EDITORIAL FOCUS
Regulation of intestinal vitamin B2 absorption
Focus on "Riboflavin uptake by human-derived colonic epithelial
NCM460 cells"
Uma
Sundaram
Division of Digestive Diseases, Department of Internal Medicine, The
Ohio State University School of Medicine, Columbus, Ohio 43210
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ARTICLE |
RIBOFLAVIN OR VITAMIN B2
(7,8-dimethyl-10-ribitylisoalloxazine) is an essential water-soluble
vitamin. Riboflavin is important for normal cell functioning and for
cell growth and development. Riboflavin is a major component of
coenzymes FAD and flavin mononuclotide (FMN) that play a key role in
the metabolism of carbohydrates, amino acids, and lipids. Riboflavin is
also intimately involved in the conversion of pyridoxine (vitamin
B6) and folic acid (vitamin B9) into their
coenzymes (2, 5).
Humans and other mammals cannot synthesize riboflavin. However, this
vitamin is found abundantly in milk, leafy green vegetables, and eggs.
The dietary riboflavin is found primarily in the forms FAD and FMN.
These coenzymes are hydrolyzed by intestinal luminal phosphatases
before they can be absorbed (1, 3, 8). Transport of riboflavin has been
extensively studied in human, rabbit, and rat small intestine.
Riboflavin absorption in the mammalian small intestine occurs on the
brush-border membrane (BBM) of the absorptive villus cells and is
carrier mediated (9, 11-15). The role of Na+
dependency for riboflavin uptake in the small intestine is not completely clear, because BBM vesicle studies suggest no
Na+ dependency, whereas intact tissue studies suggest a
role for Na+. Taken together, these findings may suggest a
secondary role for Na+ dependency in riboflavin small
intestinal absorption compared with a more direct dependency necessary
for Na+-nutrient cotransport processes such as
Na+-glucose cotransport (9, 13-15).
An equally important source of riboflavin is bacterially synthesized
riboflavin in the colon. Colonic flora synthesize a considerable amount
of this vitamin and which in the colon is available for absorption in
the free form (6, 7). Although the small intestinal assimilation of
riboflavin appears to be regulated by the amount of the vitamin
ingested (18), the colonic absorption is more dependent on the type of
diet ingested. The colonic bacteria appear to produce more riboflavin
when a more fiber-based diet (e.g., green leafy vegetables) is ingested
compared with a meat-based diet. Thus with the former type of diet
there is more of the vitamin for the colonic epithelium to absorb (6).
The wide availability of riboflavin in food sources and the redundancy
of intestinal absorption of this vitamin may point to its importance
for the overall well-being of the organism. Indeed, deficiency of
riboflavin results in a variety of pathophysiological states. Vitamin
B2 deficiency is characterized by glossitis, cheilosis, angular stomatitis, seborrhea-like dermatitis, pruritus, photophobia, visual impairment, growth retardation, alopecia, and degenerative changes of the nervous system (2, 4, 5, 10). Thus better understanding
of the intestinal assimilation of riboflavin is essential to prevent
the wide range of disease entities associated with its deficiency.
However, until recently very little was known about the cellular
regulation of riboflavin transport. This was chiefly owing to the lack
of suitable in vitro cell culture systems to study the transport of
this vitamin. Recently, Said's group in a series of studies has
elegantly detailed the cellular mechanism of regulation of riboflavin
transport in the colon. In one study using Caco-2 cells they
demonstrated that riboflavin uptake is carrier mediated, Na+ independent, and inhibitable by cation exchange (e.g.,
amiloride) but not anion exchange inhibitors (e.g., stilbene
derivatives), furosemide, or probenecid. Further, extracellular
substrate concentration appeared to regulate the transporter by
increasing the BBM transporter numbers (17). Whether this increase in
transporter numbers is secondary to altered membrane trafficking and/or
transcriptional changes of the riboflavin transporter has yet to be
deciphered. Said et al. then demonstrated that protein kinase A (PKA),
but not protein kinase C regulates the riboflavin transporter in Caco-2 cells. Increasing intracellular cAMP levels inhibited the uptake of
riboflavin in these cells. The mechanism of PKA-mediated inhibition was
not secondary to a decrease in the synthesis or membrane trafficking of
the riboflavin transporter, but most likely secondary to a decrease in
the activity of the transporter (16).
The current article in focus by Said et al. (Ref. 19, see page C270 in
this issue) describes the regulation of riboflavin transport in a human-derived nontransformed colonic epithelial cell
line, NCM460. Riboflavin uptake in these colonocytes was carrier
mediated, Na+ independent, and inhibitable by structural
analogs. The transporter is amiloride sensitive. This last observation
in both NCM460 and Caco-2 cells raises the intriguing possibility that
this may be a riboflavin/proton exchanger. Further studies will need to
be done to establish this. In NCM460 colonocytes the riboflavin
transporter number is regulated up or down by the availability of
extracellular vitamin levels. Intracellular
Ca2+/calmodulin, but not protein kinase C seems to regulate
this transporter. Specifically, inhibition of the
Ca2+/calmodulin pathway results in the inhibition of
riboflavin transport. The mechanism of inhibition is both at the level
of the transporter numbers as well as secondary to altered affinity of
the transporter for the vitamin. Future molecular studies will be
necessary to determine whether the Ca2+/calmodulin pathway
regulation of riboflavin transport is primarily at the transcriptional
or posttranslational level.
It is evident from the study in focus as well as all of the previous
work by Said's group that their effort has greatly enhanced our
knowledge of riboflavin assimilation in the intestine. Further, their
work to date has set the stage to examine the regulation of riboflavin
transport at the molecular level. Better understanding of the
regulation of this transporter in health will lead to future investigations aimed at the deregulation of this vitamin's transport in pathophysiological states. This will undoubtedly result in more
efficacious treatment modalities for disease states that result when
there is an imbalance in the homeostasis of this very important vitamin.
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FOOTNOTES |
Address for reprint requests and other correspondence: U. Sundaram,
Division of Digestive Diseases, Department of Internal Medicine, The
Ohio State Univ. School of Medicine, N-214 Doan Hall, 410 W 10th Ave.,
Columbus, OH 43210.
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Am J Physiol Cell Physiol 278(2):C268-C269
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