TITLE:
Gap Junctional Intercellular Communication Increases Cytotoxicity and Reduces Resistance to Hydroxyurea
AUTHORS:
Randall J. Ruch, Paul D. Boucher, Brian G. Gentry, Donna S. Shewach
KEYWORDS:
Bystander Effect, Drug Resistance, Gap Junctions, Hydroxyurea, Ribonucleotide Reductase
JOURNAL NAME:
Journal of Cancer Therapy,
Vol.5 No.13,
November
7,
2014
ABSTRACT: Background: Gap junctions enable small
molecules to diffuse between adjacent cells and have been associated with
greater cytotoxicity of radiation and anti-cancer drugs. We investigatedwhether this gap junctional
intercellular communication (GJIC) affected the cytotoxicity of the classicribonucleotide reductase (RR) inhibitor and anti-cancer
agent, hydroxyurea (HU). Materials and Methods: We used GJIC-proficient and
deficient, connexin 43-expressing WB rat liver epithelial cell lines. We
compared HU toxicity by crystal violet assay, effects of the drug on
deoxynucleotide pools by HPLC, and ability of GJIC to increase toxicity of
HU-resistant cells through a bystander effect in co-culture experiments.
Results: GJIC-proficient cells were three- to five-fold more sensitive (IC500.1 mM) to HU than
GJIC-deficient derivatives (IC500.3 - 0.5 mM). This
sensitivity depended upon GJIC because treatment of GJIC-proficient cells with
the GJIC blocker oleamide decreased HU toxicity by approximately 60% - 80% and
restoration of GJIC in GJIC-deficient cells by stable transduction of connexin
32-encodingGjb1increased
HU toxicity (IC500.1
mM). The effects were not due to connexin expressionper seor its localization since all
cell lines expressed comparable quantities of connexin 43 that was localized to
the plasma membrane. Also HU sensitivity was not related to differential
effects on nucleotide metabolism in the cells. Thymidine triphosphate levels
increased and deoxyadenosine triphosphate levels decreased similarly (15% -
20%) in GJIC-proficient and deficient cells over 24 h of HU treatment. More
importantly, when HU-resistant cells were co-cultured with sensitive cells, the
resistant cells were killed only when GJICwas present. Conclusion: The data suggest that GJIC enhances
cytotoxicity and decreases resistanceto HU.
These results may be important clinically if GJIC can be enhanced in
drug-resistant cells.