The promyelocytic leukemia (PML)/retinoic acid receptor (RAR) alpha fusion protein (PMLRAR?) is at the center of regulation of receptor signaling, nuclear transcription, and cancer development. PMLRAR? has been implicated in regulating death receptor signaling and apoptosis through dominant-negative effects on nuclear transcription. PML is a tumor suppressor known to stimulate apoptosis and is one of the most commonly defective (>60%) in cancers. Thus, restoration of PML function in cancer would be an effective therapy. If direct effects of PMLRAR? and PML on Fas death receptor-mediated apoptosis were to be established, this would represent a breakthrough in understanding apoptosis regulation and may open new possibilities for cancer therapy. We screened hematopoietic cancer cells for potential Fas binding inhibitors and identified PML as such a protein. Given the pro-apoptotic role of PML and pro-survival role of PMLRAR?, we tested whether PMLRAR? blocked Fas-mediated apoptosis. In preliminary studies, PMLRAR? blocked apoptosis induced by Fas ligand (FasL). PMLRAR? interacted with Fas in acute promyelocytic leukemia cells isolated directly from patients. PMLRAR? -expressing U937/PR9 cells bound cFLIP and excluded procaspase 8 in the death-induced signaling complex (DISC);thus, activation of caspase 8 was blocked. Also, knockdown of PMLRAR? in NB4 cells sensitized these cells to Fas-mediated apoptosis. Our hypothesis is that PMLRAR? diverts Fas signaling to a non-apoptosis endpoint. The results of the proposed studies will have significant implications for all cancers because PMLRAR? counters the effects of PML in apoptosis regulation. By demonstrating a direct interaction of PML and PMLRAR? with Fas, we will reveal a new target for apoptosis regulation that can be exploited for preventing and treating the many cancers that have defective PML function.
The Fas system is an important cell elimination system that has several established functions, including the elimination of autoreactive lymphocytes, infected and defective cells, and cells damaged by chemotherapy and irradiation. We have identified promyelocytic leukemia protein-retinoic acid receptor as a Fas-binding protein and potential key regulator of Fas signaling. This project will characterize the binding of PML with Fas and demonstrate how Fas is regulated in acute promyelocytic leukemia cells and in mice;thus, by showing the pivotal site of regulation, we will be able to identify a therapeutic approach for the vast majority of cancers that have defective promyelocytic leukemia protein and restore apoptosis signaling.
