Inducible protective proteins: a potentially novel approach to chemotherapy

A number of toxic chemical and physical agents elicit the induction of a series of protein species, some of which react with the agents and render them non-toxic. A few of the induced species (e.g., metallothionein) are rich in thiol groups which might be expected to react with alkylating agents and render them non-toxic. If a safe means could be found for selectively enhancing the synthesis of alkylating agent-reactive species in normal but not tumor cells, such a procedure would have ramifications in the area of cancer chemotherapy. In this report, we have utilized a variety of trace elements (Zn, Se, Cu, As) as inducers of synthesis of protective species in line CHO Chinese hamster cells and in a number of derived variants to determine whether this type of approach can be utilized to increase resistance to alkylating agent toxicity. Our results indicate that Zn, Se, and Cu elicit a protective response (increased survival) against the toxic effects of iodoacetate or melphalan, and, at least in the case of zinc, at levels which are physiologically reasonable. Arsenite appears to be a marginally effective inducer in the CHO cell. The increased survival is not attributable to metallothionein inducibility, decreased availability of the alkylating agent in the medium or decreased uptake of the drug into the trace element-pretreated cells. The protective responses induced by zinc or selenite alone are additive in cells receiving both trace element prior to exposure to alkylating agent which suggests that different domains of response are elicited by the two metals. Based upon reported differences in inducibility of protective proteins between normal and tumor cells, a possibility is raised for a novel approach to alkylating agent chemotherapy, somewhat analogous to the protocol utilized in high dose methotrexate therapy

Molecular and somatic-cell genetic analysis of metal-resistance mechanisms in mammalian cells

Combined molecular genetic analyses and somatic cell systems were utilized to delineate factors involved in metal metabolism. Somatic cells derived by selection procedures using toxic heavy metals were used to define primary factors involved in acquisition of metal resistance. Such cell variants permitted isolation of the specific genes involved in conferring heavy metal binding proteins, the metallothioneins. (MT). These isolated genes provided the molecular probes to dissect the multiple levels of control and organization of this one set of genes responsible for metal resistance. Studies on the roles of MT in metal resistance used these variants and cell lines derived from human tumors to illustrate that MTs play an important but not exclusive role in cadmium detoxification. Studies on Cd/sup + +/ responses in human tumor derived cell lines showed several orders of magnitude differences in Cd/sup + +/ sensitivity in lines having similar MT responses. Analysis of cultured normal blood cell responses showed that the most Cd/sup + +/ resistant population, the granulocytes, did not produce significant quantities of MT. The results presented here further show a lack of correlation between MT and cytotoxic responses to Cd/sup + +/ in freshly cultured human leukemic peripheral blood cells. In these, enhanced Cd/sup + +/ uptake may be a factor determining enhanced sensitivity. Theses results together indicate that an adequate understanding of cellular responses to toxic metals will not be provided by elucidation of the role(s) of one or a few known metal binding proteins such as MT. Other factors and systems that modulate cellular uptake and sensitivity must first be defined