Supplementary Figure S2. A) Quantification of gemcitabine metabolising proteins in four bladder cancer cell lines. Densitometry of bands was measured using the Licor Odyssey system and normalised to loading control (β tubulin). In order to make accurate inter-blot comparisons, protein levels were then normalised to the RT112 cell line. B) Quantification of siRNA knockdown in RT112 cells. All samples were normalised against β tubulin. Knockdown levels were quantified as a percentage of protein remaining compared to levels of nonsilencing control siRNA treated samples.
ARTICLE ABSTRACT
Purpose: In a recent phase II clinical trial, low-dose (100 mg/m2) gemcitabine showed promise as a radiosensitizer in bladder cancer, but underlying mechanisms lack elucidation. Here, we investigated the mechanism of radiosensitization by low-dose gemcitabine in bladder cancer cell lines.Experimental Design: Four bladder cancer cell lines were screened for radiosensitization by low-dose gemcitabine using clonogenic assay, and gemcitabine-resistant RT112gem and CALgem cells created by exposure to increasing gemcitabine doses. Four key gemcitabine-regulatory genes were knocked down by transient siRNA. Nude mice carrying CALgem subcutaneous xenografts were exposed to 100 mg/kg gemcitabine ± ionizing radiation (IR) and response assessed by tumor growth delay.Results: Gemcitabine was cytotoxic in the low nanomolar range (10–40 nmol/L) in four bladder cancer cell lines and radiosensitized all four lines. Sensitizer enhancement ratios at 10% survival were: RT112 1.42, CAL29 1.55, T24 1.63, and VMCUB1 1.47. Transient siRNA knockdown of deoxycytidine kinase (dCK) significantly reduced radiosensitization by gemcitabine (P = 0.02). RT112gem and CALgem cells displayed robust decreases of dCK mRNA and protein levels; reexpression of dCK restored gemcitabine sensitivity. However, CALgem xenografts responded better to combination gemcitabine/IR than either treatment alone (P < 0.001) with dCK strongly expressed in the tumor vasculature and stroma.Conclusions: Gemcitabine resistance in bladder cancer cell lines was associated with decreased dCK expression, but gemcitabine-resistant xenografts were responsive to combination low-dose gemcitabine/IR. We propose that dCK activity in tumor vasculature renders it gemcitabine sensitive, which is sufficient to invoke a tumor response and permit tumor cell kill in gemcitabine-resistant tumors. Clin Cancer Res; 20(21); 5435–45. ©2014 AACR.
