1. Regulation of GLUT transporters by flavonoids in androgen-sensitive and-insensitive prostate cancer cells.
Cancer cells show different metabolic requirements from normal cells. In prostate cancer, particularly, glycolytic metabolism differs in androgen-responsive and nonresponsive cells. In addition, some natural compounds with antiproliferative activities are able to modify glucose entry into cells by either modulating glucose transporter (GLUT) expression or by altering glucose binding.
The aim of this work was to study the regulation of some GLUTs (GLUT1 and GLUT4) in both androgen-sensitive (LNCaP) and -insensitive (PC-3) prostate cancer cells by 4 structurally different flavonoids (ie, genistein, phloretin, apigenin, and daidzein).
Glucose uptake was measured using nonradiolabeled 2-deoxyglucose. The evaluation of protein levels as well as subcellular distribution of GLUT1/4 were analyzed by Western blot and immunocytochemistry, respectively. Androgen-insensitive LNCaP-R and androgen-sensitive PC-3-AR cells were used to study the effect of androgen signaling. Additionally, a docking simulation was employed to compare interactions between flavonoids and XylE, a bacterial homolog of GLUT1 to -4.
Results show for the first time the presence of functionally relevant GLUT4 in prostate cancer cells. Furthermore, differences in GLUT1 and GLUT4 levels and glucose uptake were found, without differences on subcellular distribution, after incubation with flavonoids. Docking simulation showed that all compounds interact with the same location of transporters. More importantly, differences between androgen-sensitive and -insensitive prostate cancer cells were found in both GLUT protein levels and glucose uptake.
Thus, phenotypic characteristics of prostate cancer cells are responsible for the different effects of these flavonoids in glucose uptake and in GLUT expression rather than their structural differences, with the most effective in reducing cell growth being the highest in modifying glucose uptake and GLUT levels.
Gonzalez-Menendez P, Hevia D, Rodriguez-Garcia A, Mayo JC, Sainz RM. Regulation of GLUT transporters by flavonoids in androgen-sensitive and -insensitive prostate cancer cells. Endocrinology. 2014 Sep;155(9):3238-50. doi: 10.1210/en.2014-1260.
2. Plant flavone apigenin binds to nucleic acid bases and reduces oxidative DNA damage in prostate epithelial cells.
Oxidative stress has been linked to prostate carcinogenesis as human prostate tissue is vulnerable to oxidative DNA damage. Apigenin, a dietary plant flavone, possesses anti-proliferative and anticancer effects; however, its antioxidant properties have not been fully elucidated.
We investigated sub-cellular distribution of apigenin, it's binding to DNA and protective effects against H2O2-induced DNA damage using transformed human prostate epithelial RWPE-1 cells and prostate cancer LNCaP, PC-3 and DU145 cells. Exposure of cells to apigenin exhibited higher accumulation in RWPE-1 and LNCaP cells, compared to PC-3 and DU145 cells.
The kinetics of apigenin uptake in LNCaP cells was estimated with a Km value of 5 µmole/L and Vmax of 190 pmoles/million cells/h. Sub-cellular fractionation demonstrated that nuclear matrix retains the highest concentration of apigenin (45.3%), followed by cytosol (23.9%), nuclear membranes (17.9%) and microsomes (12.9%), respectively.
Spectroscopic analysis of apigenin with calf-thymus DNA exhibited intercalation as the dominant binding mode to DNA duplex. Apigenin exposure resulted in significant genoprotective effects in H2O2-stressed RWPE-1 cells by reduction in reactive oxygen species levels.
In addition, apigenin exposure suppressed the formation of 8-hydroxy-2' deoxyguanosine and protected exposed cells from apoptosis.
Our studies demonstrate that apigenin is readily taken up by normal prostatic epithelial cells and prostate cancer cells, and is incorporated into their nuclei, where its intercalation with nucleic acid bases may account for its antioxidant and chemopreventive activities.
Sharma H, Kanwal R, Bhaskaran N, Gupta S. Plant flavone apigenin binds to nucleic acid bases and reduces oxidative DNA damage in prostate epithelial cells. PLoS One. 2014 Mar 10;9(3):e91588. doi: 10.1371/journal.pone.0091588. eCollection 2014.
3. Apigenin induces apoptosis by targeting inhibitor of apoptosis proteins and Ku70-Bax interaction in prostate cancer.
Dysfunction of the apoptotic pathway in prostate cancer cells confers apoptosis resistance towards various therapies. A novel strategy to overcome resistance is to directly target the apoptotic pathway in cancer cells. Apigenin, an anticancer agent, selectively toxic to cancer cells induces cell cycle arrest and apoptosis through mechanisms which are not fully explored.
In the present study we provide novel insight into the mechanisms of apoptosis induction by apigenin.
Treatment of androgen-refractory human prostate cancer PC-3 and DU145 cells with apigenin resulted in dose-dependent suppression of XIAP, c-IAP1, c-IAP2 and survivin protein levels. Apigenin treatment resulted in significant decrease in cell viability and apoptosis induction with the increase of cytochrome C in time-dependent manner.
These effects of apigenin were accompanied by decrease in Bcl-xL and Bcl-2 and increase in the active form of Bax protein. The apigenin-mediated increase in Bax was due to dissociation of Bax from Ku70 which is essential for apoptotic activity of Bax.
