Certain carotenoids enhance the intracellular glutathione level in a murine cultured macrophage cell line by inducing glutamate-cysteine-ligase

Scope: Glutathione (GSH) increases in RAW264 murine macrophage cells exposed to beta-carotene or beta-cryptoxanthin, however, the underlying mechanism has not been clarified. In the present study, we investigated the expression of glutamate-cysteine-ligase (GCL), the rate-limiting enzyme in GSH synthesis, in these cells. Methods and results: Both the protein and mRNA expression of GCL increased in a beta-carotene concentration-dependent manner. Buthionine sulfoximine, a GCL inhibitor, abolished the beta-carotene-induced GSH increase without affecting the beta-carotene-induced GCL protein expression. Both cycloheximide, a translation inhibitor, and actinomycin D, a transcription inhibitor, completely suppressed the beta-carotene-induced GCL protein expression and the concomitant GSH increase. Actinomycin D inhibited the beta-carotene-induced Gcl mRNA expression as well. Similarly to beta-carotene, beta-cryptoxanthin upregulated the GCL protein expression, but lutein did not. The c-Jun N-terminal kinase (JNK) inhibitor, SP600125, suppressed the beta-carotene-induced GSH increase, whereas a p38 mitogen-activated protein kinase inhibitor or an extracellular signal-regulated kinase 1/2 inhibitor did not. The JNK inhibitor also suppressed the beta-carotene-induced GCL protein expression, and consistently beta-carotene induced JNK phosphorylation. Conclusion: These findings revealed that certain carotenoids induce the Gcl mRNA expression in RAW264 cells and subsequently the GCL protein expression, which concomitantly enhances the intracellular GSH level, in a JNK pathway-related manner.