In situ localization of enzymes and mucin in normal rat colon embedded in plastic

Metabolism of Dietary and Microbial Vitamin B Family in the Regulation of Host Immunity

Vitamins are micronutrients that have physiological effects on various biological responses, including host immunity. Therefore, vitamin deficiency leads to increased risk of developing infectious, allergic, and inflammatory diseases. Since B vitamins are synthesized by plants, yeasts, and bacteria, but not by mammals, mammals must acquire B vitamins from dietary or microbial sources, such as the intestinal microbiota. Similarly, some intestinal bacteria are unable to synthesize B vitamins and must acquire them from the host diet or from other intestinal bacteria for their growth and survival. This suggests that the composition and function of the intestinal microbiota may affect host B vitamin usage and, by extension, host immunity. Here, we review the immunological functions of B vitamins and their metabolism by intestinal bacteria with respect to the control of host immunity.

Molecular identification and functional characterization of the human colonic thiamine pyrophosphate transporter

Colonic microbiota synthesize a considerable amount of thiamine in the form of thiamine pyrophosphate (TPP). Recent functional studies from our laboratory have shown the existence of a specific, high-affinity, and regulated carrier-mediated uptake system for TPP in human colonocytes. Nothing, however, is known about the molecular identity of this system. Here we report on the molecular identification of the colonic TPP uptake system as the product of the SLC44A4 gene. We cloned the cDNA of SLC44A4 from human colonic epithelial NCM460 cells, which, upon expression in ARPE19 cells, led to a significant (p < 0.01, >5-fold) induction in [(3)H]TPP uptake. Uptake by the induced system was also found to be temperature- and energy-dependent; Na(+)-independent, slightly higher at acidic buffer pH, and highly sensitive to protonophores; saturable as a function of TPP concentration, with an apparent Km of 0.17 ± 0.064 μM; and highly specific for TPP and not affected by free thiamine, thiamine monophosphate, or choline. Expression of the human TPP transporter was found to be high in the colon and negligible in the small intestine. A cell surface biotinylation assay and live cell confocal imaging studies showed the human TPP transporter protein to be expressed at the apical membrane domain of polarized epithelia. These results show, for the first time, the molecular identification and characterization of a specific and high-affinity TPP uptake system in human colonocytes. The findings further support the hypothesis that the microbiota-generated TPP is absorbable and could contribute toward host thiamine homeostasis, especially toward cellular nutrition of colonocytes.

Recent advances in transport of water-soluble vitamins in organs of the digestive system: a focus on the colon and the pancreas

This review focuses on recent advances in our understanding of the mechanisms and regulation of water-soluble vitamin (WSV) transport in the large intestine and pancreas, two important organs of the digestive system that have only recently received their fair share of attention. WSV, a group of structurally unrelated compounds, are essential for normal cell function and development and, thus, for overall health and survival of the organism. Humans cannot synthesize WSV endogenously; rather, WSV are obtained from exogenous sources via intestinal absorption. The intestine is exposed to two sources of WSV: a dietary source and a bacterial source (i.e., WSV generated by the large intestinal microbiota). Contribution of the latter source to human nutrition/health has been a subject of debate and doubt, mostly based on the absence of specialized systems for efficient uptake of WSV in the large intestine. However, recent studies utilizing a variety of human and animal colon preparations clearly demonstrate that such systems do exist in the large intestine. This has provided strong support for the idea that the microbiota-generated WSV are of nutritional value to the host, and especially to the nutritional needs of the local colonocytes and their health. In the pancreas, WSV are essential for normal metabolic activities of all its cell types and for its exocrine and endocrine functions. Significant progress has also been made in understanding the mechanisms involved in the uptake of WSV and the effect of chronic alcohol exposure on the uptake processes.

