The prolactin-induced protein encoded by the Pip gene is secreted by apocrine cells into exocrine fluids. Prolactin-induced protein binds actin and is a biomarker for breast cancer. These knock-in mice express the GCE cassette (an EGFP/creERT2 fusion gene) from the endogenous Pip locus. When PipGCE mice are bred with mice containing loxP-flanked sequence, tamoxifen-inducible, cre-mediated recombination will result in deletion of the floxed sequences. Tamoxifen administration in double mutant mice, carrying this transgene and a reporter, induces Cre recombination in acinar cells of the submandibular gland. Although Pip is expressed as early as E14 in proacinar cells, cre activity is not detected in E15.5 or E17.5 submandibular gland. Tamoxifen inducible cre recombinase activity is not detected in parotid, sublingual or lacrimal glands, kidney, lung, pancreas, prostate, or ovary tissues. The Donating Investigator has not made the strain homozygous, although homozygotes are predicted to be viable and fertile. EGFP expression is not detected by direct fluorescence or anti-GFP antibody (immunofluorescence). During backcrossing, the Y chromosome may not have been fixed to the C57BL/6J genetic background.
The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor, which does not bind its natural ligand (17β-estradiol) at physiological concentrations, but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, Cre-ERT2 can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be coadministered.
