Turnover of mouse intestinal brush border membrane proteins and enzymes in organ culture. A direct evaluation from studies on the evolution of enzyme activities during the culture

OUP accepted manuscript

Circulating biomarkers have emerged as valuable surrogates for evaluating disease states in solid malignancies. Their relative ease of access and rapid turnover has bolstered clinical applications in monitoring treatment efficacy and cancer progression. In this review, the roles of various circulating biomarkers in monitoring treatment response are described. Non-specific markers of disease burden, tumor markers (eg CA 125, CEA, PSA, etc.), circulating tumor cells, nucleic acids, exosomes, and metabolomic arrays are highlighted. Specifically, the discovery of each of these markers is reviewed, with examples illustrating their use in influencing treatment decisions, and barriers to their application noted where these exist. Finally, opportunities for future work using these circulating biomarkers are discussed.

Structure of microvillar enzymes in different phases of their life cycles

Structural changes have been studied during the life cycles of three glycosidases: sucrase-isomaltase (EC 3.2.48-10), lactase-phlorizin hydrolase (EC 3.2.1.23-62), maltase-glucoamylase (EC 3.2.1.20); and three peptidases: aminopeptidase A (EC 3.4.11.7), aminopeptidase N (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5). The final forms of the enzymes can be divided into at least two groups: the sucrase-isomaltase type, characterized as dimers, which are asymmetric in their hydrophilic parts, have two types of active site and anchor only on one subunit; and the aminopeptidase N type, characterized as dimers, which are symmetric in their hydrophilic part, have only one type of active site and anchor on both subunits. These enzymes are likely to be synthesized on rough endoplasmic reticulum and simultaneously glycosylated into endoglycosidase H-sensitive forms. They are later reglycosylated to endoglycosidase H-resistant forms, which have relative molecular masses similar to the final forms. Enzymes of the sucrase-isomaltase type seem to be synthesized with a polypeptide-chain length corresponding to the sum of both subunits, whereas enzymes of the aminopeptidase N type seem to be synthesized with a polypeptide-chain length corresponding to the constituent subunits themselves. Not much is known about the catabolism of these enzymes. The enzyme activities and the amounts of enzyme protein decrease at the top of the villi, probably due to release into the lumen. The subunits of aminopeptidase N are cleaved by pancreatic proteases to smaller peptides, and sucrase-isomaltase may lose its sucrase polypeptide, while both enzymes remain bound to the membrane.

Microvesiculation of the microvilli provides a membranous medium within the lumen during digestion

A hypothesis is presented which states that there is a membranous medium within the intestinal lumen which occurs during digestion. The medium is thought to be generated by the microvesiculation of the microvilli.

The occurrence of aminopeptidase N and desquamated cell proteins in intestinal perfusate of the rat

The in vivo release of a microvillar enzyme, aminopeptidase N (EC 3.4.11.2), and a cytosolic enzyme, lactate dehydrogenase (EC 1.1.1.27), into the intestinal lumen was measured to gain information on the fraction of desquamated cell protein in intestinal juice and on the mechanism of release of intestinal microvillar enzymes. 1.6% and 2.9% of the mucosal activities of aminopeptidase N and lactate dehydrogenase were released to the intestinal lumen per hour, respectively. The ratios between aminopeptidase N and lactate dehydrogenase in intestinal perfusates and mucosal homogenates were similar. This result is compatible with the view that aminopeptidase N in the rat small intestine is predominantly released into the intestinal lumen by desquamation of enterocytes. This conclusion was supported by the failure to demonstrate microvesiculation of the microvilli by electron microscopy. 30-40% of the aminopeptidase N in the intestinal lumen is membrane-bound indicating that partial solubilization occurs during desquamation. The addition of calcium ions did not augment the release of aminopeptidase N or the membrane-bound fraction in the lumen. 10-20% of the protein content in the intestinal lumen is due to extruded cell protein. This includes aminopeptidase N which constitutes 1% of the luminal protein.

Intestinal tissue and cell cultures

The culture of animal cells and tissues is a widely used technique in the field of cellular and molecular biology; one of the most interesting aspect being linked to the study of the mechanisms of cell differentiation. In the specific case of intestinal epithelial cells, various tissue culture technologies have proved to be important tools for the study of precise facets related to intestinal function, pathology and differentiation. Concerning this latter aspect, organ culture experiments have brought about interesting data on the hormonal or nutritional control of intestinal maturation. Nevertheless, the study of the precise mechanisms underlying epithelial proliferation and/or differentiation at the cellular level needs more adequate cell culture model systems. One of them has been described for two cell lines derived from human colonic adenocarcinomas, in which the cells can be induced to achieve enterocytic-like differentiation. Up to date, none of the continuous cell lines starting from normal undifferentiated cells have allowed generation of morphological or functional enterocytic polarity. In contrast, primary cell cultures which allow maintenance of a more physiological environment for the epithelial cells like contacts with their in vivo counterparts, mesenchymal cells or extracellular matrix molecules, have proved to be promising approaches.

