Diffusible factor(s) controlling density inhibition of 3T3 cell growth: a new approach

In 3T3 Swiss mouse fibroblasts, incorporation of phosphate into cells and phosphorylation of small organic compounds were increased by shaking dense cultures. This response was not obtained with SV40 transformed Swiss 3T3 cells (SV-3T3). It appeared likely that these results could be accounted for by an inhibitor released from 3T3 cells but not from SV-3T3 cells. Our new method of co-incubation of sparse and dense cultures allowed us to demonstrate inhibition of growth and phosphate metabolism in sparse 3T3 cultures which were shaken in the presence of dense cultures. The inhibition was much less when the cultures were co-cultivated but not shaken. The inhibition of phosphate incorporation in acid-soluble and acid-insoluble fractions of sparse cultures was observed as early as 20 minutes of co-incubation in the presence of dense cultures, so this inhibition is not the result of depletion of growth factors in the medium. Our experiments suggest that an inhibitor(s) was released from dense cultures of 3T3 cells.

Nuclear and mitochondrial origin of rat liver double-stranded RNA

We have studied the intracellular location of double-stranded RNA from rat liver. The majority of this dsRNA is associated with nuclear and mitochondrial fractions and the remaining portion is associated with microsomes. Mitochondrial dsRNA hybridizes specifically with purified strands of mitochondrial DNA and seems therefore to arise from transcription of the latter. A fraction of nuclear dsRNA hybridizes with repetitive sequences of nuclear DNA. Because of self-hybridization of dsRNA strands, it was not possible to determine whether another fraction of DSRNA is homologous to nonrepetitive sequences of DNA. Both mitochondrial and nuclear types of dsRNA are not retained on oligo(dT) cellulose columns and therefore seem to lack long poly A sequences. The mechanism of formation of dsRNA is discussed.