Significance of TCDD-induced changes in protein phosphorylation in the adipocyte of male guinea pigs.

The chemical TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) at very low concentrations has been found to cause a rapid rise in protein phosphorylation activities in the extranuclear fraction (i.e., cytosol and cellular membranes) of the adipose tissue from male guinea pigs. The effect occurred both in vivo and when TCDD was added to adipose in tissue culture (in situ). To study the cause for such a TCDD-induced biochemical change, isolated adipose tissues were treated with TCDD in conjunction with various diagnostic agents known to affect protein synthesis (actinomycin D and cycloheximide) and protein phosphorylation activities (genistein). The stimulatory effect of TCDD on protein phosphorylation was not totally abolished by actinomycin D; however, cycloheximide and genistein partially suppressed the effect of TCDD. Such an action of TCDD was accompanied by a quick rise in ras GTP binding activity as well as a rise in phosphorylation of nuclear c-myc protein product within 10 minutes after TCDD addition, indicating that TCDD is likely to activate the signal transduction pathway for growth factors. In view of the absence of an inhibitory action of actinomycin D and the short time required for TCDD to induce these changes, we have formulated a working hypothesis that stimulation of protein phosphorylation activities by TCDD may not be mediated by a transcriptional process. To test this possibility, an extranuclear fraction was obtained from the homogenate of adipose tissue, and the effect of TCDD was studied in vitro cell- and nucleus-free conditions. It was found that TCDD was capable of activating protein phosphorylation activity under cell free conditions even in the absence of nuclei. Such an action of TCDD is dependent on the action of the Ah receptor.