Our findings suggest that the expression of PGRMC1 might be useful for predicting prognosis in patients with breast cancer.

BackgroundRecently, an intra-patient comparison demonstrated that the somatostatin (sst) ligand [68Ga]HA-DOTATATE ([68Ga]DOTA-3-iodo-Tyr3-octreotate) provides PET images comparable to or superior to those obtained with [68Ga]DOTATATE. To provide a comprehensive basis for nevertheless observed slight differences in tracer biodistribution and dosimetry, the characteristics of [68Ga]HA-DOTATATE were investigated in a detailed preclinical study.MethodsAffinities of natGa-HA-DOTATATE and natGa-DOTATATE to sst1–5 were determined using membrane preparations and [125I]SST-28 as radioligand. Internalization into AR42J cells was studied in dual-tracer studies with [125I]TOC as internal reference. Biodistribution was investigated using AR42J tumor-bearing CD1 mice, and specificity of tracer uptake was confirmed in competition studies by coinjection of 0.8 mg TOC/kg.ResultsSst2 affinities (IC50) of [natGa]HA-DOTATATE (1.4 ± 0.8 nM, logP: −3.16) and [natGa]DOTATATE (1.2 ± 0.6 nM, logP: −3.69) were nearly identical. Both compounds displayed IC50 > 1 μM for sst1,3,4, while sst5 affinity was markedly increased for natGa-HA-DOTATATE (102 ± 65 nM vs >1 μM for natGa-DOTATATE). [natLu]HA-DOTATATE and [natLu]DOTATATE showed slightly lower, identical sst2 affinities (2.0 ± 1.6 and 2.0 ± 0.8 nM, respectively) and sst3 affinities of 93 ± 1 and 162 ± 16 nM. Internalization of [68Ga]HA-DOTATATE was tenfold higher than that of [125I]TOC but only sixfold higher for [68Ga]DOTATATE and [177Lu]HA-DOTATATE. While [68Ga]HA-DOTATATE and [68Ga]DOTATATE had shown similar target- and non-target uptake in patients, biodistribution studies in mice at 1 h post injection (n = 5) revealed slightly increased non-specific uptake of [68Ga]HA-DOTATATE in the blood, liver, and intestines (0.7 ± 0.3, 1.0 ± 0.2, and 4.0 ± 0.7 %iD/g vs 0.3 ± 0.1, 0.5 ± 0.1, and 2.7 ± 0.8 %iD/g for [68Ga]DOTATATE). However, sst-mediated accumulation of [68Ga]HA-DOTATATE in the pancreas, adrenals, and tumor was significantly enhanced (36.6 ± 4.3, 10.8 ± 3.2, and 33.6 ± 10.9 %iD/g vs 26.1 ± 5.0, 5.1 ± 1.4, and 24.1 ± 4.9 %iD/g, respectively). Consequently, tumor/background ratios for [68Ga]HA-DOTATATE in the AR42J model are comparable or slightly increased compared to [68Ga]DOTATATE.ConclusionsThe present preclinical data fully confirm the general biodistribution pattern and excellent in vivo sst-targeting characteristics previously observed for [68Ga]HA-DOTATATE in patients. The effect of slightly enhanced lipophilicity on background accumulation and normal organ dose is compensated by the high uptake of [68Ga]HA-DOTATATE in tumor. Thus, [68Ga]HA-DOTATATE represents a fully adequate, freely available substitute for [68Ga]DOTATATE and, given the superb sst-targeting characteristics of [177Lu]HA-DOTATATE in vitro, potential applicability for sst-targeted PRRT.

Menopausal hormone therapy, using estrogen and synthetic progestins, is associated with an increased risk of developing breast cancer. The effect of progestins on breast cells is complex and not yet fully understood. In previous in vitro and in vivo studies, we found different progestins to increase the proliferation of Progesterone Receptor Membrane Component-1 (PGRMC1)-overexpressing MCF7 cells (MCF7/PGRMC1), suggesting a possible role of PGRMC1 in transducing membrane-initiated progestin signals.Understanding the activation mechanism of PGRMC1 by progestins will provide deeper insights into the mode of action of progestins on breast cells and the often-reported phenomenon of elevated breast cancer rates upon progestin-based hormone therapy. In the present study, we aimed to further investigate the effect of progestins on receptor activation in MCF7 and T47D breast cancer cell lines. We report that treatment of both breast cancer cell lines with the progestin norethisterone (NET) induces phosphorylation of PGRMC1 at the Casein Kinase 2 (CK2) phosphorylation site Ser181, which can be decreased by treatment with CK2 inhibitor quinalizarin. Point mutation of the Ser181 phosphorylation site in MCF7/PGRMC1 cells impaired proliferation upon NET treatment. This study gives further insights into the mechanism of differential phosphorylation of the receptor and confirms our earlier hypothesis that phosphorylation of the CK2-binding site is essential for activation of PGRMC1. It further suggests an important role of PGRMC1 in the tumorigenesis and progression of breast cancer in progestin-based hormone replacement therapy.