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alpha‐Thrombin‐induced early mitogenic signalling events and G0 to S‐phase transition of fibroblasts require continual external stimulation.

In resting Chinese hamster fibroblasts (CCL39) alpha‐thrombin rapidly stimulates several biochemical events implicated in the mitogenic response, including the breakdown of inositol phospholipids, activation of a plasma membrane Na+/H+ antiporter, phosphorylation of ribosomal protein S6 and increased expression of the proto‐oncogene c‐myc. Complete removal of the growth factor during cellular G0/G1 transit precludes the re‐initiation of DNA synthesis. The present study was designed to examine the fate of alpha‐thrombin‐activated early events following growth factor inactivation. In cells stimulated for 30 min with alpha‐thrombin, neutralization of the growth factor results in: (i) immediate arrest of inositol phosphate formation, (ii) rapid inactivation of Na+/H+ exchange, (iii) deactivation of the S6 phosphorylating system and (iv) strong reduction of c‐myc mRNA level. Our findings that commitment for DNA synthesis as well as persistent activation of ‘early’ cellular events requires continual growth factor stimulation suggest that: (i) growth factor‐induced transmembrane signals have a short life and (ii) the generation of these signals during the 8 h of the pre‐replicative phase is required for G0‐arrested cells to enter the S phase.