Common characteristics for Na+-dependent sugar transport in Caco-2 cells and human fetal colon

The recent demonstration that the human colon adenocarcinoma cell line Caco-2 was susceptible to spontaneous enterocytic differentiation led us to consider the question as to whether Caco-2 cells would exhibit sodium-coupled transport of sugars. This problem was investigated using isotopic tracer flux measurements of the nonmetabolizable sugar analog alpha-methylglucoside (AMG). AMG accumulation in confluent monolayers was inhibited to the same extent by sodium replacement, 200 microM phlorizin, 1 mM phloretin, and 25 mM D-glucose, but was not inhibited further in the presence of both phlorizin and phloretin. Kinetic studies were compatible with the presence of both a simple diffusive process and a single, Na+-dependent, phlorizin- and phloretin-sensitive AMG transport system. These results also ruled out any interaction between AMG and a Na+-independent, phloretin-sensitive, facilitated diffusion pathway. The brush-border membrane localization of the Na+-dependent system was inferred from the observations that its functional differentiation was synchronous with the development of brush-border membrane enzyme activities and that phlorizin and phloretin addition 1 hr after initiating sugar transport produced immediate inhibition of AMG uptake as compared to ouabain. Finally, it was shown that brush-border membrane vesicles isolated from the human fetal colonic mucosa do possess a Na+-dependent transport pathway(s) for D-glucose which was inhibited by AMG and both phlorizin and phloretin. Caco-2 cells thus appear as a valuable cell culture model to study the mechanisms involved in the differentiation and regulation of intestinal transport functions.

Influence of triamcinolone and somatostatin on morphometric parameters of cultured intestinal mucosa

Mucosal biopsies from rabbit ileum were organ cultured for 24 h. The influence of triamcinolone and somatostatin on villus height and diameter as well as crypt depth and the number of mitoses was measured at various times during 24 h of culture as indices of cell proliferation and tissue maintenance. It could be shown that triamcinolone reduced cell proliferation slightly but preserved mucosal structure in organ culture. Somatostatin inhibited crypt cell proliferation, without any effect on other morphological parameters.

Establishment of an animal model of ovalbumin sensitised mouse to study protein induced enteropathy

The protein induced modifications of the small bowel mucosa from ovalbumin-sensitised mouse have been studied in organ culture. A decrease in gamma-glutamyl transpeptidase, alkaline phosphatase, lactase, sucrase, and glucoamylase activities was observed in the explants cultured in the presence of ovalbumin. In contrast, a large increase of those enzymatic activities was noted in the culture media, the overall effect observed being a net stimulation of the total enzymatic activities of the culture system. The enzymes accumulated in the particulate fraction of the medium (brush border membrane fraction) suggesting an increased turnover of membrane components by a process of shedding or microvesiculation. This model serves as a useful tool in evaluating the local response of the small bowel mucosa induced by a specific protein.

Lipid esterification and secretion by the mouse intestine in organ culture

Explants of adult mouse jejunum were cultured for different time periods and incubated in presence of a lipid diet emulsified by sodium taurocholate or complexed with albumin. The esterification of fatty acids and the secretion of triglycerides and phospholipids were measured and compared to the lipid absorption observed in vivo after the perfusion of the same diet. The results show that, in vitro, the enterocytes esterify the fatty acids present in the medium and secrete them with a yield improving during the culture and even better than the absorption observed in the in vivo experiments.

Effect of dexamethasone on the fetal mouse small intestine in organ culture

The formation of intestinal villi (organogenesis phase) may be studied in organ culture with a completely synthetic medium in 15-day fetal mouse duodenal explants. However, in these explants absorptive cells remained poorly differentiated with all the hormones studied except with epidermal growth factor. In order to elucidate the role of hormones and other factors on the maturation of absorptive cells (maturation phase) in the fetal rodent in organ culture, we have taken the explants after the organogenesis phase. We have studied different culture conditions and have found that 17-day mouse duodenal explants can be cultured during 48 hours with Leibovitz L-15 medium in a 95% O2-5% CO2 atmosphere provided that the explants are relatively large (5 X 2 mm). With this method, dexamethasone (Dx) has been shown to have a direct effect on the maturation of the fetal duodenal mucosa. The addition of Dx (300 ng/ml) to the completely synthetic medium 1) improves the morphology of the explants, 2) induces a significant increase in maltase activity in the tissues, and 3) reduces significantly the labeling index of the duodenal explants after 48 hours of culture. Direct action of Dx on the duodenal mucosa is shown for the first time in organ culture using a completely synthetic medium. This method will permit us to study the effects of other intrinsic and extrinsic factors on the regulation of enzymatic maturation in fetal small intestine.

Morphometry of the small intestinal enterocytes of the fasted rat and the effects of colchicine

The function of the microtubules that are present in the villus enterocytes of the mammalian small intestine is virtually unknown. In order to advance our knowledge about enterocyte microtubules, a quantitative ultrastructural comparison was carried out on enterocytes from rats injected intraperitoneally with colchicine (0.5 mg/100 g body weight) in saline and from rats injected with saline alone. Our morphometric and stereologic study demonstrates that colchicine treatment results in 1) an absolute decrease in microtubules, 2) a reduction in microvilli, essentially in length, 3) an increased thickness of the terminal web, 4) an increase in total lysosomal volume, apparently by an increased number of smaller lysosomes, and 5) a decrease in the number of Golgi lamellae. These results along with those from other studies suggest to us that enterocyte microtubules are involved in the biogenesis of microvillus plasma membrane. Our morphometric data from the saline-treated rats essentially agree with comparable data from other studies. However, comparison with comparable data from hamster enterocytes demonstrates species differences.

Effect of pentagastrin on mouse gastric and small intestinal mucosa in vivo and in vitro

1. Single administration of various doses (62.5, 125, 250, 500 micrograms/kg bodyweight) or multiple administration of a single dose (250 micrograms/kg body wt) of pentagastrin to fasted mice do not alter the protein and DNA content, nor the DNA synthesis in fundus of stomach, distal duodenum, and proximal jejunum. 2. Pentagastrin has also been added at different doses into an organ culture of adult mouse duodenum or jejunum. 3. The explants were taken from animals fasted for 24 or 48 hr before sacrifice. 4. Protein and DNA content, protein and DNA synthesis, activities of several brush border enzymes and morphology have not been modified. 5. Therefore, the drug does not seem to have a direct effect on the mouse intestinal mucosa. 6. These observations are compared to results obtained from other species.