α-Carotene

α-Carotene is a precursor of vitamin A that has been found in various fruits and vegetables.[1] It inhibits proliferation of GOTO human neuroblastoma cells more potently than β-carotene and halts the cell cycle at the G0/G1 phase concomitantly with a reduction in the mRNA expression of the protooncogene N-Myc.[2]  It is also more potent than β-carotene in mouse models of skin and lung carcinogenesis and decreases the number of hepatomas in mice with spontaneous liver carcinogenesis when administered in drinking water at a concentration of 0.05%. [3] α-Carotene levels are increased in patients with coronary heart disease and are inversely correlated with the risk of estrogen receptor-negative breast cancer.[4][5]

Reference:
[1]. Bushway, R.J., and Wilson, A.M. Determination of α- and β-carotene in fruit and vegetables by high performance liquid chromatography. Can. Inst. Food Sci. Technol. J. 15(3), 165-169 (1982).
[2]. Murakoshi, M., Takayusa, J., Kimura, O., et al. Inhibitory effects of α-carotene on proliferation of the human neuroblastoma cell line GOTO. J. Natl. Cancer Inst. 81(21), 1649-1652 (1989).
[3]. Murakoshi, M., Nishino, H., Satomi, Y., et al. Potent preventive action of α-carotene against carcinogenesis: spontaneous liver carcinogenesis and promoting stage of lung and skin carcinogenesis in mice are suppressed more effectively by α-carotene than by β-carotene. Cancer Res. 52(23), 6583-6587 (1992).
[4]. Kontush, A., Spranger, T., Reich, A., et al. Lipophilic antioxidants in blood plasma as markers of atherosclerosis: The role of α-carotene and γ-tocopherol. Atherosclerosis 144(1), 117-122 (1999).
[5]. Zhang, X., Spiegelman, D., Baglietto, L., et al. Carotenoid intakes and risk of breast cancer defined by estrogen receptor and progesterone receptor status: A pooled analysis of 18 prospective cohort studies. Am. J. Clin. Nutr. 95(3), 713-725 (2012).