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.
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.
Casciola
L A
LA
Department of Medical Biochemistry, University of Cape Town Medical School, South Africa.
van der Westhuyzen
D R
DR
Gevers
W
W
Coetzee
G A
GA
D000906:Antibodies / Q000032:analysis; D001665:Binding Sites / Q000187:drug effects; D002478:Cells, Cultured; D003513:Cycloheximide / Q000037:antagonists & inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D006706:Homeostasis / Q000187:drug effects; D006801:Humans; D008715:Methionine / Q000494:pharmacology; D014176:Protein Biosynthesis; D011506:Proteins / Q000502:physiology*; D011691:Puromycin / Q000494:pharmacology; D011973:Receptors, LDL / Q000187:drug effects / Q000276:immunology / Q000378:metabolism* / Q000502:physiology; D012867:Skin / Q000187:drug effects / Q000378:metabolism*; D013499:Surface Properties; D013997:Time Factors
D000906:Antibodies; D011506:Proteins; D011973:Receptors, LDL; D011691:Puromycin; D003513:Cycloheximide; D008715:Methionine