WE have already presented preliminary evidence1 that crude plasma from tumour-bearing rats contains elevated concentrations of macromolecules which stimulate the release of messenger RNA (mRNA) from isolated nuclei. The detection of these putative regulatory components was facilitated by a cell-free system developed originally to study messenger2–5 and ribosomal6,7 RNA (rRNA) processing and transport in vitro. Since both maximal RNA transport and normal nuclear RNA restriction in vitro (that is, equivalence between messengers released from nuclei in vivo and in vitro) require the presence of macromolecules from homologous cytosol2,8 it is possible that the tumour cell cytoplasm is the source of the putative regulatory macromolecules in the peripheral blood of tumour-bearing animals. This possibility is supported by evidence9,10 that factors involved in the regulation of cellular proliferation are released to circulation from tumour cells, and by the observation11 that the enzyme composition of the host liver of tumour-bearing rats converges to that of the tumour. The spectrum of messengers transported from normal rat liver nuclei in homologous cytosol is significantly different from that transported in hepatoma cytosol3, suggesting that differences exist in these putative regulatory components of the two tissues.
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