Molecular Ordering of the Initial Signaling Events of CD95

Abstract

Binding of either ligand or agonistic antibodies to the death receptor CD95 (APO-1/Fas) induces the formation of the death-inducing signaling complex (DISC). We now show that signal initiation of CD95 in type I cells can be further separated into at least four distinct steps. (i) The first step is ligand-induced formation of CD95 microaggregates at the cell surface. (ii) The second step is recruitment of FADD to form a DISC. This step is dependent on actin filaments. (iii) The third step involves formation of large CD95 surface clusters. This event is positively regulated by DISC-generated caspase 8. (iv) The fourth step is internalization of activated CD95 through an endosomal pathway. The latter step is again dependent on the presence of actin filaments. The data indicate that the signal initiation by CD95 is a complex process actively regulated at various levels, providing a number of new drug targets to specifically modulate CD95 signaling.

We thank P. Krammer for providing us with anti-APO-1, H. Walczak for his generous gift of LZ-CD95L, and M. Jäättelä and A. Strasser for providing us with the MCF7-Fas and SKW6-Bcl-2 cells, respectively. We also thank E. Williamson of the University of Chicago Cancer Research Center Electron Microscopy Core.

This work was funded by NIH grant GM61712. A. A.-S. is supported by Cancer Biology Training Program 5T32CA09594. B.C.B. is supported by Molecular and Cell Biology Training grant 5T32GM07183.