Beta-cyclodextrin facilitates cholesterol efflux from larval Manduca sexta fat body and midgut in vitro

The ability of 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) and methyl-beta-cyclodextrin (MbetaCD) to promote cholesterol efflux from [3H]cholesterol-labeled larval Manduca sexta fat body and midgut was tested. In fat body, both beta-cyclodextrins induced a two-phase efflux of cholesterol. The first rapid phase depended on cyclodextrin concentration and was more rapid for MbetaCD than for HPbetaCD. The second, slower, phase was independent of cyclodextrin concentration and type. In midgut, only the concentration-dependent phase was observed; the rate constants are approximately 85% slower than for fat body. In both cases, a low activation energy for transfer was observed, consistent with a collision mechanism where cyclodextrin interacts directly with cholesterol in plasma membrane to affect transfer. In fat body, the second slower phase is suggestive of a second pool of exchangeable cholesterol and most likely represents transfer of cholesterol from internal membranes or different lateral domains of the plasma membrane. The lack of this second phase in midgut suggests that midgut has only a single pool of exchangeable cholesterol. Although the rates are somewhat different, the overall kinetic pattern for cyclodextrin-mediated cholesterol transfer in insect fat body closely resembles that for vertebrate cells, while the single pool behavior of the midgut is not found in vertebrate cells.