Molecular mechanisms of apoptosis: an overview

The Effect of Diabetes Mellitus on Apoptosis in Hippocampus: Cellular and Molecular Aspects

Background:

Diabetes mellitus is associated with cognitive deficits in humans and animals. These deficits are paralleled by neurophysiological and structural changes in brain. In diabetic animals, impairments of spatial learning, memory, and cognition occur in association with distinct changes in hippocampus, a key brain area for many forms of learning and memory and are particularly sensitive to changes in glucose homeostasis. However, the multifactorial pathogenesis of diabetic encephalopathy is not yet completely understood. Apoptosis plays a crucial role in diabetes-induce neuronal loss in hippocampus.

Methods:

The effects of diabetes on hippocampus and cognitive/behavioral dysfunctions in experimental models of diabetes are reviewed, with a focus on the negative impact on increased neuronal apoptosis and related cellular and molecular mechanisms.

Results:

Of all articles that were assessed, most of the experimental studies clearly showed that diabetes causes neuronal apoptosis in hippocampus through multiple mechanisms, including oxidative stress, inhibition of caspases, disturbance in expression of apoptosis regulator genes, as well as deficits in mitochondrial function. The balance between pro-apoptotic and anti-apoptotic signaling may determine the neuronal apoptotic outcome in vitro and in vivo models of experimental diabetes.

Conclusions:

Dissecting out the mechanisms responsible for diabetes-related changes in the hippocampal cell apoptosis helps improve treatment of impaired cognitive and memory functions in diabetic individuals.

TNFalpha-induced glutathione depletion lies downstream of cPLA(2) in L929 cells

Both glutathione (GSH) depletion and arachidonic acid (AA) generation have been shown to regulate sphingomyelin (SM) hydrolysis and are known components in tumor necrosis factor alpha (TNFalpha)-induced cell death. In addition, both have hypothesized direct roles in activation of N-sphingomyelinase (SMase); however, it is not known whether these are independent pathways of N-SMase regulation or linked components of a single ordered pathway. This study was aimed at differentiating these possibilities using L929 cells. Depletion of GSH with L-buthionin-(S,R)-sulfoximine (BSO) induced 50% hydrolysis of SM at 12 h. In addition, TNF induced a depletion of GSH, and exogenous addition of GSH blocked TNF-induced SM hydrolysis as well as TNF-induced cell death. Together, these results establish GSH upstream of SM hydrolysis and ceramide generation in L929 cells. We next analyzed the L929 variant, C12, which lacks both cytosolic phospholipase A(2) (cPLA(2)) mRNA and protein, in order to determine the relationship of cPLA(2) and GSH. TNF did not induce a significant drop in GSH levels in the C12 line. On the other hand, AA alone was capable of inducing a 60% depletion of GSH in C12 cells, suggesting that these cells remain responsive to AA distal to the site of cPLA(2). Furthermore, depleting GSH with BSO failed to effect AA release, but caused a drop in SM levels, showing that the defect in these cells was upstream of the GSH drop and SMase activation. When cPLA(2) was restored to the C12 line by expression of the cDNA, the resulting CPL4 cells regained sensitivity to TNF. Treatment of the CPL4 cells with TNF resulted in GSH levels dropping to levels near those of the wild-type L929 cells. These results demonstrate that GSH depletion following TNF treatment in L929 cells is dependent on intact cPLA(2) activity, and suggest a pathway in which activation of cPLA(2) is required for the oxidation and reduction of GSH levels followed by activation of SMases.