Showing papers by "Timothy L. Ratliff published in 2014"

Cholesteryl Ester Accumulation Induced by PTEN Loss and PI3K/AKT Activation Underlies Human Prostate Cancer Aggressiveness

Abstract: Altered lipid metabolism is increasingly recognized as a signature of cancer cells. Enabled by label-free Raman spectromicroscopy, we performed quantitative analysis of lipogenesis at single-cell level in human patient cancerous tissues. Our imaging data revealed an unexpected, aberrant accumulation of esterified cholesterol in lipid droplets of high-grade prostate cancer and metastases. Biochemical study showed that such cholesteryl ester accumulation was a consequence of loss of tumor suppressor PTEN and subsequent activation of PI3K/AKT pathway in prostate cancer cells. Furthermore, we found that such accumulation arose from significantly enhanced uptake of exogenous lipoproteins and required cholesterol esterification. Depletion of cholesteryl ester storage significantly reduced cancer proliferation, impaired cancer invasion capability, and suppressed tumor growth in mouse xenograft models with negligible toxicity. These findings open opportunities for diagnosing and treating prostate cancer by targeting the altered cholesterol metabolism.

Plk1 Inhibition Enhances the Efficacy of Androgen Signaling Blockade in Castration-Resistant Prostate Cancer

Abstract: Prostate cancer (PCa) is thought to be driven by oxidative stress, lipid metabolism, androgen receptor (AR) signaling and activation of the PI3K/AKT/mTOR pathway, but it is uncertain how they may become coordinated during progression to castration-resistant disease which remains incurable. The mitotic kinase polo-like kinase 1 (Plk1) is elevated in PCa where its expression is linked to tumor grade. Notably, Plk1 signaling and lipid metabolism were identified recently as two of the top five most upregulated pathways in a mouse xenograft model of human PCa. Herein, we show that oxidative stress activates both the PI3K/AKT/mTOR pathway and AR signaling in a Plk1-dependent manner in prostate cells. Inhibition of the PI3K/AKT/mTOR pathway prevented oxidative stress-induced activation of AR signaling. Plk1 modulation also affected cholesterol ester accumulation in PCa via the SREBP pathway. Lastly, Plk1 inhibition enhanced cellular responses to androgen signaling inhibitors (ASI) and overcame ASI resistance in both cultured PCa cells and patient-derived tumor xenografts. Given that activation of AR signaling and the PI3K/AKT/mTOR pathway is sufficient to elevate SREBP-dependent expression of key lipid biosynthesis enzymes in castration-resistant PCa, our findings argued that Plk1 activation was responsible for coordinating and driving these processes to promote and sustain the development of this advanced stage of disease. Overall, our results offer a strong mechanistic rationale to evaluate Plk1 inhibitors in combination drug trials to enhance the efficacy of androgen signaling inhibitors in castration-resistant prostate cancer.

Plk1 phosphorylation of PTEN causes a tumor-promoting metabolic state

Abstract: One outcome of activation of the phosphatidylinositol 3-kinase (PI3K) pathway is increased aerobic glycolysis, but the upstream signaling events that regulate the PI3K pathway, and thus the Warburg effect, are elusive. Increasing evidence suggests that Plk1, a cell cycle regulator, is also involved in cellular events in addition to mitosis. To test whether Plk1 contributes to activation of the PI3K pathway, and thus aerobic glycolysis, we examined potential targets of Plk1 and identified PTEN as a Plk1 substrate. We hypothesize that Plk1 phosphorylation of PTEN leads to its inactivation, activation of the PI3K pathway, and the Warburg effect. Our data show that overexpression of Plk1 leads to activation of the PI3K pathway and enhanced aerobic glycolysis. In contrast, inhibition of Plk1 causes markedly reduced glucose metabolism in mice. Mechanistically, we show that Plk1 phosphorylation of PTEN and Nedd4-1, an E3 ubiquitin ligase of PTEN, results in PTEN inactivation. Finally, we show that Plk1 phosphorylation of PTEN promotes tumorigenesis in both its phosphatase-dependent and -independent pathways, revealing potentially new drug targets to arrest tumor cell growth.

Impact of Prostate Inflammation on Lesion Development in the POET3+ Pten+/− Mouse Model of Prostate Carcinogenesis

Abstract: Evidence linking prostatitis and prostate cancer development is contradictory To study this link, the POET3 mouse, an inducible model of prostatitis, was crossed with a Pten-loss model of prostate cancer (Pten+/−) containing the ROSA26 luciferase allele to monitor prostate size Prostatitis was induced, and prostate bioluminescence was tracked over 12 months, with lesion development, inflammation, and cytokine expression analyzed at 4, 8, and 12 months and compared with mice without induction of prostatitis Acute prostatitis led to more proliferative epithelium and enhanced bioluminescence However, 4 months after initiation of prostatitis, mice with induced inflammation had lower grade pre-neoplastic lesions A trend existed toward greater development of carcinoma 12 months after induction of inflammation, including one of two mice with carcinoma developing perineural invasion Two of 18 mice at the later time points developed lesions with similarities to proliferative inflammatory atrophy, including one mouse with associated carcinoma Pten+/− mice developed spontaneous inflammation, and prostatitis was similar among groups of mice at 8 and 12 months Analyzed as one cohort, lesion number and grade were positively correlated with prostatitis Specifically, amounts of CD11b+Gr1+ cells were correlated with lesion development These results support the hypothesis that myeloid-based inflammation is associated with lesion development in the murine prostate, and previous bouts of CD8-driven prostatitis may promote invasion in the Pten+/− model of cancer