Abstract
1285
Objectives: PET imaging with 18F-fluorodeoxyglucose (FDG) is used clinically for initial staging, restaging, and assessing therapy response in breast cancer. Tumor FDG uptake in steroid hormone receptor positive breast cancer and physiologic FDG uptake in normal breast tissue can be affected by hormonal factors such as menstrual cycle phase, menopausal status, and hormone replacement therapy [1-5]. The role of progesterone and its signaling through progesterone receptor (PR) in regulating FDG uptake has not been defined. The aim of this study was to determine the role PR has in regulating FDG uptake in breast cancer cells via pharmacologic and genomic-based approaches.
Methods: Breast cancer cell lines used included MDA-MB-231 and MDA-MB-468, which are ER-/PR-/HER2-, and T47D cells, which are ER+/PR+/HER2-, and naturally express both PR-A and PR-B protein isoforms. T47D cells with knock-out of the PGR gene (PR KO) and the scrambled (Scr) control were used as matched isogenic cell lines without or with PR protein expression, respectively [6]. Variant T47D cells expressing only the PR-A or PR-B protein isoforms were also used [7]. FDG cell uptake assays were performed after 24 h treatment with PR agonists (progesterone, promegestone=R5020, medroxyprogesterone acetate=MPA, or megestrol acetate=MA), PR antagonists (mifepristone=RU486, or ulipristal acetate=UPA), or ethanol vehicle control. Steroid hormone deprived cells were incubated for 30 min in media without and with glucose (for nonspecific uptake) prior to 40 min incubation with FDG. Specific decay-corrected counts obtained from a gamma counter were normalized to protein content. Agonist and antagonist function of PR ligands were confirmed using a progesterone-response-element luciferase reporter gene assay of transcriptional activity. One-way ANOVA with multiple comparison post-tests were performed.
Results: T47D cells treated with the PR agonists, R5020, MPA, or MA, for 24 h had approximately two-fold increase in FDG uptake compared to cells treated with ethanol control (p<0.05). A dose-dependent increase in FDG uptake was observed with R5020 treatment (EC50 0.2 nM, 95%CI: 0.03-1.6 nM). Treatment with the natural PR agonist, progesterone, increased FDG uptake similar to the response observed with R5020. No change in FDG uptake was observed in T47D cells treated with the glucocorticoid receptor specific agonist, dexamethasone. There was no significant change in FDG uptake in response to PR agonist treatment in the MDA-MB-231 and MDA-MB-468 cells, which lack PR expression (p>0.99). Treatment of T47D cells with the PR antagonists, RU486 or UPA, which inhibited PR transcriptional activity, inhibited the effect of R5020 on FDG uptake (Figure 1A). In T47D Scr cells, which retain PR expression and transcriptional activity, treatment with R5020, MPA, and MA increased FDG uptake by 2.02±0.11-fold (p=0.0002), 1.88±0.10-fold (p=0.0006), and 1.84±0.13-fold (p=0.0008), respectively, relative to ethanol treated controls. However, FDG uptake remained unchanged in T47D PR KO cells, which lack PR expression and transcriptional activity, when treated with R5020, MPA, or MA (Figure 1B). Using variant T47D cells that only express either PR-A or PR-B isoforms, PR agonists increased FDG uptake in both cell types. Conclusions: Progesterone and progestins increase FDG uptake in T47D breast cancer cells expressing PR. Since the response can be blocked by pharmacologic inhibition of PR transcriptional function or genetic knock-out of PR expression, these results suggest that ligand-activated PR has a role in regulating FDG uptake in breast cancer cells. Further in vivo studies are warranted.