Apigenin treatment resulted in the inhibition of class I histone deacetylases and HDAC1 protein expression, thereby increasing the acetylation of Ku70 and the dissociation of Bax resulting in apoptosis of cancer cells.
Furthermore, apigenin significantly reduced HDAC1 occupancy at the XIAP promoter, suggesting that histone deacetylation might be critical for XIAP downregulation.
These results suggest that apigenin targets inhibitor of apoptosis proteins and Ku70-Bax interaction in the induction of apoptosis in prostate cancer cells and in athymic nude mouse xenograft model endorsing its in vivo efficacy.
Shukla S, Fu P, Gupta S. Apigenin induces apoptosis by targeting inhibitor of apoptosis proteins and Ku70-Bax interaction in prostate cancer. Apoptosis. 2014 May;19(5):883-94. doi: 10.1007/s10495-014-0971-6.
5. Apigenin inhibits prostate cancer progression in TRAMP mice via targeting PI3K/Akt/FoxO pathway.
Forkhead box O (FoxO) transcription factors play an important role as tumor suppressor in several human malignancies. Disruption of FoxO activity due to loss of phosphatase and tensin homolog and activation of phosphatidylinositol-3 kinase (PI3K)/Akt are frequently observed in prostate cancer. Apigenin, a naturally occurring plant flavone, exhibits antiproliferative and anticarcinogenic activities through mechanisms, which are not fully defined.
In the present study, we show that apigenin suppressed prostate tumorigenesis in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice through the PI3K/Akt/FoxO-signaling pathway.
Apigenin-treated TRAMP mice (20 and 50 µg/mouse/day, 6 days/week for 20 weeks) exhibited significant decrease in tumor volumes of the prostate as well as completely abolished distant organ metastasis. Apigenin treatment resulted in significant decrease in the weight of genitourinary apparatus (P < 0.0001), dorsolateral (P < 0.0001) and ventral prostate (P < 0.028), compared with the control group.
Apigenin-treated mice showed reduced phosphorylation of Akt (Ser473) and FoxO3a (Ser253), which correlated with its increased nuclear retention and decreased binding of FoxO3a with 14-3-3. These events lead to reduced proliferation as assessed by Ki-67 and cyclin D1, along with upregulation of FoxO-responsive proteins BIM and p27/Kip1.
Complementing in vivo results, similar observations were noted in human prostate cancer LNCaP and PC-3 cells after apigenin treatment. Furthermore, binding of FoxO3a with p27/Kip1 was markedly increased after 10 and 20 µM apigenin treatment resulting in G0/G1-phase cell cycle arrest, which was consistent with the effects elicited by PI3K/Akt inhibitor, LY294002.
These results provide convincing evidence that apigenin effectively suppressed prostate cancer progression, at least in part, by targeting the PI3K/Akt/FoxO-signaling pathway.
Shukla S, Bhaskaran N, Babcook MA, Fu P, Maclennan GT, Gupta S. Apigenin inhibits prostate cancer progression in TRAMP mice via targeting PI3K/Akt/FoxO pathway. Carcinogenesis. 2014 Feb;35(2):452-60. doi: 10.1093/carcin/bgt316.
6. Axl receptor tyrosine kinase is a novel target of apigenin for the inhibition of cell proliferation.
The Axl receptor tyrosine kinase (RTK), along with Tyro 3 and Mer, belongs to the TAM subfamily that promotes survival, stimulates proliferation and/or inhibits apoptosis. In various types of human cancer, including breast, lung and prostate cancer, Axl expression is increased and correlates with an advanced clinical stage.
In this study, we examined whether apigenin has an effect on Axl expression, which in turn can affect cell proliferation. The treatment of the non-small cell lung cancer (NSCLC) cells, A549 and H460, with apigenin decreased Axl mRNA and protein expression in a dose-dependent manner. Axl promoter activity was also inhibited by apigenin, indicating that apigenin suppressed Axl expression at the transcriptional level.
Upon treatment with apigenin, the viability of both the A549 and H460 cells was gradually decreased and the anti-proliferative effects were further confirmed by the dose-dependent decrease in the clonogenic ability of the apigenin-treated cells.
Subsequently, we found that the viability and clonogenic ability of the cells treated with apigenin was less or more affected by transfection of the cells with a Axl-expressing plasmid or Axl targeting siRNA, compared to transfection with the empty vector or control siRNA, respectively. In addition, apigenin increased the expression of p21, a cyclin-dependent kinase inhibitor, but reduced the expression of X-linked inhibitor of apoptosis protein (XIAP).
These cell cycle arrest and pro-apoptotic effects of apigenin were also attenuated or augmented by the up- or downregulation of Axl expression, respectively, which suggests that Axl is a novel target of apigenin through which it exerts its inhibitory effects on cell proliferation.
Taken together, our data indicate that apigenin downregulates Axl expression, which subsequently results in the inhibition of NSCLC cell proliferation through the increase and decrease of p21 and XIAP expression, respectively.
Kim KC, Choi EH, Lee C. Axl receptor tyrosine kinase is a novel target of apigenin for the inhibition of cell proliferation. Int J Mol Med. 2014 Aug;34(2):592-8. doi: 10.3892/ijmm.2014.1804.
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