Predominant cytosolic localization of type II transforming growth factor β receptors in human breast carcinoma cells

It is proposed that loss of a growth-inhibitory response in 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.