Transglutaminase induced by epidermal growth factor negatively regulates the growth signal in primary cultured hepatocytes.
Transglutaminase (TGase) activity increased 2.5-fold at 6 h after treatment of rat hepatocytes with 17 nM epidermal growth factor (EGF). In the same manner, putrescine incorporation into the proteins of cells occurred in EGF-treated cells, but not in those pretreated with monodansylcadaverine (MDC), a TGase inhibitor, even in the presence of EGF. These results suggest that EGF-induced TGase was active and catalysed some cross-linkage reaction. Cycloheximide completely blocked the increase in TGase activity induced by EGF, suggesting that EGF stimulated de novo synthesis of TGase within 6 h. Furthermore, Northern blotting analysis indicated that EGF increased the expression of TGase mRNA. Pretreatment of cells with MDC additionally increased EGF-induced DNA-synthesis and the ratio of cells in S-phase. TGase antisense oligonucleotide inhibited de novo synthesis of TGase, resulting in increases in the ratios of S- and G2/M-phase cells in the presence of EGF. This effect was the result of inhibition of EGF-induced down-regulation of high-affinity receptor expression. These results suggest that the EGF-induced increase in TGase activity is a negative regulator of a growth signal in rat hepatocytes.