Abstract
Transglutaminase (TGase) activity was measured during cell cycle progression in Chinese hamster ovary (CHO) cells synchronized by release of quiescent cultures and in CHO cells synchronized by mitotic shake off. In cells released from quiescent cultures, a greater than 2โfold increase in TGase activity occurred within 3 h of stimulation, followed by a rapid decline and a second and third peak of activity at 6 and 9 h after stimulation. The increase in TGase activity was not inhibited at any time measured by the administration of cycloheximide. Further, there was a significant increase in TGase activity detectable at 1, 5, and 6 h in CHO cells exposed to actinomycin D at time zero. TGase activity in CHO cells was not increased by the addition of either dibutyryl cAMP or 8โbromoโcAMP whereas ornithine decarboxylase (ODC) activity was increased at all times measured by the administration of cAMP analogs. These data suggest that TGase and ODC are regulated by separate hormonal pathways during cell cycle progression. In addition, ODC is induced in CHO cells by a transcriptional mechanism whereas TGase appears to be activated. In order to determine if the biphasic increase in TGase exhibited in G~1~ of cell cycle was the result of two populations of cells progressing through G~1~, we assessed TGase activity in CHO cells synchronized by mitotic shake off, which produces a greater than 95% cell synchrony. Again, there were two major peaks of TGase activity at 3 and 5 h, respectively. However, there was a marked difference in the expression of TGase within the first 2 h after release from mitosis. During this period, there was a marked and significant decrease in TGase activity within 1 h to a value about 35% of the time zero value. Another G~1~โspecific marker, ODC, also exhibited a marked decline in activity within the first hour, followed by a single peak of expression between 4 and 5 h in these cells. Both enzymes have high activity during G~2~ phase of cell cycle as previously reported for ODC. Both enzymes are rapidly decreased after release from mitosis. The peak expression of both TGase and ODC suggest they catalyze important enzymatic events required for cell cycle progression.