FLIP is Constitutively Hyperexpressed in Fas-resistant U266 Myeloma Cells, but Is Not Induced by IL-6 in Fas-sensitive RPMI8226 Cells

Abstract

Despite the expression of Fas, some clones of myeloma cells are resistant to Fas-mediated apoptosis. To define a cellular factor involved in the resistance, we performed a comparative study using two clones of myeloma cells, RPMI8226 and U266. These cells were reported to express cell surface Fas at similar levels, but only RPMI8226 cells lost their viability upon anti-Fas treatment. The resistance of U266 cells to anti-Fas did not appear to reflect dysregulation of Bcl-2, Bcl-X^L, and Bax, because these proteins were expressed in both RPMI8226 and U266 cells to similar levels. Moreover, levels of those proteins were not significantly altered by treating RPMI8226 cells with IL-6, a cytokine which suppresses the Fas-mediated death of RPMI8226 cells. Interestingly, mRNA levels of FLIP^L, an endogenous inhibitor of Fas signaling, were constitutively elevated in U266 cells. Consistent with this observation, U266 cells expressed both FLIP^L protein and its truncated 43 kDa product which is seen in FLIP^L-overexpressing cells. The truncated form of FLIP^L protein was not detected in RPMI8226. Moreover, the levels of truncated FLIP^L in U266 cells were considerably higher than those of pro-FLIP^L in RPMI8226. The overall data indicate that FLIP^L is constitutively hyperexpressed in U266 cells. However, IL-6 failed to enhance the protein levels of FLIP molecules in either of the tested cells. It appears, therefore, that FLIP^L plays a role in the intrinsic resistance of U266 cells to the apoptotic action of Fas, but is not involved in the protective action of IL-6.