Evidence for new cellular proteins which may negatively control DNA replication or cell growth in rat 3T3 fibroblasts

When separated and proliferating rat 3T3 cells are treated with butyrate (6 mM), DNA synthesis stops within 24 h, while RNA and protein synthesis proceed unaffected. This gradually converts normal cells into giant ones in the presence of butyrate (volume up to 30-fold greater). The giant cells stop growing when cell to cell contact is established. By studying the rate of synthesis of 300 cell proteins, we have identified two proteins (39 kDa, PI = 6.2, and 60 kDa, pI = 5.6) whose synthesis rises at least 10-fold when DNA replication and mitosis are prevented following intercellular contact or butyrate treatment, and another (64 kDa, pI = 5.6) whose synthesis rises at least 10-fold when cell growth stops by contact, both in the presence of butyrate and in the absence of butyrate (untreated confluent cells). The synthesis of some cellular oncogenes increases when the cell transits from G0 to S phase; the two proteins of 39 and 60 kDa described here are regulated in the opposite direction, their synthesis is enhanced when the cell leaves the proliferation cycle to enter G0.