It is proposed that loss of a growth-inhibitory response to transforming growth factor β (TGFβ) contributes to breast cancer progression. Because cellular TGFβ responsiveness often correlates with TGFβ type II receptor (TGFβ-IIR) expression, we have examined the cellular distribution of TGFβ-IIRs in tumor and nontumor mammary epithelial cells. By immunoblot analysis, TGFβ-IIR was detected both in membrane and cytosolic fractions of MDA-231 tumor cells as well as in normal human breast epithelial cells. The cytosolic protein appeared to be more abundant and was detected as a clear perinuclear staining by immunocytochemistry. The glycosylation patterns of the cytosolic and membrane form were different, indicating distinct receptor pools. The cytosolic TGFβ-IIR did not bind 125I-labeled TGFβ1 but had a detectable in vitro and in vivo kinase activity. MCF-7 breast cancer cells express the TGFβ-IIR mRNA but show undetectable cell surface TGFβ-IIR protein by affinity cross-linking. However, low levels of TGFβ-IIR were observed in MCF-7 cytosol. Sequencing of the coding region of TGFβ-IIR from MCF-7 cells indicated a point mutation (A439V) in a nonconserved region of the kinase domain. When MCF-7 cells were treated with sublethal doses of Adriamycin that induce cell differentiation, the membrane localization of TGFβ-IIR and TGFβ response were restored. Our results indicate the presence of a prominent, kinase-active TGFβ-IIR in the cytosol of several mammary cell lines. This cytosolic pool of receptors is the only detectable one in MCF-7 cells. Loss of wild-type membrane receptors due to defects in trafficking presents a potential new mechanism for escape from negative growth control.
Supported by NIH Grant CA62212 (to C. L. A.), Clinical Investigator Award from the Department of Veteran Affairs (to C. L. A.), and the T. J. Martell Foundation.