Metabolic learning in the intestine: adaptation to nutrition and luminal factors.

There is increasing evidence that the magnitude and potential of intestinal nutrient absorption (sugars, fatty acids, cholesterol and triglycerides) and intestinal defense function are regulated by metabolic learning phenomena, and are influenced by dietary energy content and exercise. Metabolic overload syndromes, mainly obesity, and chronic malabsorption disorders such as inflammatory bowel disease and celiac disease have been defined as extreme phenotypes. Metabolic learning processes depend on developmental and transcriptional control systems of intestinal epithelial cell differentiation. The physiological differentiation zone of enterocytes is linked to the beta-catenin system, apolipoprotein apoA-IV and the master transcription factors Cdx2, HNF1alpha, and GATA4. In addition to these developmental regulatory transcription factors, nuclear receptors including RXR, LXR, PPAR, PXR, and CAR have been implicated in the generation of more absorptive enterocytes with a more differentiated phenotype on the one hand, and dedifferentiated cells with reduced capacity of detoxification and defense causing loss of junction control and barrier defects on the other. Large-scale analysis of gene expression profiles and identification of key pathways and master regulatory transcription factors will help dissect the role of nutritional and environmental factors as well as pharmacological intervention on mucosal homeostasis and disease, with potential applications for diagnosis and therapy.