Pathway-specific effect of caffeine on protection against UV irradiation-induced apoptosis in corneal epithelial cells.

Exposure of mammalian cells to UV irradiation causes programmed cell death (apoptosis) and cancer. For example, intensities of UV-C and -B irradiation sufficient to induce corneal epithelial cell death are on the order of microjoules per square centimeter and millijoules per square centimeter, respectively. These intensities of UV light are comparable to that which people are exposed to on a daily basis.1–3 In response to UV irradiation, mammalian cells initiate a series of intracellular signaling cascades by activating a variety of protein kinases. UV irradiation–induced activation of signal transduction shares common signaling pathways that are induced by cytokines/stress stimulation and that are related to programmed cell death. The complex cellular events triggered by UV irradiation are initiated in the membrane and include activation of non–receptor tyrosine kinase, G proteins, transcription factors, and ligand-independent membrane receptors, such as epidermal growth factor (EGF), platelet-derived growth factor (PDGF), TNFα, and Fas ligand.4–6 In addition, these cytokine receptors can be activated by hypertonic stress.7 The effect of UV irradiation on activation of growth factors and other cytokine receptors in the cell membrane has been verified further by microarray experiments.8 Several signaling components have been identified in response to UV-induced cytokine receptor activation, including Ras-Raf, Src, and the caspases.9–12 UV irradiation induces clustering and phosphorylation of membrane receptors and subsequently activates mitogen activated protein (MAP) kinase cascades, leading to programmed cell death in various cell types.13–18 More important, portions of the MAP kinase signaling pathway, including the c-Jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) and p38 signaling pathways, mediate UV irradiation–induced apoptosis in various cell types, including corneal epithelial cells.17,19–22 Recent studies indicate that stimulation of K+ channel activity causes the rapid loss of intracellular K+, which results in the activation of the caspase cascade.18,23–27 The effect of the rapid loss of intracellular K+ on caspase activation suggests that crosstalk occurs between activation of the K+ channel and intracellular signaling pathways. We found that UV irradiation–induced activation of caspase 1 (ICE) and JNK-1 occur subsequent to the stimulation of K+ channel activity and the loss of intracellular K+ in corneal epithelial cells.17 This finding is consistent with the report that the activation of caspase 1 can affect upstream events in the JNK pathway at the JNK level.28,29 Activation of the JNK signaling pathway by UV irradiation eventually results in apoptosis. UV irradiation–induced JNK-1 activation is markedly increased after ~60 minutes, and JNK-1 activation is initially observed within 5 to 15 minutes after the end of the UV-exposure period.17 Caffeine, a natural component of coffee and tea, is used in medicine for respiratory stimulation, relief of headache, suppression of the appetite, among other uses. It also is one of the world’s most popular additives to food products. Caffeine plays multiple roles in the regulation of cell function. It is a blocker of adenosine 3,5-cyclic monophosphate (cAMP) phosphodiesterase, resulting in an increase in concentration of intracellular cAMP.30 It promotes the release of Ca2+ from intracellular stores, thus increasing cytosolic Ca2+.31 It acts as an adenosine receptor antagonist to induce dopamine release from neuronal cells in the central nervous system.32,33 It has been shown that caffeine is a direct inhibitor of members of PI3K-like kinase family, such as ATM (ataxia telangiectasia mutated kinase, ATR (ATM and Rad3-related kinase), and DNA-PK (DNA-dependent protein kinase). Suppression of ATM and ATR by caffeine affects regulation of the cell cycle checkpoint, signal transduction of DNA repair, and apoptosis.34–37 However, the role of caffeine in cell survival is still not fully understood and is rather controversial because of its multiple cellular effects. In previous studies, we found that suppression of K+ channel hyperactivity as a early response to UV irradiation (at a dosage of 42 μJ/cm2) with specific channel blockers prevents p53 phosphorylation in corneal epithelial cells. K+ channel blockers have no inhibitory effect on UV irradiation–induced p53 phosphorylation in the later response induced by continuous exposure of cells to UV irradiation.38 However, caffeine blocks both early and later phosphorylation of p53 induced by UV irradiation, indicating that there is an involvement of UV irradiation–induced DNA damage distinguishable in the effect of UV irradiation–induced activation of the membrane Kv channel-linked signaling pathway. It has been suggested that the sensors in response to DNA damage resulting in phosphorylation of p53 involve ATM and ATR.39,40 Caffeine is able to block DNA damage-induced ATM and ATR responses to inhibit subsequent phosphorylation of p53.34,41 Thus, suppression of the DNA damage–related signaling pathway by caffeine can effectively prevent UV irradiation–induced phosphorylation of p53. In the present study, we report that the action of caffeine to suppress the early cellular response induced by UV irradiation is the result of an interaction of upstream events in UV irradiation–induced activation of the JNK signaling pathway and prevents apoptosis of corneal epithelial cells. The mechanism involving cell protection by caffeine acts through the inhibition of UV-induced ASK1-SEK-JNK activation.