Journal of Lipid Research, Vol 29, 1481-1489, Copyright © 1988 by Lipid Research, Inc.
Low density lipoprotein receptor degradation is influenced by a mediator protein(s) with a rapid turnover rate, but is unaffected by receptor up- or down-regulation
LA Casciola, DR van der Westhuyzen, W Gevers and GA Coetzee
Department of Medical Biochemistry, University of Cape Town Medical School, South Africa.
Treatment of cultured human skin fibroblasts with cycloheximide retarded
the down-regulation of low density lipoprotein (LDL) receptor activity
caused by 25-hydroxycholesterol. The rate of LDL receptor degradation,
measured directly by means of [35S]methionine pulse-chase experiments, was
also markedly inhibited by cycloheximide (or puromycin), suggesting that
continuous synthesis of a short-lived mediator protein(s) was necessary for
normal LDL receptor turnover. In the absence of cycloheximide, both the up-
and down-regulation of LDL receptor activity took place with a half-time of
approximately 12 hr. Pulse-chase measurements with [35S]methionine yielded
a receptor half- life (t1/2) of 11.7 +/- 2.2 hr (n = 10) in up-regulated
cells; the t1/2 in the partially down-regulated state was similar. The
presence of LDL or 25-hydroxycholesterol did not alter this degradation
rate. Regulation of LDL receptor activity under these various culture
conditions therefore probably occurred solely as a result of changes in the
rate of receptor synthesis. The cycloheximide-sensitive factor(s) that
influences receptor turnover apparently did not play a regulatory role in
the up- or down-regulation of the LDL receptor.