Cyanobacteria have developed a light-harvesting antenna complex known as the phycobilisome. When cells are starved for nutrients or exposed to high light, the phycobilisome is rapidly degraded (bleaching). It has been suggested that in the cyanobacterium Synechococcus elongatus PCC 7942, the bleaching process is regulated by a two-component histidine kinase, NblS. To clarify the signaling pathway involving NblS, we identified the NblS-interacting response regulators RpaB and SrrA. In vitro assays using recombinant proteins showed that both RpaB and SrrA can receive phosphoryl groups from autophosphorylated NblS; the NblS-interacting protein SipA clearly enhances the phosphotransfer activity from NblS to RpaB and SrrA. In addition, NblS prefers SrrA over RpaB as the phosphotransfer target with or without SipA. Gel mobility shift assay revealed that both RpaB and SrrA can bind to the upstream region of nblA, a major regulatory factor in the bleaching process. nblA transcript accumulates in nblS or rpaB mutants even under normal growth conditions, while in the srrA disruptant the nblA transcripts are slightly up-regulated under stress conditions. These observations suggest that the bleaching signal transduction pathway via NblS is regulated by RpaB and that SrrA is partially involved.

Of late, numerous prodrugs are widely used for therapy. The hemeprotein cytochrome P450 (CYP) catalyzes the activation of prodrugs to form active metabolites. Therefore, the activation of CYP function might allow the use of lower doses of prodrugs and decrease toxicity. We hypothesized that the addition of 5-aminolevulinic acid (ALA), a precursor in the porphyrin biosynthetic pathway, enhances the synthesis of heme, leading to the up-regulation of CYP activity. To test this hypothesis, we treated a human gastric cancer cell line with ALA and determined the effect on CYP-dependent prodrug activation. For this purpose, we focused on the anticancer prodrug tegafur, which is converted to its active metabolite 5-fluorouracil (5-FU) mainly by CYP2A6. We show here that ALA increased CYP2A6-dependent tegafur activation, suggesting that ALA elevated CYP activity and potentiated the activation of the prodrug.

Recently, photodynamic therapy using 5-aminolevulinic acid (ALA-PDT) has been widely used in cancer therapy. ALA administration results in tumor-selective accumulation of the photosensitizer protoporphyrin IX (PpIX) via the heme biosynthetic pathway. Although ALA-PDT has selectivity for tumor cells, PpIX is accumulated into cultured normal cells to a small extent, causing side effects. The mechanism of tumor-selective PpIX accumulation is not well understood. The purpose of the present study was to identify the mechanism of tumor-selective PpIX accumulation after ALA administration. We focused on mitochondrial labile iron ion, which is the substrate for metabolism of PpIX to heme. We investigated differences in iron metabolism between tumor cells and normal cells and found that the amount of mitochondrial labile iron ion in cancer was lower than that in normal cells. This finding could be because of the lower expression of mitoferrins, which are the mitochondrial iron transporters. Accordingly, we added sodium ferrous citrate (SFC) with ALA as a source of iron. As a result, we observed the accumulation of PpIX only in tumor cells, and only these cells showed sensitivity to ALA-PDT. Taken together, these results suggest that the uptake abilities of iron ion into mitochondria play a key role in tumor-selective PpIX accumulation. Using SFC as a source of iron might thus increase the specificity of ALA-PDT effects.

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