Mechanisms of doxorubicin resistance in multidrug resistant human myeloma cells

Abstract

Multidrug resistance is a phenomenon associated with the emergence of simultaneous cross-resistance to the cytotoxic actions of a wide variety of structurally and functionally unrelated antineoplastic agents. One of the agents to which cross-resistance is commonly observed is doxorubicin, a commonly used antineoplastic. Studies were undertaken to determine the mechanism of doxorubicin resistance in multidrug resistant 8226 human myeloma cells. When sensitive and resistant cells were exposed to the same extracellular concentration of doxorubicin there was a decrease in the quantity of DNA lesions in the resistant subline which corresponded to a decrease in doxorubicin accumulation. When the extracellular concentration of drug was adjusted to yield equivalent intracellular levels these differences were removed. Studies utilizing an isolated nuclei system revealed no differences in the formation of DNA lesions between the sensitive and resistant cells when exposed to the same concentration of drug. Studies were undertaken to determine if the resistant subline had an increased capacity to detoxify doxorubicin via glutathione-based enzyme systems. The activities of glutathione-s-transferase and glutathione peroxidase were not found to be elevated in the resistant subline. There was a significant elevation in the nonprotein sulfhydryl content of the resistant cells as compared to the drug-sensitive line. This elevation was unstable in the absence of doxorubicin, displaying a steady decline until reaching baseline levels found in the sensitive cells. The decrease in NPSH content in the resistant line was not accompanied by an alteration in doxorubicin resistance. Thus, it appears that glutathione-based enzymatic detoxification is not causally related to doxorubicin resistance in 8226 human myeloma cells. Verapamil, an agent shown by previous studies to modulate doxorubicin resistance, led to an increase in the formation of doxorubicin-induced DNA lesions in the resistant cells secondary to an increase in intracellular drug accumulation. It had no effect on doxorubicin-induced DNA lesions or drug accumulation in the sensitive cells. Verapamil thus appears to be reversing doxorubicin resistance by increasing drug accumulation and thereby enhancing DNA damage. Under these circumstances there was a good correlation between doxorubicin accumulation, DNA damage, and cytotoxicity in the 8226 cells. The conclusion is drawn that drug accumulation accounts for the majority of doxorubicin resistance in the 8226 human myeloma cell line.