Prostaglandin-induced differentiation or dimethyl sulfoxide-induced differentiation: reduction of the neoplastic potential of a rat mammary tumor stem-cell line.

Differentiation of the rat mammary stem cell line Rama 25 to alveolar-like cells was monitored both by the increased production of domes (hemispherical blisters) in the cell monolayer and by immunoreactive casein in the tissue culture medium. In addition to the synthetic inducer dimethyl sulfoxide (DMSO), prostaglandin (PG)E1, and to a lesser extent PGE2 and PGF2 alpha (concentration range, 50--500 ng/ml) in the presence of the hormones prolactin (Pr), hydrocortisone (HC), insulin (I), and 17 beta-estradiol (E) also accelerated this step. A combination of PGE1, HC, and I was active in promoting doming even in serum-free medium, whereas PGE1, all four hormones, and serum were required for maximum production of immunoreactive casein. The DNA synthetic rate was concomitantly reduced during this differentiation step. Rama 25 readily formed tumors in young, female nu/nu mice. When Rama 25 cells were treated with PGE1 or DMSO and the four hormones yielding the droplet and doming cultures, subsequent injection of these cultures into nu/nu mice led to a reduced incidence of tumors compared with injections of untreated cultures. Variant cell lines were selected from a subclone of elongated, myoepithelial-like Rama 29 cells that had been derived from Rama 25 and directly from Rama 25 itself. The former were elongated cells and termed "Rama 521," and the latter were cuboidal epithelial cells and termed "Rama 259." Both these variants were resistant to the actions of PGE1 or DMSO and to the mammotropic hormones in accelerating the rate of formation of domes, producing immunoreactive casein, in substantially reducing the DNA synthetic rate, and in reducing the incidence of tumors when injected into nu/nu mice. We have therefore shown that conversion of Rama 24 stem cells to alveolar-like cells in culture is specifically accompanied by a reduction in their neoplastic potential in nu/nu mice.