Influence of cell proliferation and cell cycle phase on expression of estrogen receptor in MCF-7 breast cancer cells.
Kata kunci
Abstrak
In the present study, the effects of cell cycle phase and proliferation rate on the expression of specific estrogen binding activity were explored in hormone-dependent human breast cancer cells. A technique was developed to alter the proliferative rate of MCF-7 cells by plating at different densities. The doubling time ranged from 20 to 48 hr, showing a negative relation to the number of plated cells. Slowly proliferating cells had accumulated more than twice as much estrogen receptor (ER) activity as did fast-proliferating cells. Exposure of exponentially growing cells to isoleucine-deficient medium resulted in decreased thymidine incorporation and disappearance of detectable cellular ER activity. Overall protein synthesis was reduced by only 30% in cells growing in isoleucine-free medium. At 24 hr after release from isoleucine deprivation, ER levels are fully restored, although thymidine incorporation does not resume for an additional 6 to 8 hr, and increases in cell number are not seen for 24 hr. Exposure of exponentially growing cells to 2 mM thymidine for 24 hr produced partially synchronized MCF-7 cells (approximately 70%). Six hr after release from excess thymidine, cells reached S phase; after 9 hr, G2; and after 18 hr, G1. ER levels immediately and, 6 hr after release, remained unchanged, showed a slight increase at 9 hr, and showed an increase of about 50 to 60% at 18 hr. These data suggest that: (a) ER binding activity and DNA synthesis can be dissociated; (b) ongoing protein synthesis is necessary for maintenance of cellular ER activity; and (c) ER is apparently synthesized throughout the cell cycle, with some evidence that this is predominantly in G1 and G2.