The role of protein tyrosine phosphorylation in the resistance mechanism against tumor necrosis factor-mediated lysis

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995.Includes bibliographical references (leaves 115-129).Microfiche.ix, 129 leaves, bound ill. 29 cmPresently, cancer is a major cause of death of individuals in most developed countries. Despite an international effort in understanding cancer, many fundamental issues remain unresolved. Myriad treatment regimens have developed, however there is no general effective cure. Tumor necrosis factor (TNF) is among the agents tested for antitumor activity; however, the toxic side effects of TNF prevent it from being used effectively in treating cancer. TNF also plays an important role as a cytokine that modulates various immune and inflammatory responses. Thus it is important to understand the TNF signaling mechanism in order to understand its role in host protective mechanism against infectious agents and to increase the efficacy of TNF in cancer therapy. There have been numerous studies investigating the post-receptor events in the TNF signaling pathway. However, the TNF signaling pathway is not still clear. There are two major obstacles in studying TNF signaling. TNF is pleotrophic and the effects of TNF will depend on the cells studied. Further, the two receptors to TNF do not possess any catalytic domain, hence there are no direct clues for studying the signaling pathway. Protein phosphorylation and dephosphorylation play pivotal roles in several growth factors and cytokine signaling pathway. In particular, tyrosine phosphorylation has been demonstrated in various signaling pathways. Here we will present evidence that tyrosine phosphorylation is involved in resistance mechanism that prevents TNF-mediated lysis of certain cell. The addition of protein tyrosine kinase inhibitors increases lysis of TNF-mediated lysis of TNF resistant cells but not TNF sensitive cells, thus indicating the TNF resistance mechanism involves protein tyrosine kinase activity. Also, since tyrosine phosphorylation is a reversible process, we investigated the role of protein tyrosine phosphatase activity in TNF signaling. TNF-mediated lysis is decreased in TNF sensitive cells with the addition of protein tyrosine phosphatase inhibitors. The decrease in TNF-mediated lysis induced by the protein tyrosine phosphatase inhibitors was via an increase in TNF resistance. Furthermore, according to the above experiments, an increase in protein tyrosine kinase activity is detected after TNF treatment of TNF resistant cells. Likewise, an increase in protein tyrosine phosphatase activity is detected in TNF sensitive cells after TNF treatment. These findings indicate a protein tyrosine phosphatase counters the protein tyrosine kinase in the TNF resistance mechanism and it is the balance between a phosphatase(s) and a kinase(s) that determine whether a cell is killed by TNF. Lastly, when we examined the role of tyrosine phosphorylation in resistance to cell-associated TNF, signaling of soluble versus cell-associated TNF is different. Taken together, we conclude that the level of tyrosine phosphorylation in the cell after TNF treatment will influence whether the resistance mechanism will be activated to protect the cell