Abstract
In common with other tumour cell lines but in contrast to normal cells, the human adenocarcinoma cell line A549 showed a biphasic regulation of the LDL receptor activity during growth: both LDL binding and metabolism (sum of internalised and degraded LDL) increased during the log exponential growth phase and decreased when the cells approached confluence. This period of increasing LDL receptor activity coincided with a high resistance to cholesterol down-regulation which suggested a sterol-independent pathway of stimulation. Since A549 cells have an outocrine loop of growth factors, two of which have tyrosine kinase activity, the LDL receptor activity was tested in the presence of the tyrosine kinase inhibitor, genistein. When cells were incubated in the absence of cholesterol (LPDS medium), the inhibition that occurred was two-fold higher during the exponential growth phase than during the confluent phase. Moreover, the residual LDL binding and metabolism after genistein inhibition were completely resistant to down-regulation by cholesterol only during the growth phase. When cholesterol was present (FCS medium), inhibition was observed only during the growth phase. The inhibition of LDL receptor activity by genistein was found to be the result of a loss in the number of LDL binding sites, while the dissociation constant was not affected. This loss was accompanied by a disappearance of mRNA as shown by RNase mapping. By comparison, LDL receptor activity of normal cells (fibroblasts) was also affected by genistein during the exponential growth phase but was much more cholesterol-dependent. Taken together, these results suggest that the tyrosine kinase pathway is essential to up-regulate LDL receptor expression in highly dividing cells and particularly in tumour cells in which the sterol regulation is deficient.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: 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.