Increase in protein kinase C activity is associated with human fibroblast growth inhibition

Protein kinase C(PKC) activity and DNA synthesis were measured in human fetal bone marrow fibroblasts following treatment with tumor necrosis factor alpha (TNFα)(500 U/ml) or conditioned media containing natural cell proliferation inhibitor (CM-NCPI). Treatment with TNFα led to growth stimulation (120 ± 7% of control in 24 h, 141 ± 6% in 72 h). At the same time particulate PKC activity diminished, reaching 55 ± 8% of control in 24 h and remaining at this level at 72 h. CM-NCPI treatment of the cells resulted in a decrease in DNA synthesis (by 39 ± 6% in 2 h, by 58 ± 5% in 24 h, and by 78 ± 8% in 72 h). This was accompanied by a significant rise in particulate PKC activity which increased over 3-fold in 2 h, over 5-fold in 24 h, and up to 11-fold in 72 h. This 11-fold elevation was maintained after 2 week exposure of the fibroblasts to CM-NCPI. The PKC inhibitor neomycin abolished CM-NCPI induced growth inhibition, whereas PKC activator 12-O-tetradecanoylphorbol 13-acetate intensified it. These results suggest that CM-NCPI acts as PKC activator and that negative growth regulation by extracellular agents may involve stimulation of PKC activity.