Predominant cytosolic localization of type II transforming growth factor beta receptors in human breast carcinoma cells
Koli, K.M.; Arteaga, C.L.
Cancer Research 57(5): 970-977
1997
ISSN/ISBN: 0008-5472 PMID: 9041203 Document Number: 470083
It is proposed that loss of a growth-inhibitory response to transforming growth factor beta (TGF-beta) contributes to breast cancer progression. Because cellular TGF-beta responsiveness often correlates with TGF-beta type II receptor (TGF-beta-IIR) expression, we have examined the cellular distribution of TGF-beta-IIRs in tumor and nontumor mammary epithelial cells. By immunoblot analysis, TGF-beta-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 TGF-beta-IIR did not bind 125I-labeled TGF-beta-1 but had a detectable in vitro and in vivo kinase activity. MCF-7 breast cancer cells express the TGF-beta-IIR mRNA but show undetectable cell surface TGF-beta-IIR protein by affinity cross-linking. However, low levels of TGF-beta-IIR were observed in MCF-7 cytosol. Sequencing of the coding region of TGF-beta-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 TGF-beta-IIR and TGF-beta response were restored. Our results indicate the presence of a prominent, kinase-active TGF-beta-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.