Effect of pO₂ on Antitumor Drug Cytotoxicity on MDR and Non-MDR Variants Selected from the LoVo Metastatic Colon Carcinoma Cell Line

Abstract

Two chemosensitive cell lines, LoVo-fusoid (LoVo-f) and LoVo-small cells (LoVo-sc) were derived from the original LoVo cell line. These two variants and the multidrug-resistant (MDR) cell line LoVo-Dox were screened for various properties. In non-permeabilized cells, only LoVo-sc showed mucin-2 staining whereas labelling was positive in all permeabilized cell lines. As shown by electron microscopy screening and by relative resistance to trypsin detachment, only LoVo-sc cells showed strong mucus secretion. All three cell lines displayed strong staining for P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP) and lung-resistance-related protein (LRP) in different locations according to the drug resistance state. The three cell lines showed intracellular labelling of LRP and MRP. The sensitive cells showed P-gp in a large perinuclear ring and in the cytoplasm, but little (LoVo-sc cells) or no staining (LoVo-f cells) was shown at the plasma membrane level. For the Lovo-Dox cells, P-gp was located in the plasma membrane, in cellular anchorages and in the cytoplasm as well. Cell resistance against antineoplastic agents often results from mobilization of various factors, the modulation of which is linked to the culture conditions. As most of the protocols utilize cells growing in (air+5-10% CO₂) atmosphere e.g. 20% O₂, balance of the respective participants in the MDR multi-modal mechanism may not be representative of the in vivo situation and may lead to erratic pharmacological response. Indeed, cells within solid tumours were exposed to low pO₂, most of them being under hypoxic condition (0.1-5% O₂). In the absence of anticancer drugs, all LoVo cell lines grew notably faster at 20% O₂ than at 5% O₂. Moreover, respective sensitivities of both non-MDR variants to doxorubicin were altered according the pO₂. Whatever the pO₂ was, virtually none of the antioxidants tested affected the cytotoxic activity of doxorubicin for the three cell lines. By contrast, trolox showed a strong inhibitory effect on doxorubicin activity. These results underline the importance of evaluating the role of hypoxia on the cytotoxic effect of chemotherapeutic agents used either as single drugs or in combination therapy.