BAG-1

RAP46

Gene: maps to 9p12 (Takayama S. et al., 1996).
mRNA: size: 1.4 kb (Yang X. et al., 1998); 1.9 kb (Takayama S. et al., 1998).
Protein: an antiapoptotic protein capable of interacting with and enhancing the activity of bcl-2. Furthermore, it inhibits apoptosis induced by apoptotic stimuli or cytokine withdrawal in mouse NIH3T3 cells and lymphocytes. BAG-1 was shown to bind the plasma membrane-associated tyrosine kinase growth factor receptors (hepatocyte growth factor receptor and platelet-derived growth factor receptor) and enhance the ability of these receptors to inhibit apoptosis (Bardelli A. et al., 1996). In addition, BAG-1 was shown to bind several hormone receptors, such as estrogen receptor (ER), androgen receptor, and glucocorticoid receptor, and modulate their function (Zeiner M. and Gehring U., 1995). BAG-1 therefore acts as a multifunctional antiapoptotic protein capable of interacting with various cellular proteins. This could be attributable to its ability to bind and modulate the activities of the 70-kDa family of molecular chaperones, including Hsp70 and Hsc70. BAG-1 therefore may represent a novel component of the chaperone system that modulates interactions of Hsp70/Hsc70 with other proteins, inducing alterations in the conformations of these target proteins and thereby altering their biochemical and biological activities in cells (Takayama S. et al., 1997).
BAG-1 appears to be expressed as four protein isoforms p50, p46, p33, and p29, through alternative translation initiation from four different start codons through a leaky scanning mechanism. By immunofluorescence and protein fractionation analysis, these isoforms showed distinct subcellular localizations: the p50 isoform was predominantly present in the nuclear and membrane fractions and p46 was in the nuclear, cytosolic, and membrane fractions, whereas p33 and p29 were found mostly in the cytosol fraction (Yang X. et al., 1998). Their role in tumorigenesis remains unclear.
Takayama S. et al. (1998) have described three BAG-1 isoforms. These isoforms, BAG-1L, BAG-1M, and BAG-1, appear to correspond respectively to the p50, p46, and p33 isoforms described above. Only BAG-1L has a nuclear localization sequence. Immunoblotting experiments revealed that in normal tissues, BAG-1L is far more restricted in its expression and is present at lower levels than the more prevalent BAG-1 protein.
BAG-1 may promote the dissociation of ADP from heat shock cognate 70 (HSC70) protein, thereby stimulating its ATPase activity (Höhfeld J. and Jentsch S., 1997).

Cell lines:
- High BAG-1 mRNA expression was found in MCF-7, MDA-MB-436, and MDA-MB-468 breast cancer cell (BCC) lines. In Hs579T BCC and in Hs579Bst breast epithelial cells, only low levels of BAG-1 messenger were found. Expression of BAG-1 protein was low in Hs579Bst cells. It was enhanced in Hs579T, MCF-7, MDA-MB-436, and MDA-MB-468. All these BCC lines expressed the p50, p46, p33 BAG-1 isoforms. The p29 isoform was found only in MCF-7 cells (Yang X. et al., 1998).

- When stably expressed in breast cancer cell lines, BAG-1 inhibited binding of retinoic acid receptor (RAR)/retinoid X receptor (RXR) heterodimer to a number of retinoic acid (RA) response elements (RAREs) and suppressed RA-induced growth inhibition and apoptosis. In addition, RA-induced suppression of Bcl-2 expression was abrogated by overexpression of BAG-1 (Liu R. et al., 1998).

- The expression of BAG-1 mRNA was examined, by Northern-blotting, in three cell lines (Hs574, Hs578, and Hs787) derived from normal breast epithelium and in nine BCC lines (BT-20, BT-474, Hs578T, MCF-7, SK-BR-3, MDA-MB-157, -231, -436, and -468). The expression of BAG-1 mRNA was low in all of the three normal breast cell lines, but was significantly higher (increased 3.75-7.76-fold) in seven of the nine BCC lines. Only Hs578T and MDA-MB-157 expressed relatively low levels of BAG-1 mRNA (Yang X. et al., 1999).

- In contrast to normal tissues, which only rarely expressed BAG-1L, tumor cell lines commonly contained BAG-1L protein, including BT-549, MCF-7, MDA-MB-231, -435 BCC lines, suggesting that a change in BAG-1 mRNA translation frequently acccompanies malignant transformation (Takayama S. et al., 1998).

Tumors:
- The expression of two isoforms of BAG-1, p46 and p33, was much higher in breast primary tissues (invasive carcinoma, n=45) than in normal breast tissues (n=24). In contrast, the p50 isoform was not significantly more abundant in tumor tissues (Yang X. et al., 1999).

Bardelli A. et al. (1996) HGF receptor associates with the anti-apoptotic protein BAG-1 and prevents cell death. EMBO J. 15, 6205-6212.
Höhfeld J. and Jentsch S. (1997) GrpE-like regulation of the Hsc70 chaperone by anti-apoptotic protein BAG-1. EMBO J. 16, 6209-6216.
Liu R. et al. (1998) Interaction of BAG-1 with retinoic acid receptor and its inhibition of retinoic acid-induced apoptosis in cancer cells. J. Biol. Chem. 273, 16985-16992.
Takayama S. et al. (1995) Cloning and functional analysis of BAG-1: a novel bcl-2-binding protein with anti-cell death activity. Cell 80, 279-284.
Takayama S. et al. (1996) Cloning of cDNAs encoding the human BAG-1 protein and localization of the human BAG-1 gene to chromosome 9p12. Genomics 35, 494-498.
Takayama S. et al. (1997) BAG-1 modulates the chaperone activity of Hsp70/Hsc70. EMBO J. 16, 4887-4896.
Takayama S. et al. (1998) Expression and location of Hsp70/Hsc-binding anti-apoptotic protein BAG-1 and its variants in normal tissues and tumor cell lines. Cancer Res. 58, 3116-3131.
Yang X. et al. (1998) Human BAG-1/RAP46 is expressed as four isoforms by alternative translation initiation and overexpressed in human cancers. Oncogene 16, 981-989.
Yang X. et al. (1999) Differential expression of antiapoptotic gene BAG-1 in human breast cancer cell lines and tissues. Clin. Cancer Res. 5, 1816-1822.
Zapata J.M. et al. (1998) Expression of multiple apoptosis-regulatory genes in human breast cancer cell lines and primary tumors. Breast Cancer Res. Treat. 47, 129-140.
Zeiner M. and Gehring U. (1995) A protein that interacts with members of the nuclear hormone receptor family: identification and cDNA cloning. Proc. Natl. Acad. Sci. USA 92, 11465-11469.

Bcl-2, bax