Gene: OXTR (single copy) maps to 3p25-p26. It spans approximately 17 kb and contains 3 introns and 4 exons. Exons 1 and 2 correspond to the 5-prime noncoding region, followed by exons 3 and 4 encoding the amino acids of the receptor. Intron 3, which is the largest at 12 kb, separates the coding region immediately after the putative 6 transmembrane-spanning domain. The transcription start sites, demonstrated by primer extension analysis, lie 618 and 621 bp upstream of the methionine initiation codon.
mRNA: size: 3.6 kb in breast tissue.
Protein: a 388-amino acid polypeptide with 7 transmembrane domains typical of G protein-coupled receptors (see also
calcitonin receptor).
In normal human breast, OXTR is found in myoepithelial cells along ducts of normal lobules (Bussolati G. et al., 1996).
Cell lines:
- The expression of the OXTR was determined using flow cytometry in four breast cancer cell (BCC) lines (MCF-7, MDA-MB-231, MDA-MB-361, and MDA-MB-468). However, OXT had no significant effect on their growth during a short period (7 days) of culture (Ito Y. et al., 1996).
- By immunofluorescence, OXTR was found in MCF-7 and MDA-MB-231 BCC lines. By RT-PCR, OXTR mRNA was found in MCF-7, T-47D, and MDA-MB-231 BCC. MDA-MB-231 BCC showed a cyclic AMP (cAMP) increase and a reduced proliferation after treatment with oxytocin. It was shown that in these BCC, cAMP is the intracellular mediator of OT action, whereas the Ca2+-phosphoinositide system is not involved. Alone, the hormone was unable to reduce the proliferation of MCF-7 and T-47D BCC, but it prevented the proliferative effect of estradiol and enhanced the antiproliferative effect of tamoxifen on these cells (Bussolati G. et al., 1996; Cassoni P. et al., 1994; Cassoni P. et al., 1997).
- The expression of oxytocin receptor mRNA in T-47D BCC, and four additional BCC lines (BT-549, MCF-7, MDA-MB- 231, ZR-75-1), was demonstrated by RT-PCR analysis. Oxytocin-induced changes in intracellular free calcium ([Ca2+]
i) in indo-1 AM loaded T-47D BCC were monitored using flow cytometric analysis. In this cell line, oxytocin (0, 1, 10, 100, and 1,000 nM) did not induce a dose-dependent increase in the mean 405 nm/485 nm emission ratio. These results indicate that oxytocin signaling in T-47D BCC does not appear to involve an increase in [Ca2+]
i (Fay M.J. et al., 1999).
- By analyzing expression of promoter-luciferase constructs, a region between -85 and -65 was identified in OXTR promoter that was required for both basal and serum-induced expression in a BCC line (Hs578T) that expresses inducible, endogenous OXTRs. This DNA region contains an ets family target sequence (5'-GGA-3'), and a CRE/AP-1-like motif. The specific Ets factor binding to the OXTR promoter was identified, by electrophoretic mobility immunoshift assays, to be GABP alpha/beta. Co-transfection of a -85 OXTR/luciferase construct with vectors expressing GABP alpha and GABP beta1 had only a modest effect on expression, but cotransfection with GABP alpha/beta- with c-Fos/c-Jun-expressing plasmids resulted in an increase of almost 10-fold in luciferase activity. Mutation of either the GABP- or CRE-like binding sites obliterated the induction. These findings are consistent with the involvement of protein kinase C activity in serum induction of the endogenous gene in Hs578T cells (Hoare S. et al., 1999).
Tumors:
- Oxytocin and an analogue, F314, inhibited cell proliferation and tumor growth of OXTR-expressing rat and mouse mammary carcinomas (Cassoni P. et al., 1996).
- Among 57 breast cancer patients, OXTR immunoreactivity was found in 52 (91.2%) by immunohistochemistry. Of these OXTR-positive samples, 28 were diffusely positive (> 80% of cancer cells were stained), and 24 were partially positive (< 80% cells were stained). The ratio of
estrogen receptor-positive samples was slightly higher among those that were diffusely positive, but there was no apparent relationship between OXTR expression and other clinical parameters. The expression of the OXTR in positively stained samples was confirmed by means of Northern blotting and RT-PCR. The OXTR mRNA and RT-PCR product were the same size as those in the pregnant myometrium. These findings indicated that the OXTR is expressed in breast cancer derived not from the myoepithelium but from the glandular or ductal epithelium (Ito Y. et al., 1996).
