Transforming growth factor-alpha (TGF-alpha) stimulates while TGF-beta inhibits 
mammary epithelial cell growth, suggesting that when cells are treated 
concurrently with the growth factors their combined effects would result in no 
net growth. However, combined treatments stimulate proliferation and cellular 
transformation in several cell lines. The objective of this paper was to 
describe the effect of long-term (6 days) concurrent TGF-alpha and TGF-beta 
treatment on normal mammary epithelial cell growth pattern, morphology, and gene 
expression. Growth curve analysis showed that TGF-alpha enhanced while TGF-beta 
suppressed growth rate until Day 4, when cells entered lag phase. However, cells 
treated concurrently with both growth factors exhibited a dichotomous pattern of 
growth marked by growth and death phases (with no intermittent lag phase). These 
changes in growth patterns were due to a marked induction of cell death from Day 
2 (16.5%) to Day 4 (89.5%), resulting in the transition from growth to death 
phases, even though the combined treated cultures had significantly more (P < 
0.05) cells in S phase on Day 4. TGF-beta stimulated epithelial to mesenchyme 
transdifferentiation (EMT) in the presence of TGF-alpha, as characterized by 
increased expression of fibronectin and changes in TGF-beta receptor binding. 
Expression patterns of genes that regulate the cell cycle showed significant 
interaction between treatment and days, with TGF-beta overriding 
TGF-alpha-stimulated effects on gene expression. Overall, the combined 
treatments were marked by enhanced rates of cellular proliferation, death, and 
trans-differentiation, behaviors reminiscent of breast tumors, and thus this 
system may serve as a good model to study breast tumorigenesis.