Submitted to: Archives of Biochemistry and Biophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/1997
Publication Date: N/A
Interpretive Summary: Estrogen is a hormone that plays a role in the development of breast cancer. Several studies have shown that in human breast cancer cell lines, a correlation exists between the presence of the protein that is bound by estrogen (the estrogen receptor) and the protein that is bound by dioxin (the aryl hydrocarbon receptor). Dioxin is a toxic byproduct of certain industrial processes and has been shown to have estrogen-like properties. Neither the estrogen nor dioxin are able to affect the activity of cells unless they first enter the nucleus of the cell and interact with the cellular DNA (i.e., the cells genes). This study demonstrates that in some breast cell lines that are responsive to dioxin and estrogen a specific molecule called the "arnt" protein that helps shuttle dioxin into the nucleus of cells is mutated. This same protein, however, is not mutated in nonresponsive cells. The results of this study suggest that the "arnt" protein may be useful for predicting breast cancer.
Technical Abstract: Several studies have reported a correlation between expression of the estrogen receptor (ER) and aryl hydrocarbon (Ah)-responsiveness in human breast cancer cell lines. MDA-MB-231 cells are ER-negative and Ah- nonresponsive; however, initial studies showed that 2, 3, 7, 8- tetrachlorodibenzo-p-dioxin (TCDD) induced CYP1A1 mRNA levels (5.8-fold) and chloramphenicol acetyltransferase (CAT) activity (2.6 fold) in high passage (Hp, >50 passages) cells transiently transfected with an Ah- responsive plasmid. In contrast, no induction responses were observed in low passage (Lp <20 passages) cells. The Ah-responsiveness of Hp compared to Lp MDA-MB-231 cells was associated with a >2-fold increased expression of the Ah receptor in Hp cells. Further analysis revealed that the apparent molecular weight of the Ah receptor mRNA transcript and immunoreactive protein were comparable in Lp MDA-MB-231 and Ah-responsive human HepG2 cells. In contrast, RT-PCR analysis of the Ah receptor nuclear translocator (Arnt) protein showed that HepG2 cells expressed the expected 2.6-kb transcript, whereas a 1.3-kb transcript was the major product in MDA-MB-231 cells. Western blot analysis confirmed that HepG2 cells primarily expressed a 97-kDa wild-type form of Arnt, whereas a dominant 36-kDa variant was expressed in MDA-MB-231 cells. Complete sequence analysis of the variant form of Arnt revealed a major deletion of the C- terminal region of the protein (aa 330 to 789). Like HepG2 cells, the wild-type 2.6-kb transcript was detected in ER-positive (Ah-responsive) MCF-7 cells, whereas the low molecular weight variant Arnt was dominant in ER-negative MDA-MB-231, MDA-MB-435, and adriamycin-resistant MCF-7 cells. These results suggest that expression of this protein may be useful.