Background and Objectives. The extent of tumour hypoxia correlates with resistance to chemotherapeutic agents. We have previously demonstrated that in vitro hypoxia-induced resistance to various anti-cancer drugs can be attenuated by low concentrations of nitric oxide (NO) mimetic agents. The natriuretic peptides (NPs) are molecules that mediate their cellular effects by activating a signalling pathway similar to the pathway activated by NO. Thus, the aims of the present study were to determine whether atrial natriuretic peptide (ANP) is able to inhibit hypoxia-induced chemoresistance in prostate and breast carcinoma cells and to eludidate the potential mechanism through which this may occur. Methods. RT-PCR and competitive ANP binding studies revealed the presence of the three known NP receptors (NPR-A, NPR-B and NPR-C) in DU-145 human prostate and MDA-MB-231 breast carcinoma cells. Furthermore, ANP binding to these cells was unaffected by culture in 0.5% vs. 20% O2. Results. Clonogenic assays revealed that, compared with incubation in 20% O2, incubation of DU-145 and MDA-MB-231 cells in 0.5% O2 for 24 h resulted in a corresponding 4- and 10-fold increase (P < 0.001) in their survival following a 1-h exposure to doxorubicin (12.5 μM). While small concentrations of ANP (10-7 - 10-13 M) did not affect the sensitivity to doxorubicin in DU-145 and MDA-MB-231 cells incubated in 20% O2, similar concentrations of ANP inhibited the survival of these cells incubated in 0.5% O2 by up to 50% (P < 0.006). Incubation with low concentrations (1, 10 μM) of Zaprinast®, an inhibitor of cGMP-dependent phosphodiesterases 1 and 5, was also able to attenuate hypoxia-induced chemoresistance up to 47% (p<0.0001). Furthermore, the chemosensitizing effect of ANP on hypoxic DU-145 cells was inhibited by treatment with KT5823 (15 μM), an inhibitor of cGMP-dependent protein kinase G (PKG). Conclusions. These results provide evidence that NPs attenuate hypoxia-induced chemoresistance by activating a signalling pathway similar to the one activated by NO. Our findings indicate a potential use for NP’s as adjuvants in cancer chemotherapy.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]