Fatty acid metabolism in human keratinocytes cultivated at an air-medium interface

Abstract

Stratum corneum lipids, which provide the mammalian permeability barrier, display a distinctive fatty acid profile with a predominance of long chain, saturated fatty acids. In addition, linoleic acid (18:2) is present in substantial quantities, implying that it is an important structural component. To investigate selectivity of fatty acid incorporation into epidermal lipids, we examined the metabolism of exogenous fatty acids in cultured human keratinocytes, grown at the air-medium interface to enhance differentiation. Keratinocytes were pulsed with [3H] oleic, [14C] stearic, [14C] palmitic, or [14C] linoleic acids; lipids were extracted and fractionated by thin layer chromatography. All fatty acids were taken up and incorporated into complex lipids in a dose-dependent manner that was linear over the first 60 min. These fatty acids were incorporated predominantly into phospholipids and triacylglycerols; their incorporation could be rank ordered: linoleic greater than oleic greater than or equal to palmitic greater than stearic acid. Less than 2% of each fatty acid taken up by keratinocytes was oxidized to CO2; therefore, these differences in utilization cannot be ascribed to differences in rates of beta-oxidation. In pulse-chase studies fatty acids incorporated initially into triacylglycerols, subsequently chased into phospholipids. [14C]Palmitic acid and [14C] acetate were incorporated into sphingolipids more efficiently than the other fatty acids studied. These studies demonstrate that 1) keratinocytes have the ability to incorporate exogenous fatty acids preferentially into complex lipids; 2) triacylglycerols provide a pool of fatty acids for phospholipid synthesis; and 3) palmitate and de novo synthesized fatty acid are preferably utilized for sphingolipid synthesis.