Drp1 Mediates Caspase-Independent Type III Cell Death in Normal and Leukemic Cells

Abstract

Ligation of CD47 triggers caspase-independent programmed cell death (PCD) in normal and leukemic cells. Here, we characterize the morphological and biochemical features of this type of death and show that it displays the hallmarks of type III PCD. A molecular and biochemical approach has led us to identify a key mediator of this type of death, dynamin-related protein 1 (Drp1). CD47 ligation induces Drp1 translocation from cytosol to mitochondria, a process controlled by chymotrypsin-like serine proteases. Once in mitochondria, Drp1 provokes an impairment of the mitochondrial electron transport chain, which results in dissipation of mitochondrial transmembrane potential, reactive oxygen species generation, and a drop in ATP levels. Surprisingly, neither the activation of the most representative proapoptotic members of the Bcl-2 family, such as Bax or Bak, nor the release of apoptogenic proteins AIF (apoptosis-inducing factor), cytochrome c, endonuclease G (EndoG), Omi/HtrA2, or Smac/DIABLO from mitochondria to cytosol is observed. Responsiveness of cells to CD47 ligation increases following Drp1 overexpression, while Drp1 downregulation confers resistance to CD47-mediated death. Importantly, in B-cell chronic lymphocytic leukemia cells, mRNA levels of Drp1 strongly correlate with death sensitivity. Thus, this previously unknown mechanism controlling caspase-independent type III PCD may provide the basis for novel therapeutic approaches to overcome apoptotic avoidance in malignant cells.

CD47 deficiency confers cell and tissue radioprotection by activation of autophagy

“…1 Ligation of CD47 is known to induce apoptotic and nonapoptotic death in several cell types including activated T cells, 39 B-cell chronic lymphocytic leukemia, 40,41 promyelocytic leukemia, 42 multiple myeloma, 43 breast carcinoma, 44 and monocytes and dendritic cells. 45 Jurkat T cells deficient of CD47 are resistant to Fas-mediated cell death and are sensitized to death by ligation of Fas and re-expression of CD47.…”

Pim-1 preserves mitochondrial morphology by inhibiting dynamin-related protein 1 translocation

“…In addition to steadystate activity, Drp1 is also associated with early apoptosis (4,(7)(8)(9)(10). Cellular stress increases Drp1 shuttling, which leads to fragmented mitochondrial networks, whereas prolonged or severe insult promotes stabilization of Drp1 to the mitochondria, which increases the likelihood of progression to apoptotic cell death (7,8,(11)(12)(13). Phosphorylation of Drp1-S637 prevents translocation to mitochondria, whereas overexpression of Drp1 K38A (a dominant-negative mutation lacking GTPase activity) prevents Drp1 translocation, attenuates a fragmented mitochondrial phenotype, and decreases cell death (4,14).…”

Caspase-independent type III PCD: a new means to modulate cell death in chronic lymphocytic leukemia

“…5 In this study, we reported that the engagement of CD47 efficiently killed CLL cells. CD47 is a glycoprotein composed of a single IgV-like extracellular domain, a transmembrane region and a short intracytoplasmic tail.…”

“…These molecules regulate a variety of biological phenomena in the immune system, such as platelet activation, leukocyte and dendritic cell migration, macrophage multinucleation and cell death. Concerning PCD, we reported in our recent paper that CD47-mediated type III PCD is exclusively characterized by cytoplasmic hallmarks, 5 including an exposure of phosphatydilserine in the outer leaflet of the plasma membrane, a decrease in mitochondrial transmembrane potential (DC m ) and reactive oxygen species production. Caspase-independent type III PCD involves the serpases, a family of serine proteases, and is related to the mitochondrial redeployment of the cytosolic protein Drp1.…”