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.

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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.

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