Abstract
Apolipoprotein B transports cholesterol in plasma as low density lipoprotein (LDL) and targets its delivery to cells by binding to a specific plasma membrane receptor. The cellular consequences of apoB binding to its receptor were investigated to determine whether it suppresses cholesterol biosynthesis and reduces the number of cellular receptors for the apoprotein. Upon preincubation of fibroblasts with lipoprotein-deficient medium alone or supplemented with either LDL or apoB complexed to BSA (apoB-BSA), LDL suppressed cholesterol biosynthesis, but apoB enhanced it. Similarly, fibroblasts preincubated in medium supplemented with LDL bound decreased amounts of either (125)I-labeled LDL or (125)I-labeled apoB-BSA to their receptors, while preincubation with apoB-BSA increased the binding relative to the controls. These latter results occurred in association with a decrease in cellular cholesterol content, indicating that apoB in the medium bound cholesterol and removed it from the cells, thus stimulating both cholesterol synthesis and cellular binding of apoB. Accordingly, fibroblast cholesterol synthesis and the number of functional LDL receptors are not suppressed by the binding of the apoprotein to the receptor, and the known role of apoB remains that of transporting cholesterol in plasma and delivering it to the cell. A possible physiologic role for apoB in depleting cells of cholesterol is presently unknown since apoB is not known to exist free in plasma; however, these findings demonstrate such a functional capability for this apoprotein.-Shireman, R. B., and W. R. Fisher. Apolipoprotein B: its role in the control of fibroblast cholesterol biosynthesis and in the regulation of its own binding to cellular receptors.
Highlights
Apolipoprotein B transports cholesterol in plasma as low density lipoprotein (LDL) and targets its delivery to cells by binding to a specific plasma membrane receptor
We have recently shown that the binding of LDL to its plasma membrane receptor results from the direct interaction of apolipoprotein B with the receptor, and that once bound, apoB is internalized and undergoes proteolytic hydrolysis in a manner analagous to the internalization and hydrolysis of native apoB when it exists within the LDL macromolecule [4].’
To measure the effect of preincubation of fibroblasts with apoB on the functional expression of the cellular LDL receptor, fibroblasts were preincubated for 20 hr in tissue culture medium containing 5% lipoprotein-deficient serum only or supplemented with LDL or apoB complexed with bovine serum albumin (BSA)
Summary
Apolipoprotein B transports cholesterol in plasma as low density lipoprotein (LDL) and targets its delivery to cells by binding to a specific plasma membrane receptor. The question may be posed as to whether the regulation of these metabolic reactions is solely under the control of the cellular content of cholesterol, which is delivered by LDL, or whether the metabolic regulation results in part as a direct consequence of the binding of LDL with its plasma membrane receptor.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
