Studies on experimental cancer immunosuppressive activity and mechanisms of action of mithramycin

Abstract

As a first step in evaluating the chemotherapeutic efficacy of mithramycin, an antitumor antibiotic, and correlating its toxic, antitumor, and immunosuppressive properties, the following studies were carried out:

The effect of this antibiotic on Yoshida ascites sarcoma (YAS) of rats.

Its effect on the immune response to sheep erythrocytes in rats.

Its capacity to inhibit DNA and RNA synthesis in vivo and in vitro in normal rat liver and YAS cells, and its effect on transcription in isolated nuclei.

The interaction of the antibiotic with DNA and chromatin from normal (rat liver) and neoplastic (YAS) tissues.

Mithramycin inhibits the growth of Yoshida ascites sarcoma (YAS), and anti-YAS immunity is observed in drug-treated tumor-free rats. This immunity can be adoptively transferred to syngeneic animals by the transfer of lymphocytes.

The antibiotic suppresses both humoral and cellular immune responses against sheep erythrocytes in rats. At low doses, mithramycin suppresses the haemolytic response, whereas at higher doses it enhances the haemolytic response, and a concomitant increase in the levels of serum IgM is observed. However, haemagglutinin levels, at the doses employed, are not affected.

The number of rosette-forming cells is decreased after antibiotic treatment.

Mithramycin inhibits DNA and RNA synthesis in vivo and in vitro, both in normal rat liver and YAS cells. DNA synthesis is inhibited at higher concentrations than those required for inhibition of RNA synthesis. In vitro protein synthesis is not affected by the antibiotic.

The antibiotic inhibits RNA synthesis in isolated nuclei from rat liver and YAS, and no specificity for nuclei from tumor tissue is observed. Furthermore, the antibiotic inhibits both Mg² - and Mn² -stimulated RNA polymerases, and again no preference for either of the enzymes is shown.

Keeping in view the suggested mode of action of this antibiotic-namely, inhibition of DNA-dependent RNA synthesis by binding of the drug to DNA-the binding of mithramycin with DNA and chromatin from rat liver (normal) and YAS (tumor) cells was investigated.

The apparent association constant (Kapp) and the number of binding sites for this antibiotic on DNA and chromatin have been calculated. There are more binding sites on DNA than on chromatin. The number of binding sites on YAS and rat liver chromatin for the antibiotic are similar, i.e., 0.065 per nucleotide, whereas on YAS and rat liver DNA they are 0.094 and 0.125 per nucleotide, respectively.