The tocotrienol-rich fraction (TRF) of palm oil consists of tocotrieno
ls and some alpha-tocopherol (alpha-T). Tocotrienols are a form of vit
amin E having an unsaturated sidechain, rather than the saturated side
-chain of the more common tocopherols. Because palm oil has been shown
not to promote chemically-induced mammary carcinogenesis, we tested e
ffects of TRF and alpha-T on the proliferation, growth, and plating ef
ficiency (PE) of MDA-MB-435 estrogen-receptor-negative human breast ca
ncer cells. TRF inhibited the proliferation of these cells with a conc
entration required to inhibit cell proliferation by 50% of 180 mu g/mL
, whereas alpha-T had no effect at concentrations up to 1000 mu g/mL a
s measured by incorporation of [H-3]thymidine, The effects of TRF and
alpha-T also were tested in longer-term growth experiments, using conc
entrations of 180 and 500 mu g/mL. We found that TRF inhibited the gro
wth of these eel Is by 50%, whereas alpha-T did not, Their effect on t
he ability of these cells to form colonies also was studied, and it wa
s found that TRF inhibited PE, whereas alpha-T had no effect. These re
sults suggest that the inhibition is due to the presence of tocotrieno
ls in TRF rather than alpha-T.