Inhibition of melanogenesis as a radiation sensitizer for melanoma therapy

Abstract

Melanin pigment displays strong photo-and radioprotective properties, suggesting that inhibition melanogenesis could increase sensitivity of melanoma to ionizing radiation. We tested this concept in human melanoma cells cultured in either Ham's F10 medium to maintain amelanotic phenotype or DMEM to induce/stimulate melanin production, respectively; N-phenylthiourea (PTU) and Dpenicillamine were used as an inhibitor of melanogenesis. Melanogenic activity was evaluated by visual inspection (color of cell pellets) or by measurement of tyrosinase (dopa oxidase) activity assay. Amelanotic cells or cells with various melanin content were exposed to gamma radiation and tested for viability and colony forming capability. Gamma radiation at doses of 2-15 Gy inhibited cell viability and colony forming efficiency in dose-and timedependent manner, but pigmented melanoma cells were significantly more resistant to gamma radiation than nonpigmented cells (p < 0.05-0.001). Both PTU and D-penicillamine inhibited strongly tyrosinase activity and melanin production in melanoma cells (p < 0.05-0.001). Furthermore, inhibition of melanogenesis by PTU or D-penicillamine resulted in enhancement of melanoma cells sensitivity to killing by gamma rays. In conclusion, the results of these cell culture experiments give support to a clinical trial of pharmacologically induced decrease in melanin synthesis to enhance the efficacy of radiotherapy in advanced melanomas. ' 2008 Wiley-Liss, Inc.Key words: melanogenesis; melanin; melanoma; tyrosinase; gamma radiation Melanoma is the tumor with the most rapidly increasing incidences worldwide and it is predicted that by 2010, 1 person out of 50 white persons will be affected by this disease.1 Surgical excision is an effective treatment for early-stage melanomas (localized to the skin), and it is widely accepted that melanoma is resistant to all therapeutic modalities once the metastatic process started. 1-4The use of radiotherapy for advanced melanoma is controversial, and it serves mainly as palliative treatment. [5][6][7][8][9] Most likely the radioresistance of melanoma is due to the presence of melanin pigment, which acts as endogenous radioprotector. 10,11The major role of cutaneous melanin is protection against the harmful actions of solar radiation, by acting as a filter against ultraviolet radiation (UVR), preventing its penetration and genotoxicity (formation of UV-induced DNA dimmers).11-16 Melanin also neutralizes reactive oxygen species (ROS) generated mostly by UVA, through its antioxidative and free-radical scavenging actions. 13,16,17 Overall, melanin protects against epidermal carcinogenesis or melanomagenesis. 11,12,14,16 Nevertheless, the same positive capacity of melanin to scavenge free radical and ROS becomes undesirable in the response of melanoma to radio-, photo-or chemotherapy 11,16,[18][19][20] ; furthermore, the pro-oxidant properties of melanin can, in certain conditions, stimulate melanoma progression. 21Melanotic and amelanotic melanomas differ in their sensitivity to...

Targeting protein-trafficking pathways alters melanoma treatment sensitivity

“…The data here indicate that melanomas can be protected from apoptosis by melanosomal sequestration of treatment agents, consistent with our and other groups' previous findings that melanosome function plays a role in melanoma treatment resistance (13,(27)(28)(29)(30)(31). Gottesman and colleagues (29) have shown that chemosensitivity is independent of melanoma pigment content, consistent with a melanosome function alternative to that of melanin synthesis (e.g., sequestration) influencing chemosensitivity.…”

The role of melanogenesis in regulation of melanoma behavior: Melanogenesis leads to stimulation of HIF-1α expression and HIF-dependent attendant pathways

“…Medium containing high concentrations of melanin precursors induces rapid melanization of Bomirski hamster amelanotic AbC1 melanoma [57] and SKMEL-188 human melanoma [28, 30]. When cells are switched from Ham's F10 (containing 10 μM of L-tyrosine) to DMEM media (containing 400 μM L-tyrosine), the production of melanin pigment is observed after two days of culture, with an increase in melanization levels observed on day 3 that is accompanied by rounding of heavily melanized cells and their detachment from the culture surface into the media as described previously [57, 83, 84].…”

“…The melanin synthesis apparatus serves not only as a differentiation marker but also, and more importantly, protects normal and malignant melanocytes and epidermal keratinocytes containing melanin from a myriad of physical and chemical insults [12, 25]. These latter properties can lead to melanotic melanomas that are relatively resistant to chemo-, radio- and phototherapy [12, 25, 27, 28]. Melanogenesis also influences the behavior of normal and malignant melanocytes and their surrounding microenvironment [26, 27, 29] through the action of intermediates of melanogenesis [30-32], oxygen consumption and stimulation of aerobic glycolysis [26, 33] or interaction with other metabolic pathways [12, 34-36].…”

Therapy Preferences in Melanoma Treatment - Willingness to Pay and Preference of Quality versus Length of Life of Patients, Physicians and Healthy Controls

“…A study in advanced ovarian cancer showed that if the therapy was equally effective, patients were willing to pay for avoiding side effects, as they chose a more expensive drug with a better adverse event profile accepting co-payments [18]. New complementary approaches in melanoma include adjuvant drugs like melatonin [19] or vitamin D [20] since it could play a role in melanoma progression [21], [22] and -as investigated in in vitro models- inhibitors of melanogenesis [23], [24].…”