Table 3.

Changes in DNA topoisomerase I levels quantitatively or quantitatively affected cellular irinotecan sensitivity

Cells	Change in sensitivity to irinotecan and structurally related Top1 inhibitors and relationship with changes in Top1 expression/primary structure		Ref.
Human lung (A549), colon (HT29), stomach (St-4), and murine leukemia P388	The three human solid tumor cell lines were treated with a sublethal concentration of camptothecin and camptothecin-resistant P388 cells were established in vivo by treatment with 30 mg/kg of irinotecan over repeated transplantations	Top1 protein levels were significantly lower in resistant HT29/CPT, St-4/CPT, and P388/CPT, but not in resistant A549/CPT. Nuclear extracts (4.1 ng protein) from HT29 showed slightly higher Top1 activity than those (12.5 ng protein) from the resistant HT29/CPT cells. Resistance to topotecan is attributable to qualitative difference of Top1 proteins	Sugimoto et al.[31]
Human breast (MB MDA436, MB MDA231, ZR75-1, and MCF7) and a colon (HT29) cancer cell line	Change in Top1 gene copy number: 1 and 4 in breast carcinoma cells, 5 in a breast carcinoma, 5 in a colon cancer; and 6 in a breast carcinoma	Cells with low Top1 gene copy number show the highest resistance to SN-38 Copy number and Top1 protein expression highly correlated (r = 0.92)	McLeod and Keith[32]
A human colon cancer cell line, DLD-1	Missense Top1 mutation (Gly365Ser)	Cells exposed continuously to SN-38 were resistant to SN-38, camptothecin, and topotecan (10-100-fold) and carried a missense 365Ser allele	Arakawa et al.[33]
A human glioblastoma cell line, SF295	Two different camptothecin-resistant sublines were established by stepwise selection by BN80915 and homocamptothecin (both 50 nM)	Both sublines were 7-27-fold resistant to topotecan and camptothecin. Reduced Top1 mRNA (< 50%) and protein expressions were observed	Liao et al.[34]
A human colon cancer cell line, HCT116	Missense mutations of Top1 were found in 5 different resistant cell clones obtained by stepwise increased SN-38 concentrations: R621H, E710G and L617I	No difference in Top1 expression in the 5 resistant cells. All resistant cells treated with 1 mM SN-38 showed lower amounts of Top1-DNA complex as compared to the drug-sensitive HCT116	Gongora et al.[35]
Three human colon cancer cell lines, HCT116, HT29, and LoVo	Drug-resistant cells induced by exposing to increasing concentrations of SN-38 HCT116/SN-38 carried two Top1 mutations, R364K and G717R HT29/SN-38 showed 20% loss of Top1 compared with sensitive HT29 cells through chromosome 20q rearrangement	HT29/SN-38 and LoVo/SN-38 exhibited upregulations of ABCG2, by 25- and 60-fold, respectively. All SN-38-resistant cells were cross-resistant to an indenoisoquinoline non-camptothecin Top1-targeting drug in clinical trials	Jensen et al.[36]
Four human colon cancer cell-lines, HCT116, HT29, DLD1, and LoVo	HT29 cells express the least amount of carboxyl-terminal domain RNA polymerase II polypeptide small phosphatase 1 (CTDSP1) and were the most resistant to SN-38. HCT116 cells expressed CTDSP1 in a larger amount than HT29	siRNA-mediated downregulation of CTDSP1 in HCT116 cells activates the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which phosphorylated Top1 at Serine 10. The phosphorylated Top1 is apt to undergo SN-38-mediated proteasomal degradation. CTDSP1-downregulated HCT116 cells acquired a small degree of SN-38 resistance. HT29 cells showed a higher phosphorylation and activated status of DNA-PKcs. Top1 in HT29 cells was spontaneously and more easily degraded	Matsuoka et al.[37]

Top1: Topoisomerase I; CPT: camptothecin.