A compound in potato juice called Patatin was found to have potent antiproliferative effects in a mouse model of melanoma. Melanoma is an often deadly form of skin cancer and the risk of melanoma is actually increased by the use of sunscreen (2). The best policy is to avoid sunscreen and use your skin as an indicator of how long to be out in the sun. If you cannot avoid being overexposed, then the best way to avoid burning is wearing protective clothing. Chinese herbal medicine by the way, has also been studied and a number of compounds have been discovered that inhibit the proliferation of melanoma (3).


Sources:
(1) Food Funct. 2013 Oct 25;4(10):1502-11. doi: 10.1039/c3fo60248f
(2) West J Med. 2000 July; 173(1): 58
(3) e.g., Cryptotanshinone, a component of Salvia M.; Cancer Chemother Pharmacol. 2011 July; 68(1): 17–27