Cancer cells, including melanoma, often metastasize regionally through lymphatics before metastasizing systemically through the blood1-4; however, the reason for this is unclear.,Here we show that melanoma cells in lymph experience less oxidative stress and form more metastases than melanoma cells in blood.,Immunocompromised mice with patient-derived melanomas and immunocompetent mice with mouse melanomas had more melanoma cells per microliter of tumor-draining lymph than tumor-draining blood.,Cells metastasizing through blood, but not lymph, became dependent on the ferroptosis inhibitor GPX4.,Cells pre-treated with chemical ferroptosis inhibitors formed more metastases than untreated cells after intravenous, but not intralymphatic, injection.,We observed multiple differences between lymph fluid and blood plasma that may contribute to decreased oxidative stress and ferroptosis in lymph, including higher levels of glutathione and oleic acid, and less free iron, in lymph.,Oleic acid protected melanoma cells from ferroptosis in an Acsl3-dependent manner and increased their capacity to form metastatic tumors.,Melanoma cells from lymph nodes were more resistant to ferroptosis and formed more metastases after intravenous injection than melanoma cells from subcutaneous tumors.,Exposure to the lymphatic environment thus protects melanoma cells from ferroptosis and increases their ability to survive during subsequent metastasis through the blood.
Melanoma is the most deadly form of cutaneous malignancy, and its incidence rates are rising worldwide.,In melanoma, constitutive activation of the BRAF/MEK/ERK (MAPK) and PI3K/AKT/mTOR (PI3K) signaling pathways plays a pivotal role in cell proliferation, survival and tumorigenesis.,A combination of compounds that lead to an optimal blockade of these critical signaling pathways may provide an effective strategy for prevention and treatment of melanoma.,The phytochemical fisetin is known to possess anti-proliferative and pro-apoptotic activities.,We found that fisetin treatment inhibited PI3K signaling pathway in melanoma cells.,Therefore, we investigated the effect of fisetin and sorafenib (an RAF inhibitor) alone and in combination on cell proliferation, apoptosis and tumor growth.,Combination treatment (fisetin + sorafenib) more effectively reduced the growth of BRAF-mutated human melanoma cells at lower doses when compared to individual agents.,In addition, combination treatment resulted in enhanced (i) apoptosis, (ii) cleavage of caspase-3 and PARP, (iii) expression of Bax and Bak, (iv) inhibition of Bcl2 and Mcl-1, and (v) inhibition of expression of PI3K, phosphorylation of MEK1/2, ERK1/2, AKT and mTOR.,In athymic nude mice subcutaneously implanted with melanoma cells (A375 and SK-MEL-28), we found that combination therapy resulted in greater reduction of tumor growth when compared to individual agents.,Furthermore, combination therapy was more effective than monotherapy in: (i) inhibition of proliferation and angiogenesis, (ii) induction of apoptosis, and (iii) inhibition of the MAPK and PI3K pathways in xenograft tumors.,These data suggest that simultaneous inhibition of both these signaling pathways using combination of fisetin and sorafenib may serve as a therapeutic option for the management of melanoma.