A kinetic model to evaluate cholesterol efflux from THP-1 macrophages to apolipoprotein a-1

The kinetics (0 to 3 h) of cholesterol efflux to delipidated apolipoprotein A-1 were investigated, and the experimental data were best fitted to a mat hematical model that involves two independent pathways of cholesterol efflu x. The first pathway with a rate constant of 4.6 h(-1) is fast but removes only 3-5% of total cholesterol. After preconditioning apoA-1, it was found that this pathway remains, and hence it is a property of the cholesterol-lo aded cells rather than due to modification on the apolipoprotein. This fast initial efflux does not seem to contribute to cholesterol efflux at later stages (> 1 h) where a second pathway predominates. However, the fast initi al efflux pool can be restored if apoA-1 is withdrawn. The second slower pa thway (kmembrane-media = 0.79 h(-1)) is associated with cholesterol ester h ydrolysis whose rate constant could be experimentally verified (k(cal) = 0. 43, k(exp) = 0.38 +/- 0.05). The model suggests that two different plasma m embrane domains are involved in the two pathways. Loading of the cells with an oxysterol, 7-ketocholesterol (7K), inhibits efflux from both pathways. The model predicts that 7K decreases the initial efflux by decreasing the a vailable cholesterol (by possibly affecting lipid packing), while all rate constants in the second pathway are decreased. In conclusion, the kinetic m odel suggests that cholesterol efflux to apoA-1 is a two-step process. In t he first step, some of the plasma membrane cholesterol contributes to a fas t initial efflux (possibly from lipid rafts) and leads to a second pathway that mobilizes intracellular cholesterol mobilization.