Biosynthesis of nuclear proteins after stimulation of quiescent Swiss mouse 3T3 cells

Stimulation of quiescent Swiss mouse 3T3 fibroblasts either by serum or by the low molecular weight hormones, prostaglandin F2 alpha and insulin, induces DNA synthesis after a lag period of about 15 h. Following restimulation by serum or these pure hormones there is an overall increase of two- to fourfold in the rate of biosynthesis of nuclear proteins. In addition, there is a relative decrease in some proteins (Mr = 200 X 10(3), pI 6.0-6.5), while others increase (e.g. actin). Two polypeptides show specific correlations with the exit from G0. The synthesis of p30 (Mr = 30 X 10(3), pI 5.2) is at a maximum within 5 h of restimulation, while the synthesis of p36 (Mr = 36 X 10(3), pI = 4.25) is first seen at 10–20 h after restimulation. Synthesis of p36 correlates well with the initiation of DNA synthesis. The synthesis of both proteins is stimulated by serum and by the hormones. Thus there are common biosynthetic responses to different stimuli indicating convergent pathways leading to DNA biosynthesis. Addition of hydrocortisone with the growth-stimulatory hormones inhibits both entry into the S phase and biosynthesis of p36. In contrast, hydrocortisone does not alter the biosynthesis of p30. This ‘early’ protein, p30, is different from the products of both c-fos and c-myc. Therefore, we have identified two specific components that might participate in the regulation of cell proliferation.