Human crypt intestinal epithelial cells are capable of lipid production, apolipoprotein synthesis, and lipoprotein assembly

The recent availability of spontaneously proliferating, non-transformed hum an crypt intestinal epithelial cells (HIEC) affords an opportunity to inves tigate lipid metabolism in undifferentiated enterocytes. The major purpose of this study Tvas to explore the capability of undifferentiated crypt cell s to synthesize, assemble, and secrete lipids and apolipoproteins. HIEC wer e cultured in medium with 5% fetal bovine serum for 5 to 21 d, The cells we re clearly able to incorporate [C-14]oleic acid (dpm/mg protein) into trigl ycerides (128,279 +/- 16,988), phospholipids (30,278 +/- 2,107), and choles teryl esters (2,180 +/- 207), AG though improvement in lipid secretion was noted with prolongation of cell culture periods, low efficiency of Lipid ex port (10.3 +/- 2.2% of intracellular content) characterized the HIEC. All p hospholipid classes were elaborated, with phosphatidylcholine accounting fo r 79.3 +/- 1.3% of cellular phospholipids. Chylomicrons were the dominant ( 46.4%) lipoproteins secreted, followed by high, low; and very low density L ipoproteins (HDL, LDL, and VLDL) comprising 22.5, 20.2, and 10.8% of the to tal, respectively. HIEC elaborated most of the major apolipoprotein (apo) c lasses (A-I, A-IV, B-100, C, and E), but were less efficient in producing a poB-48, In contrast to the production of apoA-I and C as early as 5 days af ter confluence, apoA-I and A-IV were maximally expressed at 11 d, Culture m edia 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 endog enous cholesterol synthesis and LDL receptor expression, Cholesterol synthe sis 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 sol id evidence that immature HIEC are capable of key fat absorptive functions of well-differentiated enterocytes. The intracellular mechanisms required f or lipid and apolipoprotein synthesis as well as for lipoprotein assembly a re already present in intestinal crypt cells. These cells also retain the c apacity for sterol enzyme and receptor expression. However, certain Limitat ions, especially apoB-48 production and lipoprotein secretion as well as un responsiveness of cholesterol synthesis to LDL, may be ascribed to the lack of differentiation.