Although transforming growth factor β (TGF-β) is known to be a potent growth inhibitor of breast cancer cells (BCCs), the signaling mechanisms mediating TGF-β responses have not been defined. We have demonstrated previously that TGF-β can activate Ras and extracellular signal-regulated kinase (ERK) 1 in untransformed epithelial cells (K. M. Mulder and S. L. Morris, J. Biol. Chem., 267: 5029–5031, 1992; M. T. Hartsough and K. M. Mulder, J. Biol. Chem., 270: 7117-7124, 1995). We have also shown that TGF-β signaling is altered in epithelial cells when Ras activation is blocked (Hartsough et al., J. Biol. Chem., 271: 22368–22375). Here we demonstrate the ability of the TGF-β3 isoform to activate the signaling component ERK2 in TGF-β-sensitive BCCs but not in TGF-β-resistant cells. The ERK2 isoform was activated by 6-fold within 10 min of TGF-β3 addition to the TGF-β-sensitive BCC line Hs578T. Moreover, the IC50 for inhibition of DNA synthesis by TGF-β3 in this cell line correlated with the EC50 for TGF-β3 activation of ERK2. In contrast, TGF-β3 had little effect on either DNA synthesis or ERK2 activation in ZR-75 BCCs lacking the type-II TGF-β receptors (RII), or in ZR-75 BCCs stably transfected with RII yet still TGF-β resistant. In addition, our data demonstrate that TGF-β3 affected a sustained activation of the stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK) type of mitogen-activated protein kinase (MAPK); maximal induction levels were 2.5-fold above basal values and were attained at 30 min after TGF-β3 treatment. In contrast, TGF-β3 did not increase SAPK/JNK activity in the TGF-β-resistant ZR-75 RII BCCs. Our data provide the first evidence that TGF-β activation of ERK2 and SAPK/JNK is associated with negative growth control of BCCs. This is also the first demonstration that TGF-β can activate the SAPK/JNK type of MAPK and that the TGF-β3 isoform can regulate MAPK activity.
This work was supported by NIH Grants CA51452, CA54816, and CA68444 to K. M. M. K. M. M. is the recipient of NIH Research Career Development Award KO4 CA59552.