Topoisomerase I is differently phosphorylated in two sublines of L5178Y mouse lymphoma cells.
Two sublines of LY murine lymphoma, differing in sensitivity to CPT, served as source of topoisomerase I in order to compare the enzyme's properties. The activity of topoisomerase I isolated from LY-S cells of reduced sensitivity to CPT increased about 2-times more upon phosphorylation with casein kinase but was inhibited to a lesser extent upon dephosphorylation with alkaline phosphatase than the enzyme from the CPT-sensitive LY-R cells. The in vitro phosphorylation of LY-S enzyme restored its sensitivity to CPT. The in vitro incorporation of 32P into topoisomerase protein was about 1.7-times higher in LY-S than in LY-R enzyme. A reversed incorporation ratio was observed upon metabolic labelling. The level of topoisomerase I protein, determined by Western blot analysis using scleroderma anti-topoisomerase I antibodies, was about 1.5-times higher in LY-S than in LY-R cells. The level of topoisomerase I mRNA was similar in both sublines. These results indicate that the reduced sensitivity of LY-S cells to CPT is based on the lowered phosphorylation of topoisomerase I protein but does not depend on the expression of topoisomerase I gene.
Staron
K
K
Institute of Biochemistry, Warsaw University, Poland.
Kowalska-Loth
B
B
Zabek
J
J
Czerwinski
R M
RM
Nieznanski
K
K
Szumiel
I
I
D000818:Animals; D004264:DNA Topoisomerases, Type I / Q000235:genetics / Q000378:metabolism*; D008223:Lymphoma / Q000201:enzymology; D051379:Mice; D010766:Phosphorylation; D012333:RNA, Messenger / Q000378:metabolism; D014407:Tumor Cells, Cultured
D012333:RNA, Messenger; D004264:DNA Topoisomerases, Type I