**2.2 ERβ splice variants**

The presence of ERβ isoforms has been confirmed in various human cell lines as well as in a broad range of tissues at different levels (Leung *et al.*, 2006; Moore *et al.*, 1998), which provides another possible mechanism of tissue-dependent modulation of the ER response. Therefore distribution of particular isoforms of both ERs should be taken into account when considering tissue response to estrogens as they have differential and sometimes antagonistic properties and their differential distribution might significantly influence biological response to hormone.

Different isoforms of ERβ have been described (figure 5) with a variable C-terminus, and which were cloned from a testis cDNA library (Moore *et al.*, 1998). At present their functional significance is poorly understood. The ERβ isoform whose function has been described in most detail of all ERβ isoforms studied is **ERβ1**, which is a full length protein with LBD and active AF-2 domain. **ERβ2, 4 and 5** have a shortened Helix 11 and a full length Helix 12 is present only in ERβ1 and β2. In ERβ2, Helix 12 has a different orientation than in ERβ1 due to the shorter Helix 11. It has been reported that the displaced Helix 12 in ERβ2 limits ligand access to the binding pocket. As a consequence of their altered structure, ERβ2, 4 and 5 cannot form homodimers and have no transcriptional activity on their own, although they have been shown to heterodimerize with ERβ1 upon E2 treatment and enhance its AF-2 mediated transcriptional activity (Leung *et al.*, 2006). Studies of interactions between different ERβ isoforms with ERα are very limited. However **ERβ2** (also named **ERβcx**) was shown to limit DNA binding of ERα66 and inhibit its transcriptional activity in similar manner to ERβ1 (Ogawa *et al.*, 1998b).

Fig. 5. Comparison between full length ERβ and it is most referenced truncated isoforms.

Two new exon-deleted variants were detected in the cancer cell line MDA-MB-231, **ERβΔ1,2,5** and **ERβΔ1,2,5,6** of approximately 35 and 28 kDa, respectively (Treeck *et al.*, 2008). Both proteins are predicted not to contain AF-1, and to have deletions in the DBD and LBD. Therefore, these two variants are expected to be devoid of or have significantly reduced ligand-dependent and ligand independent activities, and their expression did not affect growth of cancer cell lines tested. A list of selected ERβ splice variants and their

The presence of ERβ isoforms has been confirmed in various human cell lines as well as in a broad range of tissues at different levels (Leung *et al.*, 2006; Moore *et al.*, 1998), which provides another possible mechanism of tissue-dependent modulation of the ER response. Therefore distribution of particular isoforms of both ERs should be taken into account when considering tissue response to estrogens as they have differential and sometimes antagonistic properties and their differential distribution might significantly influence

Different isoforms of ERβ have been described (figure 5) with a variable C-terminus, and which were cloned from a testis cDNA library (Moore *et al.*, 1998). At present their functional significance is poorly understood. The ERβ isoform whose function has been described in most detail of all ERβ isoforms studied is **ERβ1**, which is a full length protein with LBD and active AF-2 domain. **ERβ2, 4 and 5** have a shortened Helix 11 and a full length Helix 12 is present only in ERβ1 and β2. In ERβ2, Helix 12 has a different orientation than in ERβ1 due to the shorter Helix 11. It has been reported that the displaced Helix 12 in ERβ2 limits ligand access to the binding pocket. As a consequence of their altered structure, ERβ2, 4 and 5 cannot form homodimers and have no transcriptional activity on their own, although they have been shown to heterodimerize with ERβ1 upon E2 treatment and enhance its AF-2 mediated transcriptional activity (Leung *et al.*, 2006). Studies of interactions between different ERβ isoforms with ERα are very limited. However **ERβ2** (also named **ERβcx**) was shown to limit DNA binding of ERα66 and inhibit its transcriptional activity in

Fig. 5. Comparison between full length ERβ and it is most referenced truncated isoforms.

Two new exon-deleted variants were detected in the cancer cell line MDA-MB-231, **ERβΔ1,2,5** and **ERβΔ1,2,5,6** of approximately 35 and 28 kDa, respectively (Treeck *et al.*, 2008). Both proteins are predicted not to contain AF-1, and to have deletions in the DBD and LBD. Therefore, these two variants are expected to be devoid of or have significantly reduced ligand-dependent and ligand independent activities, and their expression did not affect growth of cancer cell lines tested. A list of selected ERβ splice variants and their

**2.2 ERβ splice variants** 

biological response to hormone.

similar manner to ERβ1 (Ogawa *et al.*, 1998b).

expression in various breast tissues (normal and tumor) and breast cancer cell lines is given in Table 3.

Various studies reveal that physiological levels of ERα and ERβ may vary depending on the cell or tissue type (Enmmark *et al.*, 1997; Bonkhoff *et al.*, 1999; Makinen *et al.*, 2001; Pearce *et al.*, 2004) and as a consequence the biological response to endogenous or exogenous ligands can differ significantly.


Table 3. List of selected ERβ splice variants and their expression in various breast tissues (normal and tumour) and breast cancer cell lines.
