Differences in resistance to 5-fluorouracil as a function of cell cycle delay and not apoptosis

Abstract

Summary A series of human embryo fibroblasts has previously been shown to display increasing resistance to the antimetabolites methotrexate (MTX) and N-phosphonacetyl-L-aspartate (PALA) with increasing tumorigenicity. This increased resistance was found to be further increased as a result of salvage pathway activity for purine and pyrimidine biosynthesis. A similar pattern of increasing resistance paralleling increasing tumorigenicity has now been shown to occur with 5-fluorouracil (5-FU), which is independent of salvage pathway activity. The KMS normal cell line was found to be more sensitive to 5-FU than either the immortalised KMST or tumorigenic KN-NM cell lines. Immunohistochemical analysis of the three cell lines demonstrated high levels of p53 protein in the KMST and KN-NM cell lines, but undetectable p53 levels in the KMS cell line. From these data it was hypothesised that a difference in p53 function may be causing the difference in the patterns of sensitivity observed in the three cell lines. P53 is now believed to function as a regulator of a GI to S cell cycle checkpoint and as an inducer of apoptosis following DNA damage to the cell.The differences in sensitivity of the cell lines could not be explained by differences in the levels of apoptosis but could be attributed to differences in cell cycle response. Our evidence suggests that loss of cell cycle control, possibly through loss of p53 function, is an important factor in increasing the drug resistance of fibroblast cell lines.

S-phase accumulation precedes apoptosis induced by preoperative treatment with 5-fluorouracil in human colorectal carcinoma cells

“…Three mechanisms of the cytotoxic action of 5‐FU have been proposed: 1) 5‐FU is metabolized to 5‐fluorodeoxyuridine monophosphate(FdUMP), which suppresses thymidylate synthetase (TS) and subsequently inhibits DNA synthesis18; 2) the incorporation of FdUMP into nuclear RNA has important metabolic consequences for the cell that lead to cytotoxity19, 20; and 3) 5‐FU residues are incorporated directly into DNA, resulting in DNA damage as a consequence of ineffective repair 21, 22. Of the three mechanisms, the third may account for 5‐FU‐induced apoptosis,23, 24 whereas the first likely predominates in cell cycle effects, including partial synchronization of cycling cells in the S‐phase and subsequent retarded progression through the rest of the cell cycle 25–29…”

Preoperative treatment with tegafur suppositories enhances apoptosis and reduces the intratumoral microvessel density of human colorectal carcinoma

“…Nabeya et al and Pickard et al reported that 5‐FU induced expression of the wild‐type p53 gene, followed by the induction of transcription of a variety of genes, including transcriptional factors such as Bax and p21; this resulted in arrest of cell growth and induction of apoptosis 2, 3. We demonstrated that 5‐FU induced apoptosis in human gastric carcinoma cell lines MKN‐45 and MKN‐74, which carry the wild‐type p53 gene, but not in MKN‐28 (which carries a mutated p53 gene) or in KATO‐III (which lacks the gene) 4.…”

Expression of thymidine phosphorylase in malignant ovarian tumors: correlation with microvessel density and an ultrasound‐derived index of angiogenesis

“…5-FdUMP in turn can inhibit thymidylate synthase, restricting de novo synthesis of thymidine monophosphate, and can ultimately inhibit the synthesis of DNA 26 . It has also been reported that 5-FU can induce apoptosis in some cancer cells 27,28 which in turn might stimulate the immune system. It is possible that treatment with 5′-DFUR might be selective for the inhibition of angiogenesis in ovarian tumors with high thymidine phosphorylase expression or high PSV.…”