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Human natural killer (NK) cells are lymphocytes that destroy tumor and virally infected cells. Previous studies have shown that exposure of NK cells to tributyltin (TBT) greatly diminishes their ability to destroy tumor cells (lytic function) while activating mitogen-activated protein kinases (MAPK) (p44/42, p38, and JNK) in NK cells. The signaling pathway that regulates NK lytic function appears to include activation of protein kinase C(PKC) as well as MAPK activity. TBT-induced activation of MAPKs would trigger a portion of the NK lytic signaling pathway, which would then leave the NK cell unable to trigger this pathway in response to a subsequent encounter with a target cell. In the present study we evaluated the involvement of PKC in inhibition of NK lysis of tumor cells and activation of MAPKs caused by TBT exposure. TBT caused a 2–3-fold activation of PKC at concentrations ranging from 50 to 300 nM (16–98 ng/ml),indicating that activation of PKC occurs in response to TBT exposure. This would then leave the NK cell unable to respond to targets. Treatment with the PKC inhibitor, bisindolylmaleimide I, caused an 85% decrease in the ability of NK cells to lyse tumor cells, validating the involvement of PKC in the lytic signaling pathway. The role of PKC in the activation of MAPKs by TBT was also investigated using bisindolylmaleimide I. The results indicated that, in NK cells where PKC activation was blocked, there was no activation of the MAPK, p44/42 in response to TBT.However, TBT-induced activation of the MAPKs, p38 and JNK did not require PKC activation. These results indicate the pivotal role of PKC in the TBT-induced loss of NK lytic function including activation of p44/42 by TBT in NK cells.

Destruction of tumor cells is a key function of natural killer (NK) cells. Previous studies have shown that tributyltin (TBT) can significantly reduce the lytic function of the human NK cells with accompanying increases in the phosphorylation (activation) states of the mitogen activated protein kinases (MAPKs), p44/42. The current studies examine the role of p44/42 activation in the TBT-induced reduction of NK-lytic function, by using MAPK kinase (MEK) inhibitors, PD98059 and U0126. A 1 h treatment with PD98059 or U0126 or both decreased the ability of NK cells to lyse K562 tumor cells. PD98059, U0126 or a combination of both inhibitors were able to completely block TBT-induced activation of p44/42. However, when p44/42 activation was blocked by the presence of PD98059, U0126 or the combination, subsequent exposure to TBT was still able to decrease the lytic function of NK cells. These results indicate that TBT-induced activation of p44/42 occurs via the activation of its upstream activator, MEK, and not by a TBT-induced inhibition of p44/42 phosphatase activity. Additionally, as lytic function was never completely blocked by MEK inhibitors, the results indicate that activation of p44/42 pathway is not solely responsible for the activation of lytic function of freshly isolated human NK cells. Finally, the results showed that TBT-induced activation of p44/42 is not solely responsible for the loss of lytic function.