Intestinal-type and liver-type fatty acid-binding protein in the intestine. Tissue distribution and clinical utility
Introduction
The analysis of specific tissue proteins or enzymes in plasma is a common approach for the detection of tissue necrosis and is widely used for detection of liver, muscle and heart injury [1]. However, for the early detection of intestinal injury due to decreased perfusion of the small bowel, inflammation or rejection, no protein is routinely analyzed. Several studies have investigated the use of biochemical markers like hexoaminidase and lactate dehydrogenase [2] or physiologic markers like mucosal pH [3], but these were nonconclusive. By analogy to the use of cytoplasmic heart-type fatty acid-binding protein (H-FABP) for detection of heart- and muscle injury, the cytoplasmic intestinal fatty-acid binding protein (I-FABP) is a newly proposed marker. I-FABP is part of a family of nine different FABP types, each named after the tissue of its first detection [4], [5]. FABPs are 15 kD cytoplasmic proteins, involved in the intracellular buffering and transport of long chain fatty-acids. I-FABP occurs in the enterocytes of small intestine and constitutes 2% of enterocyte protein [5], [6]. Several studies have described the use of I-FABP for the detection of rat intestinal injury after acute ischemic diseases [2], rejection [7], [8] and necrotic enterocolitis [9], but with different outcomes in the human setting [10], [11], [12], [13], [14], [15], [16], [17], [18]. Furthermore, the tissue content of I-FABP is still unknown, so that plasma I-FABP cannot yet be used for quantitation of the amount of tissue injury.
Interestingly, intestinal cells also express liver-type FABP (L-FABP), which occurs in liver and additionally in kidney [19], [20]. However, the possibility of using L-FABP and/or a combination of I-FABP and L-FABP as plasma marker for detection of intestinal injury has not yet been explored. When intestinal ischemia is limited to a period of less than 2 h, only the villi are affected while the crypt cells remain intact, and there is a rapid recovery of function [21]. Because I-FABP en L-FABP are mainly expressed in the villi and not in the crypt [4], [5], [11], we hypothetised that both proteins may be early and sensitive plasma markers of intestinal ischemia.
The aim of our study was to investigate the tissue distribution and contents of both I-FABP and L-FABP in human intestine along the duodenal to colonal axis in surgery and autopsy samples and to study the potential of these proteins as plasma marker for the detection of intestinal injury in patients. Because the proteins may show a distinct pattern of tissue distribution, expressing their ratio in plasma after intestinal injury might be useful to locate the necrotic stage and improve surgical procedures. Heart-type FABP (H-FABP), which is expressed in smooth muscle cells [22], was measured to investigate whether intestinal autopsy samples are comparable in their amount of muscle cells. Finally, for establishing upper reference levels for both marker proteins, the influence of age, gender and circadian rhythm on normal plasma concentrations was studied in healthy individuals.
Section snippets
Human tissue
Human intestinal tissue samples were obtained from autopsies of 23 subjects (Medical Academy, Bialystok and Mental Hospital, Choroszcz, Poland), performed at 17 ± 4 h after death (mean ± SD), and divided into duodenum, jejunum, ileum, proximal colon and distal colon. The samples then were directly frozen in liquid nitrogen. Human liver tissue, used for investigation of the tissue content of FABP, was obtained during routine surgery (Academic Hospital Maastricht, Maastricht, the Netherlands) and
I-FABP and L-FABP ELISA
The sandwich-type ELISAs for I-FABP and L-FABP each showed a detection limit of 0.1 μg/L (measured with standard of 0 μg/L; mean + 3 sd for n = 10). The calibration curve was linear up to 5 μg/L for I-FABP and up to 20 μg/L for L-FABP. For both assays, using standards of 2 and 5 μg/L, the intra-assay and inter-assay coefficients of variation (CV) were <5% and <15%, respectively.
Intestinal and liver tissue I-FABP contents
The tissue content of I-FABP varies along the small intestine (Fig. 1, upper panel). Colon contains the lowest
Discussion
This study documents, for the first time, the contents of the cytoplasmic fatty acid-binding proteins I-FABP, L-FABP, and H-FABP in segments of human intestine. In addition, it is shown that in cases of intestinal injury both I-FABP and L-FABP can be found in plasma, whereby the markedly higher intestinal tissue content of L-FABP when compared to that of I-FABP is reflected in proportional differences in plasma concentrations of these proteins.
Acknowledgements
The authors wish to thank Dr. Gorski, Bialystok, Poland for stimulating discussions, Mrs D. van der Voort for technical assistance during part of the studies and dr. W. Buurman and Mrs G. Francot, HyCult biotechnology, Uden, the Netherlands for providing the FABP immunoassay kits. This work was supported by the Ministry of Economic Affairs of the Netherlands, grant BTS 97.188.
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