FABPs as useful markers for the assessment of intestinal epithelial cell damage

I-FABP is a promising marker to assess enterocyte injury

Evaluation of intestinal pathology in patients of all age-groups has been a challenge for clinicians and researchers. Currently, it is still difficult to diagnose gut pathology in patients presenting with abdominal complaints and in volunteers participating in intestinal studies ^1,2. The current standard technique for assessing intestinal status is bowel biopsy, which is often inadequate as it is invasive, sometimes requires sedation, is expensive and only assesses the function of the tissue at the biopsy site. Moreover, for patients who are neutropenic and/or thrombocytopenic, the procedure is physically hazardous and often ethically unacceptable. A major delay in diagnosis is often present in patients with e.g. necrotizing enterocolitis (NEC), inflammatory bowel disease, acute mesenteric ischemia and celiac disease ^1,3. A diagnostic delay results in postponed correct treatment, which is accompanied by higher morbidity and mortality rates. In line with these diagnostic concerns, the follow-up of numerous intestinal diseases is hampered by the absence of non-invasive, rapid diagnostic means to assess intestinal damage for evaluation of the effects of treatment on the recovery of the disorder ¹. Measurement of endogenous cytosolic enterocyte proteins in urine or plasma has been shown to be useful to estimate enterocyte damage.

Fatty Acid Binding Proteins (FABP’s) comprise a class of low molecular weight (14-15 kDa) cytosolic proteins found in high concentrations in tissues involved in the uptake and consumption of fatty acids. Enterocytes contain Liver-Fatty Acid Binding Protein (L-FABP) and Intestinal-FABP (I-FABP). Enterocytes contain L-FABP (Figure 1), which is localized in small amounts in the mature enterocytes of the small and large intestine but in high amounts in the liver ⁵. I-FABP is primarily limited to mature enterocytes of the small and large intestine ^4,5. It circulates in low amounts in the blood stream of healthy individuals. I-FABP is a useful plasma marker for early enterocyte cell death and levels rise rapidly after episodes of acute intestinal ischemia and inflammation ^4-7. The level of circulating I-FABP has been reported to correlate with the histological status of the epithelium after intestinal ischaemia-reperfusion in experimental studies ^8-10.

Figure 1: Human L-FABP in the colon epithelial layer of a 9 month old baby (mAb L2B10, Cat.# HM2049).

Because of their low molecular weight, FABP present in the systemic circulation pass through the glomerular filter (fractional renal excretion 28%; half life time 11 minutes) and can readily be detected in urine 11. I-FABP is not expressed in the urinary tract mucosa. Thus urinary values of I-FABP provide specific information about the number of dying intestinal epithelial cells. Especially in neonates and children it is of importance that the test can be performed in non-invasively collected material. Blood collection for diagnostic purposes is traumatic for all children and a major cause for anaemia in neonates. NEC is a serious intestinal neonatal disease with high morbidity and mortality rates, among a population of preterm infants who presented with gastrointestinal symptoms 12. A recent study showed that urinary I-FABP, measured using ELISA, prospectively identified neonates with NEC (Figure 2).

Figure 2: In the first urine sample of neonates with suspected NEC, the mean urinary I-FABP:Cr ratio was significantly higher in neonates who ultimately developed NEC or intestinal necrosis (3.9 pg/nmol, range 2.3 to 6.6) than in those without NEC (1.2 pg/nmol, range 0.1 to 1.8) (p = 0.001).
Cr, creatine; I-FABP, intestinal fatty acid binding protein ¹².

In conclusion, I-FABP is a promising marker to assess enterocyte injury since this protein is specifically expressed in the gut on the tops of the villi. This protein is present in the cells at the initial site of destruction in numerous intestinal diseases, and is upon cell damage released immediately into the circulation and subsequently into urine.

 References

1. Dewar, D et al; Clinical features and diagnosis of celiac disease. Gastroenterology 2005, 128: S19
2. Nikolaus, S et al; Diagnostics of inflammatory bowel disease. Gastroenterology 2007, 133: 1670
3. Oldenburg, W, et al; Acute mesenteric ischemia: a clinical review. Arch Intern Med 2004, 164: 1054
4. Lieberman, J et al; Human intestinal fatty acid binding protein: report of an assay with studies in normal volunteers and intestinal ischemia. Surgery 1997, 121: 335
5. Pelsers, M et al; Fatty acid-binding proteins as plasma markers of tissue injury. Clin Chim Acta 2005, 352: 15
6. Kanda, T et al; Intestinal fatty acid-binding protein is a useful diagnostic marker for mesenteric infarction in humans. Gastroenterology 1996, 110: 339
7. Derikx, J et al; Evidence for intestinal and liver epithelial cell injury in the early phase of sepsis. Shock 2007, 28: 544
8. Gollin, G et al; Intestinal fatty acid binding protein in serum and urine reflects early ischemic injury to the small bowel. Surgery 1993, 113: 545
9. Kanda, T et al; Intestinal fatty acid-binding protein as a sensitive marker of intestinal ischemia. Dig Dis Sci 1992, 37: 1362
10. Marks, W et al; Biochemical detection of small intestinal allograft rejection by elevated circulating levels of serum intestinal fatty acid binding protein. Surgery 1993, 114: 206
11. Poll, M et al; Liver manipulation causes hepatocyte injury and precedes systemic inflammation in patients undergoing liver resection. World J Surg 2007, 31:2033.
12. Derikx, J et al; Urine based detection of intestinal mucosal cell damage in neonates with suspected necrotising enterocolitis. Gut 2007, 56:1473
13. Beuk, R et al; Total warm ischemia and reperfusion impairs flow in all rat gut layers but increases leukocyte–vessel wall interactions in the submucosa only. Annals of Surgery 2000, 231: 96