Mitogen‐stimulated events in nuclei of Swiss 3T3 cells Evidence for a direct link between changes of inositol lipids, protein kinase C requirement and the onset of DNA synthesis

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Abstract
Two different clones of Swiss 3T3 cells belonging to the same original cell line have been obtained, one of which was unresponsive to mitogenic stimulation (e.g. insulin‐like growth factor‐I, bombesin, insulin‐like growth factor‐I + bombesin), while the other clone showed a very high rate of DNA synthesis under identical conditions as demonstrated by 5‐bromodeoxyuridine incorporation. Both types of cells expressed the IGF‐I receptor and showed high contact inhibition. When highly purified nuclei from responsive cells, treated for a short time with bombesin and insulin‐like growth factor‐I or insulin‐like growth factor‐I alone, were incubated with [γ‐32P]adenosine triphosphate, the labelling of phosphatidylinositol‐mono‐ und diphosphate decreased when compared to controls, while this transient effect did not appear in the nuclei from unresponsive cells. Similarly nuclear protein kinase C is activated only in responsive cells. Therefore, it seems that a direct link exists between polyphosphoinositide metabolism, protein kinase C activation and the early events leading to cell division, since the rapid changes in the labelling of both phosphatidylinositol mono‐ and di‐phosphate occur only in nuclei from Swiss 3T3 cells, which respond to the mitogenic stimulus determined by insulin‐like growth factor‐I on its own, or in combination with bombesin.

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