No abstract

We studied the intracellular events associated with pancreatic β cell apoptosis by IFN-γ/TNF-α synergism. IFN-γ/TNF-α treatment of MIN6N8 insulinoma cells increased the amplitude of high voltage-activated Ca2+ currents, while treatment with IFN-γ or TNF-α alone did not. Cytosolic Ca2+ concentration ([Ca2+]c) was also increased by IFN-γ/TNF-α treatment. Blockade of L-type Ca2+ channel by nifedipine abrogated death of insulinoma cells by IFN-γ/TNF-α. Diazoxide that attenuates voltage-activated Ca2+ currents inhibited MIN6N8 cell death by IFN-γ/TNF-α, while glibenclamide that accentuates voltage-activated Ca2+ currents augmented insulinoma cell death. A protein kinase C inhibitor attenuated MIN6N8 cell death and the increase in [Ca2+]c by IFN-γ/TNF-α. Following the increase in [Ca2+]c, calpain was activated, and calpain inhibitors decreased insulinoma cell death by IFN-γ/TNF-α. As a downstream of calpain, calcineurin was activated and the inhibition of calcineurin activation by FK506 diminished insulinoma cell death by IFN-γ/TNF-α. BAD phosphorylation was decreased by IFN-γ/TNF-α because of the increased calcineurin activity, which was reversed by FK506. IFN-γ/TNF-α induced cytochrome c translocation from mitochondria to cytoplasm and activation of caspase-9. Effector caspases such as caspase-3 or -7 were also activated by IFN-γ/TNF-α treatment. These results indicate that IFN-γ/TNF-α synergism induces pancreatic β cell apoptosis by Ca2+ channel activation followed by downstream intracellular events such as mitochondrial events and caspase activation and also suggest the therapeutic potential of Ca2+ modulation in type 1 diabetes.

We have previously shown that mouse microglial cells undergo apoptosis upon inflammatory activation and that nitric oxide (NO) is the major autocrine mediator in this process (Lee, P., Lee, J., Kim, S., Yagita, H., Lee, M. S., Kim, S. Y., Kim, H., and Suk, K. (2001) Brain Res. 892, 380 -385). Here, we present evidence that interferon regulatory factor-1 (IRF-1) and caspase-11 are the essential molecules in activation-induced cell death of microglial cells. The apoptogenic action of inflammatory stimuli such as lipopolysaccharide (LPS) and interferon-␥ (IFN␥) was mediated through the induction of IRF-1 and caspase-11 expression in two separate events. Although IRF-1 was required for NO synthesis, caspase-11 induction was necessary for NO-independent apoptotic pathway. Microglial cells from IRF-1-deficient mice showed markedly decreased NO production, and they were partially resistant to apoptosis in response to LPS/IFN␥ but were sensitive to NO donor exposure. LPS/IFN␥ treatment resulted in the induction of caspase-11 followed by activation of caspase-11, -1, and -3. Inactivation of caspase-11 by the transfection of dominant-negative mutant or treatment with the caspase inhibitors rendered microglial cells partially resistant to LPS/IFN␥-induced apoptosis. Inhibition of both NO synthesis and caspase-11 completely blocked LPS/IFN␥-induced cytotoxicity. These results indicated that LPS/ IFN␥ not only induced the production of cytotoxic NO through IRF-1 but also initiated the NO-independent apoptotic pathway through the induction of caspase-11 expression.