Regulation of Porphyrin Synthesis and Photodynamic Therapy in Heavy Metal Intoxication

Abstract

Protoporphyrin IX (PpIX) synthesis by malignant cells is successfully exploited for photodynamic therapy (PDT) following administration of 5-aminolevulinic acid (ALA) and light irradiation. The influence of two environmental heavy metal poisons, lead and gallium, on PpIX-synthesis and ALA-PDT was studied in two neu-ronal cell lines, SH-SY5Y neuroblastoma and PC12 pheochromocytoma. The heavy metal intoxication affected two of the heme-synthesis enzymes, ALA-dehydratase (ALAD) and porphobilinogen deaminase (PBGD). The present results show that lead poisoning significantly decreased the PBGD cellular level and inhibited its enzymatic activity, whereas the effects of gallium were less prominent. Although, the protein levels were reduced, the mRNA levels of PBGD remained unchanged during metal intoxication. These findings show additional inhibitory activity of lead on top of its classical effect on ALAD. Proteasome activity was enhanced during lead treatment, as measured by the AMC fluorigenic proteasome assay. The reduction in PBGD levels was not a consequence of PBGD mRNA reduced synthesis, which remained unchanged as shown by RT-PCR analysis. As a result of the lead poisoning, marked alterations in the cell cycle were observed, including a decreased G1 phase and an increased number of S phase cells. The efficacy of ALA-PDT was reduced in correlation with decreased activities of the enzymes during lead intoxication. We may conclude that lead poisoning adversely affects the outcome of ALA photodynamic therapy of cancer.

Aminolevulinic Acid (ALA) as a Prodrug in Photodynamic Therapy of Cancer

“…who showed that silencing this enzyme with shRNA decreased synthesis of protoporphyrin IX in K562 erythroleukemic cells and resulted in impaired PDT outcome [ 45 ]. Similar effects have been presented for inhibition of PBGD with Pb 2+ [ 46 ]. Furthermore, it has been revealed that pretreatment of LNCaP tumor cells with methotrexate significantly improved ALA-PDT efficacy.…”

Silencing of ALA dehydratase affects ALA-photodynamic therapy efficacy in K562 erythroleukemic cells

“…ALAD inhibition by lead was shown to decrease PpIX formation and to reduce ALA-PDT efficacy following ALA administration. 5 In addition, Gibson et al showed a reduced PpIX formation after incubation with succinyl acetone, a potent inhibitor of ALAD. 3 Therefore it was crucial to elucidate the role of ALAD expression on PpIX synthesis.…”

“…7 In addition, we have previously demonstrated that lead poisoning induced acceleration of proteasomal activity concomitant with the inhibition of ALAD enzymatic activity. 5 Thus, ALAD is involved in two physiological processes: the one for PBG synthesis and the other to regulate the proteasome. These processes might compete with each other for availability of ALAD.…”

“…We have shown previously that lead poisoning adversely reduces the efficacy of ALA-PDT, as a result of ALAD inhibition and reduced expression of PBGD. 5 In the present study, we further characterize the effect of ALAD expression on PpIX production and ALA-PDT efficacy by specific silencing of this enzyme.…”

Roles of Porphyrin and Iron Metabolisms in the δ‐Aminolevulinic Acid (ALA)‐induced Accumulation of Protoporphyrin and Photodamage of Tumor Cells

“…Thus, the elevation of the level of heme‐biosynthetic enzymes, including PBGD, UROS and CPOX could be responsible for the high accumulation of protoporphyrin in tumor cells. In addition, considering that silencing of ALA‐dehydratase caused the decrease of ALA‐induced accumulation of protoporphyrin (36), ALA‐dehydratase seems to play a role for ALA‐PDT.…”