Transforming growth factor βs (TGF-βs) are a family of polypeptide growth factors that regulate cellular growth, phenotype, and differentiation. TGF-βs are synthesized as latent high molecular weight complexes that include the NH2-terminal remnant of the TGF-β precursor (latency-associated protein) and, frequently, latent TGF-β binding protein. After activation, TGF-βs act as local mediators of hormonal responses in target tissues. TGF-β functions as a negative growth regulator for both breast cancer cells and normal mammary epithelial cells. Vitamin D3 is growth inhibitory for the estrogen receptor-negative breast cancer cell line BT-20 and regulates TGF-β expression in cultured keratinocytes. We studied here the effects of vitamin D3 and its analogues on TGF-β expression and activity in BT-20 cells. It was found that vitamin D3 enhanced both TGF-β1 mRNA and secretion of the protein in a time- and dose-dependent manner. Analyses of the vitamin D3 responses in the presence of cycloheximide or actinomycin D indicated that the TGF-β1 mRNA induction was dependent on both protein and RNA synthesis. The amounts of latent TGF-β binding protein were also increased in the conditioned medium but not in the pericellular matrix of vitamin D3-treated cultures. The amounts of active TGF-β were enhanced in vitamin D3-treated cultures as well, suggesting autocrine or paracrine functions for the secreted growth factor. Some analogues of vitamin D3 (EB 1089, MC 903, and KH 1060) that are known to be potent inhibitors of breast cancer cell growth both in vitro and in vivo had similar or more pronounced inducing effects on TGF-β1 mRNA levels. The present results indicate that vitamin D3 and its analogues are potent inducers of both active and latent forms of TGF-β1 in BT-20 breast carcinoma cells and provide evidence for coordinated regulation of latent TGF-β binding protein and TGF-β1.
This work was supported by the Medical Research Council of the Academy of Finland, the Finnish Cancer Foundation, Sigrid Juselius Foundation, and the Finnish Society of Sciences and Letters. Presented in part at the “9th Workshop on Vitamin D,” Orlando, Florida, 1994.