Journal of the National Cancer Institute Advance Access originally published online on August 24, 2009
JNCI Journal of the National Cancer Institute 2009 101(18):1259-1271; doi:10.1093/jnci/djp259
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Published by Oxford University Press 2009.
ARTICLES |
Influence of Melanosome Dynamics on Melanoma Drug Sensitivity
Affiliations of authors: Laboratory of Cell Biology, National Cancer Institute (KGC, JCV, COC, WDV, VJH, MMG), and Laboratory of Bioengineering and Physical Science, National Institute of Biomedical Imaging and Bioengineering (RDL, GZ), National Institutes of Health, Bethesda, MD; Advanced Photon Source, Argonne National Laboratory, Argonne, IL (BL)
Correspondence to: Michael M. Gottesman, MD, Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bldg 37, Rm 2108, Bethesda, MD 20892 (e-mail: mgottesman{at}nih.gov).
Background: Malignant melanomas are intrinsically resistant to many conventional treatments, such as radiation and chemotherapy, for reasons that are poorly understood. Here we propose and test a model that explains drug resistance or sensitivity in terms of melanosome dynamics.
Methods: The growth and sensitivity to cisplatin of MNT-1 cells, which are melanotic and enriched with mature stage III and IV melanosomes, and SK-MEL-28 cells, which have only immature stage I and II melanosomes, were compared using clonogenic assays. Differences in pigmentation, melanosome stages, melanosome number, and cellular structures in different cell lines in response to various treatments were examined by electron microscopy. The relative numbers of melanosomes of different stages were compared after treatment with 1-phenyl-2-thiourea. The relationship between drug transporter function and endogenous melanogenic toxicity was assessed by treating cells with the cyclosporin analog PSC-833 and by assessing vacuole formation and cell growth inhibition. All statistical tests were two-sided.
Results: Endogenous melanogenic cytotoxicity, produced by damaged melanosomes, resulted in pronounced cell growth inhibition in MNT-1 cells compared with amelanotic SK-MEL-28 cells. The sensitivity to CDDP of MNT-1 cells was 3.8-fold higher than that of SK-MEL-28 cells (mean IC50 for SK-MEL-28 and MNT-1 = 2.13 µM and 0.56 µM, respectively; difference = 1.57 µM, 95% confidence interval = 1.45 to 1.69; P = .0017). After treatment with 6.7 µM CDDP for 72 hours, the number of stage II-III melanosomes in surviving MNT-1 cells was 6.8-fold that of untreated cells. Modulation of MNT-1 cells to earlier-stage (II, II-III, III) melanosomes by treatment with the tyrosinase inhibitor 1-phenyl-2-thiourea dramatically increased CDDP resistance. Furthermore, PSC-833 principally suppressed MNT-1 melanotic cell growth via an elevation of autophagosome-like vacuolar structures, possibly by inhibiting melanosome membrane transporters.
Conclusions: Melanosome dynamics (including their biogenesis, density, status, and structural integrity) regulate the drug resistance of melanoma cells. Manipulation of melanosome functions may be an effective way to enhance the therapeutic activity of anticancer drugs against melanoma.
| CONTEXT AND CAVEATS Prior knowledge Melanoma is intrinsically resistant to treatments such as radiation and conventional chemotherapy. In addition to cytotoxic effects from by-products of melanin synthesis, melanosomes are involved in drug trapping and export. Study design The relationship between melanosome dynamics and drug resistance was studied using microscopy methods and growth assays in a variety of melanoma cell lines. Numbers of melanosomes and their distribution according to stage were altered pharmacologically. Contribution This work suggested that stage IV melanosomes increase drug sensitivity through cytotoxic effects and that melanosomes at stages II and III may decrease sensitivity to a chemotherapy drug used to treat melanoma. The results raise the possibility that interventions that alter melanosome numbers and stages could provide a means to increase cellular sensitivity to chemotherapy drugs. Implications The usefulness of interventions to increase chemotherapy drug sensitivity by altering melanosome dynamics should be investigated. Limitations The results reported in this study were based on in vitro studies using immortalized cell lines. Further validation in animal models is needed. From the Editors
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Manuscript received October 23, 2008; revised June 9, 2009; accepted July 14, 2009.
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J Natl Cancer Inst 2009 101: 1221.