title: Smad4 inhibits tumor growth by inducing apoptosis in estrogen receptor-alpha-positive breast cancer cells.
abstract: Estrogen is a mitogen in most estrogen receptor-alpha (ERalpha)-positive breast cancers. We have found that Smad4, a common signal transducer in the transforming growth factor-beta superfamily, acts as an ERalpha transcriptional corepressor. Here, we show that Smad4 induces apoptosis in ERalpha-positive MCF-7 breast cancer cells, but not in ERalpha-negative MDA-MB-231 cells. Smad4 induced expression of short Bim isoforms (by alternative splicing) and Bax and release of cytochrome c in ERalpha-positive cells only, and expression of these apoptotic marker genes was reduced when ERalpha small interfering RNA was introduced. Notably, Smad4 was able to induce apoptosis in MDA-231 cells with acquired ERalpha expression. Furthermore, Smad4 inhibited ERalpha-positive tumor growth by inducing apoptosis in tumor xenografts in nude mice. The sizes of tumors expressing Smad4 were only one-tenth the size of those expressing green fluorescent protein, whereas in ERalpha-negative cells, Smad4 did not reduce the tumor size. Notably, Smad4 also promoted short Bim isoform and Bax expression and release of cytochrome c only in ERalpha-positive MCF-7 tumor xenografts. Bim was sufficient for induction of apoptosis, and the short form was the most potent inducer. Our results demonstrate that Smad4 induces apoptosis by regulating Bim splicing as an initial intrinsic signal in ERalpha-positive cells. Smad4-induced apoptosis in ERalpha-positive breast cancer cells may explain the invasive nature of ERalpha-negative breast tumors, thereby providing a potential target for breast cancer intervention.
authors.affiliation: Department of Pathology, University of Alabama at Birmingham, Birmingham, Alabama 35294-0019, USA.
authors.forename: Qingnan
authors.initials: Q
authors.lastname: Li
authors.forename: Liyu
authors.initials: L
authors.lastname: Wu
authors.forename: Denise K
authors.initials: DK
authors.lastname: Oelschlager
authors.forename: Mei
authors.initials: M
authors.lastname: Wan
authors.forename: Cecil R
authors.initials: CR
authors.lastname: Stockard
authors.forename: William E
authors.initials: WE
authors.lastname: Grizzle
authors.forename: Ning
authors.initials: N
authors.lastname: Wang
authors.forename: Huaiqing
authors.initials: H
authors.lastname: Chen
authors.forename: Yi
authors.initials: Y
authors.lastname: Sun
authors.forename: Xu
authors.initials: X
authors.lastname: Cao
mesh_terms: D017398:Alternative Splicing; D000818:Animals; D017209:Apoptosis / Q000235:genetics*; D051017:Apoptosis Regulatory Proteins; D000072224:Bcl-2-Like Protein 11; D001943:Breast Neoplasms / Q000737:chemistry / Q000473:pathology*; D002352:Carrier Proteins / Q000235:genetics; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D045304:Cytochromes c / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000502:physiology*; D047628:Estrogen Receptor alpha / Q000235:genetics*; D005260:Female; D015316:Genetic Therapy; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics; D051379:Mice; D008819:Mice, Nude; D009374:Neoplasms, Experimental / Q000473:pathology / Q000628:therapy; D011518:Proto-Oncogene Proteins / Q000235:genetics; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics; D051901:Smad4 Protein; D015534:Trans-Activators / Q000502:physiology*; D014183:Transplantation, Heterologous; D051028:bcl-2-Associated X Protein
chemical_list: D051017:Apoptosis Regulatory Proteins; C494826:BAX protein, human; C000606775:BCL2L11 protein, human; C494827:Bax protein, mouse; D000072224:Bcl-2-Like Protein 11; C000606776:Bcl2l11 protein, mouse; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D047628:Estrogen Receptor alpha; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; C497796:SMAD4 protein, human; D051901:Smad4 Protein; D015534:Trans-Activators; D051028:bcl-2-Associated X Protein; D045304:Cytochromes c