Lipid esterification and synthesis by the IEC-6 intestinal epithelial crypt cell line: effect of transforming growth factor β

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Abstract
The capacity of immature intestinal epithelial crypt cells to synthesize lipids and the factors that promote their differentiation remain largely unknown. We examined the profile of lipids synthesized by a normal rat intestinal epithelial crypt cell line (IEC-6) and determined the effects of transforming growth factor β (TGFβ), a putative crypt cell differentiating factor, on their lipid handling. Incubation of IEC-6 cells with [14C]oleic acid (20 h) resulted in lipid esterification and synthesis, mainly as triglycerides (TGs, 57 ± 0.6%) and phospholipids (PLs, 30 ± 0.6%), with a PL/TG ratio of 0.53. When cells were pulsed (2.5 h) with [14C]oleic acid and then maintained 20 h in medium alone, a significant elevation of the PL/TG ratio (10.2 ± 1.3, p < 0.01) was observed, primarily accounted for by a significant decrease of the TG fraction (p < 0.01). IEC-6 cells secreted only trace amounts of lipids under the latter experimental condition. Incubation with TGFβ (20 h) significantly inhibited IEC-6 cell proliferation but did not promote the expression of cell sucrase activity. TGFβ induced a significant increase in the cellular composition of PL (p < 0.05) and a decrease in the TG fraction (p < 0.02), after a 2.5-h pulse of [14C]oleic acid. Lipid production was unaffected by TGFβ during the 20-h incubation with [14C]oleic acid. Lipid secretion into the medium remained negligible in the presence of TGFβ, after 2.5 h of incubation with substrate as above. Our findings suggest that immature crypt IEC-6 cells are capable of lipid esterification and synthesis but secrete minute amounts of lipoproteins. The predominant lipid fraction synthesized depended upon the availability of fatty acid substrate, with longer incubations favoring TG synthesis and brief cultures generating mainly PLs. The latter tendency was enhanced by TGFβ, which did not induce crypt cell maturation under the conditions tested.Key words: intestinal crypt epithelial cells, transforming growth factor β, triglycerides, phospholipids, enterocyte differentiation.

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