Date of Award

August 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Biomedical Sciences

First Advisor

Jennifer A Doll

Committee Members

Dean T Nardelli, Janis T Eells, Elizabeth S Liedhegner, Xuexia Wang

Keywords

Obesity, High fat diet (HFD), ob/ob mice, Pigment epithelium-derived factor (PEDF), Prostate cancer (PCa), Serine palmitoyltransferase (SPT), Superoxide dismutase 2 (SOD2), Tumor microenvironment

Abstract

Background: Pigment epithelium-derived factor (PEDF), a multifunctional protein, is a potent tumor suppressor and anti-angiogenic factor. PEDF has been identified as a novel regulator of lipid metabolism via binding and activating adipose triglyceride lipase, which stimulates lipolytic pathways. PEDF-deficient mice develop prostatic hyperplasia that progresses to cancer when these mice are fed a high fat diet (HFD). PEDF expression is decreased in prostate cancer (PCa) patient tissues. Conversely, PEDF treatment inhibits PCa growth in mouse models. Recently, our lab has demonstrated that excess oleic acid (OA) treatment, an in vitro obesity model, suppresses secreted PEDF expression in PCa cell lines (LNCaP, DU145, and PC-3) and suppresses cellular PEDF expression in androgen-independent PCa cell lines (DU145 and PC-3 cells). This is of interest because PCa patients who are obese are at a higher risk of aggressive disease and more likely to die from the disease. However, the molecular mechanisms through which excess OA down-regulates PEDF in vitro are unknown. For the in vitro studies, we first investigated one OA receptor, G protein-coupled receptor 40 (GPR40), and tested if OA-mediated PEDF suppression was dependent on calcium (Ca2+) and/or peroxisome proliferator-activated receptor gamma (PPAR) signaling pathways. In addition, it is unknown if obese conditions suppress PEDF in vivo within PCa tissues or within PCa xenograft tumors. Thus, we hypothesize that excess lipids, in obesity and HFD microenvironments, alter lipid metabolism, which in turn, increases pro-tumorigenic activity and decreases PEDF expression in PCa. To study this hypothesis, we used an excess OA model in vitro and two in vivo PCa models.

Methods: We treated RWPE-1, LNCaP, DU145, PC-3, and TRAMPC2 with excess OA at 1 mM for 48 hours. Then, cells were counted, viability was assessed, and PEDF levels were quantified by enzyme-linked immunosorbent assay (ELISA). For in vivo studies, two obese models were used: (1) a xenograft model using the TRAMPC2 cell line injected in male WT mice and obese (ob/ob) mice and (2) male Pbsn-Cre+-PTENfl/fl mice fed either a control diet (CD) or a HFD. All mice were in a C57Bl/6J background. Blood samples, TRAMPC2 tumor xenograft tissue, mouse PCa tissues, liver, and adipose tissues were collected. PEDF levels and pro-tumorigenic pathway mediators were evaluated by ELISA and by Western blot, respectively.

Results: For the in vitro study, we found that OA did not interact with the GPR40 receptor and/or PPAR to rescue secreted PEDF levels in LNCaP, DU145, and PC-3 cells. In Pbsn-Cre+-PTENfl/fl model, the prostate weight of HFD-fed mice was increased as compared to CD-fed mice (P-value <0.042). In the TRAMPC2 xenograft model, we observed a significant increase in tumor volume and weight in TRAMPC2 tumors grown in ob/ob mice as compared to tumors grown in lean mice (P-value <0.018). Tumor PEDF levels were significantly decreased in TRAMPC2 xenograft tumors grown in ob/ob mice compared to TRAMPC2 xenograft tumors grown in WT mice (P-value <0.024). Lastly, in an effort to identify other signaling pathways altered in TRAMPC2 tumor tissues grown in ob/ob mice versus WT mice, we found a significant reduction in superoxide dismutase 2 (SOD2; P-value <0.018) and serine palmitoyltransferase (SPT; P-value <0.050). Interestingly, while previous studies showed that obesity increased circulating PEDF, here we did not find an increase in circulating PEDF levels in ob/ob mice with TRAMPC2 xenograft tumors as compared to WT mice. Moreover, circulating PEDF levels were actually significantly decreased in HFD-fed Pbsn-Cre+-PTENfl/fl mice compared to CD-fed Pbsn-Cre+-PTENfl/fl mice (P-value <0.001).

Conclusion: This is the first study to report that an obese microenvironment suppresses circulating PEDF and suppresses PEDF, SOD2, and SPT in mouse PCa tissues. Therefore, these data indicate that reduced PEDF, SOD2, or SPT levels could be mechanisms of obesity-driven PCa progression.

Available for download on Friday, August 27, 2021

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