A high-affinity and specific carrier-mediated mechanism for uptake of thiamine pyrophosphate by human colonic epithelial cells

All mammals require exogenous sources of thiamine (vitamin B1), as they lack the ability to synthesize the vitamin. These sources are dietary and bacterial (the latter is in reference to the vitamin, which is synthesized by the normal microflora of the large intestine). Bacterially generated thiamine exists in the free, as well as the pyrophosphorylated [thiamine pyrophosphate (TPP)], form. With no (or very little) phosphatase activity in the colon, we hypothesized that the bacterially generated TPP can also be taken up by colonocytes. To test this hypothesis, we examined [(3)H]TPP uptake in the human-derived, nontransformed colonic epithelial NCM460 cells and purified apical membrane vesicles isolated from the colon of human organ donors. Uptake of TPP by NCM460 cells occurred without metabolic alterations in the transported substrate and 1) was pH- and Na(+)-independent, but energy-dependent, 2) was saturable as a function of concentration (apparent K(m) = 0.157 ± 0.028 μM), 3) was highly specific for TPP and not affected by free thiamine (or its analogs) or by thiamine monophosphate and unrelated folate derivatives, 4) was adaptively regulated by extracellular substrate (TPP) level via what appears to be a transcriptionally mediated mechanism(s), and 5) appeared to be influenced by an intracellular Ca(2+)/calmodulin-mediated regulatory pathway. These findings suggest the involvement of a carrier-mediated mechanism for TPP uptake by colonic NCM460 cells, which was further confirmed by results from studies of native human colonic apical membrane vesicles. The results also suggest that the bacterially synthesized TPP in the large intestine is bioavailable and may contribute to overall body homeostasis of vitamin B1 and, especially, to the cellular nutrition of the local colonocytes.

Prostatic intraepithelial neoplasia and other changes during promotion and progression

Preneoplastic lesions, early neoplastic lesions and carcinomas in situ have been demonstrated to be of great value for many purposes in many organ systems. Their recognition can be useful in epidemiological studies and can facilitate the selection of patients for therapeutic interventions. They can be used as "surrogate endpoint biomarkers" in studies aimed at the chemoprevention of cancers. In the lung, colon and various other organs, such markers are well recognized to be associated with the development of cancer in man. In the livers and colons of experimental animals, there has been detailed characterization of "enzyme-altered foci" (EAF) as "putative preneoplastic markers." The words "surrogate" and "putative" are important; the biological potential of these lesions needs to be elucidated in much greater detail. The quantification of early lesions that are associated with and sometimes precursors of neoplasia is of particular value because they are much more numerous, in most organ systems, than the carcinomas that develop in the same organs. The most abundant of these lesions show minimal or no morphological alterations. For example, EAF and aberrant crypts are more numerous than polyps in the colons of patients and experimental animals with cancer or precancerous conditions that affect the colon. Currently, there are few well documented putative or surrogate markers that are highly associated with the development of prostatic carcinoma in man. The best documented among these is prostatic intraepithelial neoplasia. We have recently reported the identification of EAF in the human prostate. While they share many phenotypic alterations with prostatic intraepithelial neoplasia, much remains to be accomplished if their biological fate is to be understood.

Aberrant crypts in human colonic mucosa: putative preneoplastic lesions

Aberrant crypts are recognized in methylene blue-stained, unsectioned, colonic mucosa by their increased size, elliptical lumenal opening, thicker epithelial layer, and increased pericryptal region. Aberrant crypt foci in rodents are observed as early as 2 weeks and for at least 9 months after a single dose of carcinogen, have a distribution that parallels that of tumors, and have an increased number of aberrant crypts per focus with time after the carcinogen dose. The ability to quantify these lesions in the entire colon of rodents in less than an hour suggests that aberrant crypts may provide a highly efficient in vivo bioassay for colon carcinogens. Since aberrant crypt foci appear to be the earliest identifiable putative precursors of colon cancer, they represent lesions that can be characterized further for the earliest genetic and biochemical alterations. In F344 rats, we have demonstrated that aberrant crypts have multiple histochemically-detectable enzyme alterations. Using similar techniques, we were the first to demonstrate aberrant crypts in unsectioned human mucosa. After embedding and sectioning, these microscopic aberrant crypts resemble rare lesions described earlier in the literature after extensive serial sectioning. In rats and humans, aberrant crypts may be histologically normal or display varying degrees of dysplasia and histochemically-detectable altered enzyme activities. These putative, preneoplastic lesions should reveal early changes that precede colon cancer and ways to alter their progression.