Involvement of extracellular signal-regulated kinase 2 and stress- activated protein kinase/Jun N-terminal kinase activation by transforming growth factor β in the negative growth control of breast cancer cells

Although transforming growth factor β (TGF-β) is known to be a potent growth inhibition 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 (R(II)), or in ZR-75 BCCs stably transfected with R(II) 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 R(II) 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.