Resistance to TNF-alpha cytotoxicity can be achieved through different signaling pathways in rat mesangial cells.

We reported previously that Ro-318220 blocked expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) induced by tumor necrosis factor-alpha (TNF-alpha) and subsequently caused apopotosis in mesangial cells (Y.-L. Guo, B. Kang, and J. R. Williamson. J. Biol. Chem. 273: 10362-10366, 1998). These data support our hypothesis that a TNF-alpha-inducible phosphatase may be responsible for preventing sustained activation of c-Jun NH2-terminal protein kinase (JNK) and consequent cell death in these cells (Y.-L. Guo, K. Baysal, B. Kang, L.-J. Yang, and J. R. Williamson. J. Biol. Chem. 273: 4027-4034, 1998). In this study, we investigated the involvement of protein kinase C (PKC) in regulation of MKP-1 expression in mesangial cells together with effects on viability. Although originally characterized as a PKC inhibitor, Ro-318220 inhibited TNF-alpha-induced MKP-1 expression through a mechanism other than blocking the PKC pathway. Furthermore, inhibition of the PKC pathway neither significantly affected TNF-alpha-induced MKP-1 expression nor made cells susceptible to toxic effect of TNF-alpha. Thus PKC activation is not essential for cells to achieve the resistance to TNF-alpha cytotoxicity displayed by normal mesangial cells. However, activation of PKC by phorbol 12-myristate 13-acetate (PMA) dramatically increased cellular resistance to the apoptotic effect of TNF-alpha. Coincidentally, PMA stimulated MKP-1 expression and suppressed JNK activation. Therefore, PMA-induced MKP-1 expression may contribute to the protective effect of PMA. These results provide a mechanistic explanation for previous documentation that PKC activation can rescue some cells from apopotosis.