Human crypt intestinal epithelial cells are capable of lipid production, apolipoprotein synthesis, and lipoprotein assembly.
The recent availability of spontaneously proliferating, non-transformed human crypt intestinal epithelial cells (HIEC) affords an opportunity to investigate lipid metabolism in undifferentiated enterocytes. The major purpose of this study was to explore the capability of undifferentiated crypt cells to synthesize, assemble, and secrete lipids and apolipoproteins. HIEC were cultured in medium with 5% fetal bovine serum for 5 to 21 d. The cells were clearly able to incorporate [(14)C]oleic acid (dpm/mg protein) into triglycerides (128,279 +/- 16,988), phospholipids (30, 278 +/- 2,107), and cholesteryl esters (2,180 +/- 207). Although improvement in lipid secretion was noted with prolongation of cell culture periods, low efficiency of lipid export (10.3 +/- 2.2% of intracellular content) characterized the HIEC. All phospholipid classes were elaborated, with phosphatidylcholine accounting for 79. 3 +/- 1.3% of cellular phospholipids. Chylomicrons were the dominant (46.4%) lipoproteins secreted, followed by high, low, and very low density lipoproteins (HDL, LDL, and VLDL) comprising 22.5, 20.2, and 10.8% of the total, respectively. HIEC elaborated most of the major apolipoprotein (apo) classes (A-I, A-IV, B-100, C, and E), but were less efficient in producing apoB-48. In contrast to the production of apoA-I and C as early as 5 days after confluence, apoA-I and A-IV were maximally expressed at 11 d. Culture media accumulated much more apoB-100 than apoB-48 (B-48/B-100 ratio 0.21 +/- 0.03), reflecting limited apoB mRNA editing. HIEC demonstrated both endogenous cholesterol synthesis and LDL receptor expression. Cholesterol synthesis was sensitive to 25-hydroxycholesterol and mevinolin, but unresponsive to LDL treatment, suggesting independent regulation pathways. In contrast, LDL inhibited receptor activity. The present findings provide the first solid evidence that immature HIEC are capable of key fat absorptive functions of well-differentiated enterocytes. The intracellular mechanisms required for lipid and apolipoprotein synthesis as well as for lipoprotein assembly are already present in intestinal crypt cells. These cells also retain the capacity for sterol enzyme and receptor expression. However, certain limitations, especially apoB-48 production and lipoprotein secretion as well as unresponsiveness of cholesterol synthesis to LDL, may be ascribed to the lack of differentiation.
Levy
E
E
Groupe CRM sur le Développement Fonctionnel et la Physiologie du Tube Digestif, Université de Montréal, Montréal, Québec H3T 1C5, Canada.
Beaulieu
J F
JF
Delvin
E
E
Seidman
E
E
Yotov
W
W
Basque
J R
JR
Ménard
D
D
D000818:Animals; D001053:Apolipoproteins / Q000096:biosynthesis*; D001692:Biological Transport; D002250:Carbon Radioisotopes; D002417:Cattle; D002784:Cholesterol / Q000096:biosynthesis; D002853:Chromatography, Liquid; D019084:Fluorescent Antibody Technique, Indirect; D006801:Humans; D007413:Intestinal Mucosa / Q000166:cytology / Q000378:metabolism*; D008055:Lipids / Q000096:biosynthesis*; D008854:Microscopy, Electron; D019301:Oleic Acid / Q000378:metabolism; D011973:Receptors, LDL / Q000378:metabolism
D001053:Apolipoproteins; D002250:Carbon Radioisotopes; D008055:Lipids; D011973:Receptors, LDL; D019301:Oleic Acid; D002784:Cholesterol