Cytotoxic Cell Granule-Mediated Apoptosis

Abstract

The mechanism underlying perforin (PFN)-dependent delivery of apoptotic granzymes during cytotoxic cell granule-mediated death remains speculative. Granzyme B (GrB) and perforin were found to coexist as multimeric complexes with the proteoglycan serglycin (SG) in cytotoxic granules, and cytotoxic cells were observed to secrete exclusively macromolecular GrB-SG. Contrary to the view that PFN acts as a gateway for granzymes through the plasma membrane, monomeric PFN and, strikingly, PFN-SG complexes were shown to mediate cytosolic delivery of macromolecular GrB-SG without producing detectable plasma membrane pores. These results indicate that granule-mediated apoptosis represents a phenomenon whereby the target cell perceives granule contents as a multimeric complex consisting of SG, PFN, and granzymes, which are, respectively, the scaffold, translocator, and targeting/informational components of this modular delivery system.

Extracellular Matrix Remodeling by Human Granzyme B via Cleavage of Vitronectin, Fibronectin, and Laminin

“…It is well established that cationic GrB interacts with both chondroitin sulfate and heparin sulfate (30,(75)(76)(77). Heparin sulfate does not inhibit GrB enzymatic activity; thus, it probably prevents GrB binding to the ECM.…”

“…Specialized lysosomes (granules) within these cells contain multiple cytotoxins, including the pore-forming molecule, perforin. These cytotoxins are stored in complex with the proteoglycan, serglycin (1,2). Release of cytotoxic granule contents from CLs into the target cell results in the rapid induction of apoptosis and is the primary mechanism used to destroy abnormal cells (3).…”

A Novel Mechanism for Protein Delivery

“…Our laboratory originally speculated that cytotoxic granulemediated apoptosis represents a form of cell death in which the target cell internalizes a multi-component effector system of various cationic cytotoxins ionically linked to SG (4,36). The data reported here suggest that SG may serve a more specialized function than simply being an extracellular carrier of granule proteins.…”

“…The target cell is then postulated to bind and internalize the granzymes to endosomes, whereas PFN is thought to disrupt the vesicles to deliver the granzymes to the cytosol (3)(4)(5)(6)(7). GrA and GrB are considered the predominant initiators of apoptosis.…”

“…The original model proposed that granzymes would diffuse to the cytosol through perforininduced plasma membrane pores. We have, however, recently demonstrated that GrB is secreted as a multi-molecular complex with the chondroitin sulfate (CS) proteoglycan SG (4,16). Furthermore, in the presence of non-permeabilizing concentrations of PFN, preformed complexes readily induce apoptosis in target cells.…”