Endogenous and exogenous factors affecting ribosomal RNA release from rat liver nuclei in a cell-free system.

rRNA release from isolated liver nuclei has been analyzed in a reconstituted cell-free system using density-gradient analysis and hybridization to a specific recombinant DNA probe to monitor the process. The cell-free system was shown previously to be energy- and cytosol-dependent and to support the formation and release of functional ribosomal subunits. The release of rRNA is now shown to have an absolute dependence on a 70000 dalton cytosol protein. Although in vivo studies suggest that chronic administration of thioacetamide may block formation of a protein involved in the nucleocytoplasmic transfer of ribosomes, the 70000 dalton transport factor is not affected by the treatment. Rather the defect appears to be localized to the nucleus, since it cannot be reversed with normal cytosol from a homologous source. Early stages of nRNA processing appear to be affected by thioacetamide, although additional effects on transport are not ruled out.

Absence of the cancer-associated factor with a molecular weight of 60,000 from the plasma of patients with a spectrum of nonneoplastic conditions.

We demonstrated previously (Cancer Res., 42: 4964-4969, 1982) that a tumor-associated factor was consistently present in the plasma of over 100 human cancer patients with tumors at 31 different sites. The plasma of healthy controls had very low activity in the biochemical assay. In the present study, we show by a combination of molecular sieving and assay of nuclear RNA transport that the tumor-associated factor, which has a molecular weight of 60,000, is undetectable in the plasma of healthy adults. The low activity reported earlier is due to three normal cell factors of markedly different molecular weight. Furthermore, the tumor factor is shown to be absent from the plasma of male and female patients hospitalized for a variety of nonmalignant surgical conditions. Only the plasma from patients who were pregnant, suffered from chronic renal failure, or had recent myocardial infarction gave false positives in the biochemical assay. However, in these cases, the activity was due to an increase in the normal tissue-associated factors and not to the appearance of the Mr 60,000 tumor-associated factor. The factor is present in amniotic fluid, confirming that it is a fetal factor which does not cross the placental barrier. Thus, it may be classified as an oncofetal factor. All four factors found in the plasma were identified in the cytosol from a human tumor. In summary, the tumor-associated factor appears to be tumor specific and can be unambiguously identified by bioassay of the plasma factors eluting from Sepharose CL-6 B columns in the Mr 60,000 region. It can also be identified by examination of sodium dodecyl sulfate:polyacrylamide gel electrophoretograms of the appropriate Sepharose CL-6 B fractions after removal of albumin.