In vitro effects of lipoprotein-associated cytotoxic factor on rat prostate adenocarcinoma cells.

As assayed by the colony inhibition technique, sera from rats at an advanced stage of pregnancy were cytotoxic to a number of tumorigenic and "nontransformed" cell lines. The cytotoxic effect is not species specific. Primary fetal rat kidney cells were not susceptible to this cytotoxic effect. The level of demonstrable cytotoxicity in sera rose gradually as gestation advanced; it peaked at 48 hr before parturition and disappeared thereafter. Sera from control nonpregnant rats were not cytotoxic. The cytotoxic factor(s) in sera was demonstrable only in the very low-density lipoprotein (VLDL) (hydrated density less than 1.006 g/ml) fraction. Reconstitution studies suggest that: (a) inhibitors which usually mask the cytotoxic actitivity of VLDL in whole serum, are present; and (b) pregnancy initiates enhanced VLDL synthesis, which exceeds the masking effect. The possible role of this cytotoxic VLDL in host defense against neoplasia is discussed.