Intrinsic Pcd Pathway

In contrast to the extrinsic pathway, the intrinsic pathway (Fig. 1) is mediated primarily by Mt and Mt stress (3,16-21). One of the pivotal events in the process is Mt outer membrane permeabilization. This leads to release of several Mt inducers, for example, cytochrome-c, which are normally found in the space between the inner and outer Mt membrane. Under high glucose conditions and in the diabetic state, Mt outer membrane permeabilization is often preceded by hyperpolarization of the inner Mt membrane potential (A¥M), followed by a depolarization step, an event associated with induction of PCD (16,20,22,23). In dorsal root ganglion (DRG) neurons, the hyperpolarization wave is observed early after an added glucose load and this corresponds to early cleavage of caspase-3 at the same point of time. One of the key events preceding apoptosis is a change in the Mt permeability transition. Mt permeability transition is associated with opening of the adenine nucleotide transporter (ANT)/voltage-dependent anion channel (VDAC) spanning the inner and outer Mt membranes. This change results in osmotic

Fig. 1. Model for PCD pathways in neurons. Following inner Mt membrane depolarization, cytochrome-c (Cyt C) is released and combines with cell death pathway components to form the apoptosome complex, consisting of caspase-9 and apoptosis protease-activating factor 1 (Apaf-1). The formation of this complex leads to cleavage of caspase-9 and downstream activation of effector caspases-3, -6 and -7. The activation of the effector caspases is blocked by inhibitor of apoptosis proteins (IAPs). The IAPs in turn might be inhibited by second Mt activator of caspase (SMAC/DIABLO) that is released by apoptotic stimulation of the Mt. The effector caspases damage structural proteins, inhibit the DNA repair cycle, DNA transcription and translation, and cleave poly-ADP-ribose-polymerase (PARP). Cleavage of PARP facilitates the degradation of DNA. Apoptosis inducing factor (AIF) is released from the Mt with induction of apoptosis through a caspase independent pathway. AIF translocates to the nucleus causing DNA fragmentation. Genes that regulate apoptosis (both activators and inhibitors) are listed. (Reprinted from ref. 115.)

Fig. 1. Model for PCD pathways in neurons. Following inner Mt membrane depolarization, cytochrome-c (Cyt C) is released and combines with cell death pathway components to form the apoptosome complex, consisting of caspase-9 and apoptosis protease-activating factor 1 (Apaf-1). The formation of this complex leads to cleavage of caspase-9 and downstream activation of effector caspases-3, -6 and -7. The activation of the effector caspases is blocked by inhibitor of apoptosis proteins (IAPs). The IAPs in turn might be inhibited by second Mt activator of caspase (SMAC/DIABLO) that is released by apoptotic stimulation of the Mt. The effector caspases damage structural proteins, inhibit the DNA repair cycle, DNA transcription and translation, and cleave poly-ADP-ribose-polymerase (PARP). Cleavage of PARP facilitates the degradation of DNA. Apoptosis inducing factor (AIF) is released from the Mt with induction of apoptosis through a caspase independent pathway. AIF translocates to the nucleus causing DNA fragmentation. Genes that regulate apoptosis (both activators and inhibitors) are listed. (Reprinted from ref. 115.)

swelling that in turn disrupts the integrity of the outer Mt membrane (24), and is associated with release of proapoptotic factors into the cytoplasm that activate the caspase cascade (17). In contrast, inhibition of the ANT/VDAC channel by bongkrekic acid or with cyclosporine stabilizes the A¥m (20,25,26), and inhibits downstream cleavage of cas-pase-3 indicating that stabilization of the ATM is important in preventing PCD. Moreover, kinetic data show that Mt undergo major changes in membrane permeability, polarity, and volume before other well-recognized signs of apoptosis such as caspase activation and chromatin condensation (18). All these changes have been described in models of diabetic neuropathy (16,20,27-31).

The BCL-2 family of apoptosis-regulating proteins might also prevent Mt outer membrane permeabilization. Certain Bcl proteins inhibit PCD. For example, BCL-2, BCL-xL, and BCL-2-associated athanogene (Bag-1) are part of the basic machinery that controls PCD by inhibiting a variety of stimuli including reactive oxygen species (ROS) generation and loss of trophic support (32-34). BCL-2 and related proteins act directly on the outer Mt membrane to prevent permeabilization (3,17,35,36), and suppress PCD (37,38) by regulating calcium fluxes through the Mt and endoplasmic reticulum (39). BCL-xL appears to function in a similar manner to BCL-2. Bag-1 shares no significant homology with BCL-2, but interacts functionally and additively with BCL-2 to prevent PCD (33).

In contrast, proapoptotic BCL proteins, such as BAX and BAK can bind to VDAC and might result in permeabilization of the outer Mt membrane (40,41). In the presence of an apoptosis-inducing signal, BCL-2-associated protein (BAX) and BCL-2-homologous antagonist killer (BAK) oligomerize and insert into the outer Mt membrane (42). In support of this concept, mice lacking BAX and BAK fail to undergo outer membrane permeabilization and apoptosis following numerous apoptotic signals (43,44). When there is loss of outer Mt membrane permeability, this leads to release of proteins from the intermembrane space, although rather than being released simultaneously, release is likely dependent on secondary events for example remodeling of the matrix or change in polarity of the inner Mt membrane (45).

Despite the intrinsic and extrinsic apoptosis pathways exhibiting well-characterized hierarchies, a potential interaction between the pathways exists. The extrinsic pathway interacts with the intrinsic pathway through caspase-8-mediated cleavage of BCL-2 interacting protein (BID), which triggers the release of proapoptotic proteins from the intermembrane space of Mt into the cytoplasm (46).

Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

Get My Free Ebook


Post a comment