Abstract
Transforming growth factor-beta2 (TGF-beta2) and -beta3 mRNA expressions were studied in ductal hormone-dependent (HD) and -independent (HI) in vivo lines of the medroxyprogesterone acetate (MPA)-induced mammary tumor model in Balb/c mice. MPA treatment of HD tumors induced a significant decrease in TGF-beta2 and -beta3 mRNA levels. Progression to an HI phenotype of ductal tumors was associated with reduced TGF-beta2 and -beta3 expressions, as compared with their HD counterparts. Exogenously added TGF-beta1, -beta2, and -beta3 (1 ng/ml) inhibited the proliferation of primary cultures of epithelial cells from ductal HD and HI tumors. In addition, TGF-beta expression and effects were studied in the other type of MPA-induced mammary tumors, which are of lobular origin and lack steroid hormone receptors and evidence an HI behavior. These lobular HI lines showed TGF-beta2 levels similar to those found in HD lines growing in MPA-treated mice. In contrast, TGF-beta3 mRNA levels were 12- to 20-fold higher than in HD tumors. Primary cultures of lobular HI epithelial cells required either TGF-beta concentrations of 10 ng/ml to show an inhibitory response, or were completely resistant to TGF-beta inhibition. Studies of the molecular mechanisms involved in reduction or loss of TGF-beta responsiveness in lobular HI tumors showed that cell surface type II TGF-beta receptor levels were lower in these tumors than those present in HD tumors. Our results support the hypothesis that TGF-beta could play a role as an autocrine growth inhibitor in HD and HI ductal tumors. Autonomous growth of lobular HI tumors could be favored by undetectable or low TGF-beta1 and -beta2 expressions and by reduced or lost sensitivity of epithelial cells to TGF-beta's antiproliferative effects. However, the extremely high levels of TGF-beta3 expression in lobular HI tumors, in spite of reduced sensitivity to TGF-beta3 inhibitory growth effect in tumor epithelial cells, suggest a net positive role for TGF-beta3 in these tumors.
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