Effect of L-thioproline on the progression and events of the cell cycle of HeLa cells.

L-thioproline (thiazolidine-4-carboxylic acid, L-Tp) reduces DNA synthesis in cultured HeLa cells by about 50% when added to synchronized cultures at the beginning of G1 phase. This inhibition is not observed when the drug is added at mid-S, in G2 or in M. L-Tp almost annuls the burst in 22Na uptake occurring at the beginning of G1 and is as potent an inhibitor as amiloride. It is also able to delay the apparition of the early S-specific cAMP peak and to elevate its intracellular concentration. These results indicate that L-Tp could decrease tumor cell growth by acting at the G1-S boundary.