Tamoxifen Suppresses Clusterin Level through Akt Inactivation and Proteasome Degradation in Human Prostate Cancer Cells

The crosstalk between ubiquitination and endocrine therapy

Endocrine therapy (ET), also known as hormone therapy, refers to the treatment of tumors by regulating and changing the endocrine environment and hormone levels. Its related mechanism is mainly through reducing hormone levels and blocking the binding of hormones to corresponding receptors, thus blocking the signal transduction pathway to stimulate tumor growth. However, with the application of ET, some patients show resistance to ET, which is attributed to abnormal accumulation of hormone receptors (HRs) and the production of multiple mutants of HRs. The targeted degradation of abnormal accumulation protein mediated by ubiquitination is an important approach that regulates the protein level and function of intracellular proteins in eukaryotes. Here, we provide a brief description of the traditional and novel drugs available for ET in this review. Then, we introduce the link between ubiquitination and ET. In the end, we elaborate the clinical application of ET combined with ubiquitination-related molecules. KEY MESSAGES: • A brief description of the traditional and novel drugs available for endocrine therapy (ET). • The link between ubiquitination and ET. • The clinical application of ET combined with ubiquitination-related molecules.

Tamoxifen inhibits the proliferation of non‑melanoma skin cancer cells by increasing intracellular calcium concentration

Tamoxifen is an estrogen receptor (ER) antagonist used as first-line chemotherapy in breast cancer. Recent studies suggest that tamoxifen may be effective not only for ER‑positive but also for ER‑negative cancer cases. The aim of the present study was to investigate the antiproliferative effect of tamoxifen against human non‑melanoma skin cancer cells. Tamoxifen inhibited the proliferation of the skin squamous cell carcinoma (SCC) cell lines A431, DJM‑1 and HSC‑1. A431 cells did not express ER‑α or -β, suggesting that tamoxifen may exert antiproliferative effects on skin SCC cells via a non‑ER‑mediated pathway. Tamoxifen increased the intracellular calcium concentration of skin SCC cells, and this increase in intracellular calcium concentration by calcium ionophore A23187 suppressed the proliferation of skin SCC cells. These data indicate that tamoxifen inhibited the proliferation of human skin SCC cells via increasing intracellular calcium concentration. Voltage-gated calcium channels and non‑selective cation channels are involved in the increase in intracellular calcium concentration induced by tamoxifen. The broad-spectrum protein kinase C (PKC) inhibitor phloretin significantly attenuated the antiproliferative effect of tamoxifen on skin SCC cells. From these data, it may be concluded that tamoxifen inhibits the proliferation of skin SCC cells by induction of extracellular calcium influx via calcium channels in the plasma membrane and by subsequent activation of PKC.

Knockdown of clusterin expression increases the in vitro sensitivity of human prostate cancer cells to paclitaxel

Clusterin/apolipoprotein J is a secreted heterodimeric glycoprotein that is implicated in several pathophysiological processes, including tissue remodeling, reproduction, lipid transport, and apoptosis. Although previous studies demonstrated that clusterin is able to protect against apoptosis, the role of the clusterin in cellular proliferation remains elusive. To determine whether clusterin plays an important role in cellular proliferation, the function of clusterin was examined using a small interfering RNA (siRNA) in PC3 human prostate cancer cells. Transient transfection with clusterin siRNA resulted in significant suppression of clusterin mRNA and protein expression. Clusterin knockdown resulted in a decrease in protein expression of phospho-Akt and an increase in expression of proteins phosphatase type 2AC (PP2AC) and phosphorylation of p38. However, treatment with PP2AC siRNA exerted minimal effects on clusterin expression. Interestingly, clusterin mRNA expression was reduced in paclitaxel-treated cells, and the cytotoxic effect of paclitaxel was more potent when cells were incubated with clusterin siRNA. In addition, co-treatment with paclitaxel and clusterin siRNA significantly enhanced PP2AC levels. Taken together, these results indicate that clusterin plays a crucial role in PC3 cell proliferation and that clusterin depletion may contribute to enhanced sensitivity of PC3 cells to anticancer agents such as paclitaxel.

Cross-regulation between protein L-isoaspartyl O-methyltransferase and ERK in epithelial mesenchymal transition of MDA-MB-231 cells

AIM

Protein L-isoaspartyl O-methyltransferase (PIMT) regulates cell adhesion in various cancer cell lines through activation of integrin αv and the PI3K pathway. The epithelial mesenchymal transition (EMT) enables epithelial cells to acquire the characteristics of mesenchymal cells, and to allow them to migrate for metastasis. Here, we examined the relationship between PIMT and EMT with attached or detached MDA-MB 231 cells.

METHODS

Human breast cancer cell line MDA-MB-231 cells were maintained in a suspension on poly-HEMA in the presence or absence of PIMT siRNA or ERK inhibitor PD98059. The mRNAs and proteins were analyzed using RT-PCR and immunoblotting, respectively.

RESULTS

During cellular incubation under detached conditions, PIMT, integrin αv and EMT proteins, such as Snail, Slug and matrix metalloproteinase 2 (MMP-2), were significantly increased in correlation with the phosphorylation of ERK1/2. The ERK inhibitor PD98059 (25 μmol/L) strongly suppressed the expression of the proteins and PIMT. Interestingly, PIMT siRNA blocked the phosphorylation of ERK and the expression of the EMT proteins. Additionally, PIMT and ERK phosphorylation were both co-activated by treatment with TGF-β (10 ng/mL) and TNF-α (10 ng/mL).

CONCLUSION

A tight cross-regulation exists between ERK and PIMT in regards to their activation and expression during the EMT.