Rapid Inhibition of Intercellular Communication between BALB/c 3T3 Cells by Diacylglycerol, a Possible Endogenous Functional Analogue of Phorbol Esters

Abstract

Intercellular communication between cultured cells is reversibly inhibited by phorbol ester tumor promoters, which have been shown to activate protein kinase C directly, replacing the role of diacylglycerol. In order to see whether a presumed endogenous functional analogue, a diacylglycerol, could inhibit intercellular communication in the same way as phorbol esters, we compared the effects of 1-oleoyl-2-acetyl-glycerol (OAG) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on intercellular communication between BALB/c 3T3 cells, using a fluorescent dye transfer method. When cells were treated with OAG, dose-dependent inhibition of dye transfer between cells was observed, which was almost complete with OAG at 50 µg/ml. The effect was rapid, a maximal effect occurring within 30 min after addition. The inhibitory effect of both compounds was maintained for at least for 4 h when the cells were in the growing phase; thereafter, the capacity to transfer dye recovered gradually and then returned to the control level after 8 or 12 h of treatment with OAG or TPA, respectively. Further additions of OAG or TPA had no effect. When OAG was added to cultures during a refractory period produced by TPA, the culture was also refractory to OAG; however, TPA could induce at least 60% inhibition of dye transfer in cultures that had been made refractory to OAG. However, when cultures that had been made refractory to TPA were washed and then OAG was added, it induced extensive inhibition of dye transfer at any time after removal of TPA, whereas addition of TPA to the culture caused no significant reinhibition by 6 h and was detectable only 9 h after removal of TPA. These results indicate that OAG can inhibit dye transfer in a similar manner to TPA, suggesting that activation of protein kinase C may be a mechanism by which phorbol esters inhibit intercellular communication. Our results also suggest that there is some difference between the mechanisms by which OAG and TPA inhibit intercellular communication.