Abstract
The efficacy of aminolevulinic acid (5-ALA)-based photodynamic diagnosis (5-ALA-PDD) and photodynamic therapy (5-ALA-PDT) is dependent on 5-ALA-induced cancer-specific accumulation of protoporphyrin IX (PpIX). We previously reported that inhibition of oncogenic Ras/MEK increases PpIX accumulation in cancer cells by reducing PpIX efflux through ATP-binding cassette sub-family B member 1 (ABCB1) and ferrochelatase (FECH)-catalysed PpIX conversion to haem. Here, we sought to identify the downstream pathways of Ras/MEK involved in the regulation of PpIX accumulation via ABCB1 and FECH. First, we demonstrated that Ras/MEK activation reduced PpIX accumulation in RasV12-transformed NIH3T3 cells and HRAS transgenic mice. Knockdown of p90 ribosomal S6 kinases (RSK) 2, 3, or 4 increased PpIX accumulation in RasV12-transformed NIH3T3 cells. Further, treatment with an RSK inhibitor reduced ABCB1 expression and increased PpIX accumulation. Moreover, HIF-1α expression was reduced when RasV12-transformed NIH3T3 cells were treated with a MEK inhibitor, demonstrating that HIF-1α is a downstream element of MEK. HIF-1α inhibition decreased FECH activity and increased PpIX accumulation. Finally, we demonstrated the involvement of RSKs and HIF-1α in the regulation of PpIX accumulation in human cancer cell lines. These results demonstrate that the RSK-ABCB1 and HIF-1α-FECH axes are the downstream pathways of Ras/MEK involved in the regulation of PpIX accumulation.
Highlights
The haem biosynthesis pathway, which is present in all cells, plays essential roles in cellular metabolism, including oxygen transport, regulation of cellular oxidation, and drug m etabolism[1]
Treatment with the ATP-binding cassette sub-family B member 1 (ABCB1) inhibitor increased protoporphyrin IX (PpIX) accumulation in all cell lines except MDA MB 231 cells. These results demonstrate that the ribosomal S6 kinases (RSK)-ABCB1 and HIF-1α-FECH axes, which are the downstream elements of MEK responsible for reducing PpIX accumulation, are present in human cancer cells as well as Ras-transformed mouse fibroblast cells
We previously demonstrated that inhibiting oncogenic Ras/MEK increases PpIX accumulation in cancer cells and promotes the efficacy of 5-Aminolevulinic acid (ALA)-PDD and PDT in vitro and in vivo[14,15]
Summary
The haem biosynthesis pathway, which is present in all cells, plays essential roles in cellular metabolism, including oxygen transport, regulation of cellular oxidation, and drug m etabolism[1]. We identified that the activation of MEK, a downstream element of Ras, reduces 5-ALA-induced PpIX accumulation through two independent pathways ‒ increased expression of ATP-binding cassette Treatment with all three MEK inhibitors significantly increased 5-ALA-induced PpIX accumulation in RasV12 cells in a dose-dependent manner (Fig. 2).
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