidase N measurements of cells grown on conventional substrates and on modified, micro/nano-patterned surfaces. A comparable shift of ZO-1 isoforms was not observed in other tumour cell lines of non-intestinal origin (A549, Calu-3). Pancreatic stem cells, propagated without exogenous differentiation stimuli, displayed a slight, stable prevalence of the alpha(-) isoform. Of the intestinal cell lines examined (Caco-2 and T84), only Caco-2 cells displayed a dramatic shift in isoform expression. This suggests that this tumour cell line retains to a higher degree a developmental programme related to the dynamic of enterocytic differentiation in vivo.

Epithelial cell surface polarity: the early steps

Establishment and maintenance of epithelial cell surface polarity is of vital importance for the correct function of transporting epithelia. To maintain normal cell function, the distribution of apical and basal-lateral proteins is highly regulated and defects in expression levels or plasma membrane targeting can have severe consequences. It has been shown recently that initiation of cell-surface polarity occurs immediately upon cell-cell contact, and requires components of the lateral targeting patch, the Exocyst and the lateral SNARE complex to specify delivery of basolateral proteins to the site of cell-cell adhesion. The Exocyst and SNARE complex are present in the cytoplasm in single epithelial cells before adhesion. Upon initial cell-cell adhesion, E-cadherin accumulates at the forming contact between cells. Shortly hereafter, components of the lateral targeting patch, the Exocyst and the lateral SNARE complex, co-localize with E-cadherin at the forming contact, where they function in specifying the delivery of basal-lateral.

Coordinated protein sorting, targeting and distribution in polarized cells

The polarized distribution of functions in polarized cells requires the coordinated interaction of three machineries that modify the basic mechanisms of intracellular protein trafficking and distribution. First, intrinsic protein-sorting signals and cellular decoding machineries regulate protein trafficking to plasma membrane domains; second, intracellular signalling complexes define the plasma membrane domains to which proteins are delivered; and third, proteins that are involved in cell-cell and cell-substrate adhesion orientate the three-dimensional distribution of intracellular signalling complexes and, accordingly, the direction of membrane traffic. The integration of these mechanisms into a complex and dynamic network is crucial for normal tissue function and is often defective in disease states.

JunD represses transcription and translation of the tight junction protein zona occludens-1 modulating intestinal epithelial barrier function

The AP-1 transcription factor JunD is highly expressed in intestinal epithelial cells, but its exact role in maintaining the integrity of intestinal epithelial barrier remains unknown. The tight junction (TJ) protein zonula occludens (ZO)-1 links the intracellular domain of TJ-transmembrane proteins occludin, claudins, and junctional adhesion molecules to many cytoplasmic proteins and the actin cytoskeleton and is crucial for assembly of the TJ complex. Here, we show that JunD negatively regulates expression of ZO-1 and is implicated in the regulation of intestinal epithelial barrier function. Increased JunD levels by ectopic overexpression of the junD gene or by depleting cellular polyamines repressed ZO-1 expression and increased epithelial paracellular permeability. JunD regulated ZO-1 expression at the levels of transcription and translation. Transcriptional repression of ZO-1 by JunD was mediated through cAMP response element-binding protein-binding site within its proximal region of the ZO-1-promoter, whereas induced JunD inhibited ZO-1 mRNA translation by enhancing the interaction of the ZO-1 3'-untranslated region with RNA-binding protein T cell-restricted intracellular antigen 1-related protein. These results indicate that JunD is a biological suppressor of ZO-1 expression in intestinal epithelial cells and plays a critical role in maintaining epithelial barrier function.

Parallels between global transcriptional programs of polarizing Caco-2 intestinal epithelial cells in vitro and gene expression programs in normal colon and colon cancer

Posttranslational mechanisms are implicated in the development of epithelial cell polarity, but little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized temporal patterns of gene expression during cell-cell adhesion-initiated polarization of cultured human Caco-2 cells, which develop structural and functional polarity resembling enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts. Comparison to gene expression patterns in normal human colon and colon tumors revealed that the pattern in proliferating, nonpolarized Caco-2 cells paralleled patterns seen in human colon cancer in vivo, including expression of genes involved in cell proliferation. The pattern switched in polarized Caco-2 cells to one more closely resembling that in normal colon tissue, indicating that regulation of transcription underlying Caco-2 cell polarization is similar to that during enterocyte differentiation in vivo. Surprisingly, the temporal program of gene expression in polarizing Caco-2 cells involved changes in signaling pathways (e.g., Wnt, Hh, BMP, FGF) in patterns similar to those during migration and differentiation of intestinal epithelial cells in vivo, despite the absence of morphogen gradients and interactions with stromal cells characteristic of enterocyte differentiation in situ. The full data set is available at http://microarray-pubs.stanford.edu/CACO2.