Loss of TP53 function may contribute to 5fluorouracil (5-FU) resistance in colorectal cancer since TP53-deficient cells may be unable to undergo apoptosis in response to 5-FU-induced DNA damage. 5-FU treatment of TP53-deficient cells would provide useful information on the apoptotic response to drug-induced DNA damage in the absence of TP53 and its transcriptional targets. We investigated apoptosis induction and cell cycle alterations in response to short-term treatment with two different 5-FU concentrations following siRNA-mediated knockdown of TP53 in the TP53-proficient HCT116 colon cancer cell line. We focused on high-dose 5-FU treatment to investigate the apoptotic phenotype in 5-FU-treated cultures since this dose resulted in apoptosis induction at 24 h of treatment, whereas clinically-relevant bolus 5-FU treatment of HCT116 cultures did not. Gene expression alterations were also assessed in 5-FU-treated HCT116 cultures using whole genome expression arrays. Compared to 5-FU-treated TP53-proficient HCT116 cultures, 5-FU-treated TP53depleted HCT116 cultures showed lack of CDKN1A induction, decreased apoptotic levels, decreased FAS and TNFRSF10B transcript levels and cleaved PARP protein levels, G 1 /S transition arrests, decreased CCND1 protein levels, and smaller intra-S phase arrests. Alterations in gene expression in 5-FU-treated TP53-depleted HCT116 cultures confirmed previously-reported TP53 target genes and suggested potentially novel TP53 target genes (e.g. APOBEC3C, BIRC3, JMJD2B, LAMP3, MYO1E, PRRG1, SULF2, TACSTD2, TncRNA, ZFYVE20) that may play a role in mediating the 5-FU-induced DNA damage response in TP53-proficient cells. Abrogation of TP53 function in 5-FUtreated HCT116 cultures results in reduced apoptosis, TP53and CDKN1A-independent G 1 /S phase arrests that may be protective against apoptosis, smaller intra-S phase arrests, and transcript level decreases of both reported TP53 target genes as well as potentially novel TP53 target genes.

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The cellular response to chemoradiotherapy was investigated in cells of the HCT116 (wild-type TP53) and HT29 (mutated TP53) human colorectal cancer cell lines to better understand how the chemotherapeutic agent 5-fluorouracil (5-FU) acts as a radiosensitizer in vitro and how it contributes to the well-documented greater efficacy of chemoradiotherapy compared to radiotherapy (or chemotherapy) alone. A bolus 5-FU treatment protocol that simulated actual clinical clearance kinetics was used with a radiation dose given within 90 min after drug addition. The involvements of key signaling pathways (DNA damage response, cell cycle progression, cell proliferation, cell death) in cell responses were investigated concurrently, allowing for direct correlations of numerous treatment response phenotypes. Early DNA damage response, substantial cell death, loss of clonogenicity, and senescence characterized both radiotherapy- and chemoradiotherapy-treated cultures but not chemotherapy-treated cultures. The largest G(2)/M arrests and strongest correlation of senescence with non-clonogenicity were seen in radiotherapy- and chemoradiotherapy-treated HCT116 cell cultures, suggesting that functional TP53 could play a role in maintaining/inducing these cellular phenotypes. Overall, chemoradiotherapy proved to be the most effective treatment modality since it resulted in the strongest growth inhibitions, largest G(2)/M arrests, largest fractions of senescent cells, and complete loss of clonogenicity in both cell lines.

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Acid back diffusion into the rat stomach mucosa leads to gastric vasodilation. We hypothesized that histamine, if released from the rat mucosa under such conditions, is mast cell derived and involved in the vasodilator response. Gastric blood flow (GBF) and luminal histamine were measured in an ex vivo chamber. Venous histamine was measured from totally isolated stomachs. Mucosal mast cells (MMC), submucosal connective tissue mast cells (CTMC), and chromogranin A-immunoreactive cells (CgA IR) were assessed morphometrically. After mucosal exposure to 1.5 M NaCl, the mucosa was subjected to saline at pH 5.5 (control) or pH 1.0 (H(+) back diffusion) for 60 min. H(+) back diffusion evoked a marked gastric hyperemia, increase of luminal and venous histamine, and decreased numbers of MMC and CTMC. CgA IR cells were not influenced. Depletion of mast cells with dexamethasone abolished (and stabilization of mast cells with ketotifen attenuated) both hyperemia and histamine release in response to H(+) back diffusion. GBF responses to H(+) back diffusion were attenuated by H(1) and abolished by H(3) but not H(2) receptor blockers. Our data conform to the idea that mast cells are involved in the gastric hyperemic response to acid back diffusion via release of histamine.