It has been demonstrated that SNCG is involved in tumorigenesis and metastasis of a wide range of malignancies
[18–23]. SNCG-positive breast cancers have worse clinical outcome compared with SNCG-negative breast cancers. Only one report has documented a correlation between SNCG and prostate cancer metastasis
. The precise biological functions of SNCG and the mechanisms of its gene regulation in prostate cancer are still unknown. Metastasis is one of the hallmarks of advanced prostate cancer, and contributes to the high rates of morbidity and mortality in patients. In our studies, we found that SNCG expression in human prostate cancer cells results in a more malignant phenotype with increased cellular proliferation and motility in vitro. SNCG interacts with AR and enhances PSA expression mediated by androgen-induced transcriptional activity of AR. We also demonstrated that SNCG regulates androgen-dependent tumor size in vivo, and evaluated the clinical value of detection of SNCG protein expression in diagnosis of androgen-dependent prostate cancer.
To clarify the functional roles of SNCG in prostate cancer cells, we knocked down SNCG expression by siRNA in LNCaP cells and investigated its effects on cellular biological behaviors. Our data showed that silencing of SNCG in LNCaP cells contributes to suppression of cellular growth and proliferation, induction of cell cycle arrest at G1 phase and inhibition of cellular migration and invasion in vitro. These results are consistent with observations in many other human cancer cells and indicate that the functions of SNCG are not cell type-specific. The signaling pathways controlling SNCG gene regulation are still unknown. Some studies in other cancers reported SNCG is implicated in regulation of key steps of cellular proliferation, invasion and metastasis as well as survival. It may be activated through several cellular mechanisms, including reducing BubR1 protein levels
[11, 26, 27], increasing ER-α transcription
[28, 29], activating RHO GTPase
, MAPK and ElK1
, inducing MMP expression
, and constitutive activation of ERK1/2
It was reported that SNCG expression was strongly correlated with the stages and Her-2 status
[22, 33]; however, this was not associated with ER and PR expression status in breast cancer studies
. We found SNCG expression is dependent on androgen status in human prostate cancer cells. Anti-androgen treatment mostly blocked DHT-induced SNCG expression, indicating that DHT modulates SNCG expression through AR signaling. This may account for our observations that SNCG expression was at an undetectable level in AIPC tissues and overexpression of SNCG did not affect tumorigenesis in the castrated male mice. To our knowledge, this is the first report that SNCG expression is dependent on androgen and plays an important role in prostate cancer progression. Consequently, SNCG may be closely associated with hormone-related tumors, and provide a new strategy for these tumors.
AR is a ligand-dependent transcription factor and a member of the class I subgroup of the nuclear receptor superfamily
. The androgen/AR signaling pathway is demonstrated to play a central role in prostate cancer development and progression. AR is activated by a ligand-dependent or a ligand-independent manner
[36–38]. Subsequently, the activated receptor homodimerizes with AREs in the promoters of androgen target genes, resulting in activation of downstream gene expression
. Previous studies have demonstrated that AR signaling could be modulated by AR cofactors, such as heat-shock protein 27
, peroxiredoxin 1
, peroxisome proliferator-activated receptor coactivator-1 (PGC-1)
 and human heterochromatin protein 1isoform HP1β
. SNCG has been shown to interact with ER and enhance ER transcriptional activity
[29, 30]. To explore whether SNCG is involved in mediation of AR signaling, we investigated the interaction between SNCG and AR proteins in LNCaP cells. Our results revealed that SNCG interacts with AR and its interaction is strengthened by DHT treatment. Although silencing of SNCG had no significant effect on AR expression in LNCaP cells, its suppression influenced PSA expression and AR transcriptional activity. We suggest that SNCG is a novel co-activator of AR and may play an important role in the molecular interaction with AR signaling in prostate cancer cells. The mechanisms need further exploration, including how they interact with each other, what downstream factor they promote or suppress, and so on.
Furthermore, we showed that silencing of SNCG by siRNA in LNCaP cells reduced tumor growth when the cells were injected into nude mice. These in vivo studies were consistent with the previously investigated functions of SNCG in prostate cancer cells in vitro. Our results indicate that aberrantly high expression of SNCG is partly responsible for tumor growth and invasion. Since SNCG expression of prostate cancer cells was regulated by androgen in vitro, we investigated stable SNCG-overexpressing LNCaP tumor growth in the castrated host mice. However, there was no significant difference between two groups with different expression levels of SNCG, indicating that SNCG regulates androgen-dependent prostate tumorigenesis. When prostate cancer patients are diagnosed at an advanced stage of the disease, androgen-deprivation therapy (ADT) has become the standard therapy. While the controversial topic remains, doctors believe that declining serum levels of testosterone and aging represent the most significant risk factors for prostate cancer progression
[47–49]. A previous study claimed that exposure to reduced androgens may promote prostate tumorigenesis by activating special molecular events that drive more aggressive hormone-refractory tumors
. However, our data suggest that ADT therapy regimen in the treatment of advanced prostate cancer patients might effectively reduce some androgen-induced risk factors such as SNCG.
Abate-Shen showed that prostate tumors from low-testosterone mutant mice shared a similar gene expression profile to androgen-independent prostate tumors
. They suggested that declining serum levels of testosterone associated with aging is the main aggressive factor for prostate cancer. We raised the question whether it is necessary to perform androgen-deprivation (ADT) therapy on aged patients if they have high expression levels of SNCG protein. To address this issue, we overexpressed SNCG in androgen-independent LNCaP cells (LNCaP-AI). We found SNCG-overexpressing LNCaP-AI cells enhanced AR transcriptional activity and promoted PSA expression and cellular proliferation in response to DHT treatment. This suggested that SNCG may be a malignant risk factor in older men with prostate cancer.
Our results from a tissue microarray with immunohistochemical staining indicated SNCG protein is highly expressed in androgen-dependent (AD) prostate tumors, but is rarely expressed in benign tissues. In 122 radical prostatectomy specimens, 63.9% of SNCG-positive patients developed peripheral invasion. Only 20% of SNCG-high-positive (++ or +++) patients have local cancer. A total of 84.6% of SNCG-high positive (++ or +++) with lymph node metastasis indicate that SNCG expression is strongly associated with prostate cancer metastasis. The proportion of SNCG-positive metastasis is nearly consistent with previous reports
. Although SNCG protein expression was not associated with AR status, the high risk in malignant progression may help us establish efficient treatment strategies and reduce inappropriate or unnecessary treatments. SNCG expression was reported in 20% of preneoplastic lesions in the ovary
, and was observed in various cancer types with lymph node invasion
. We found SNCG was not expressed in benign epithelium cells, but aberrantly expressed in advanced malignant states, suggesting that SNCG may be a tumor-oriented chaperone. The cases with high SNCG protein expression were found to be strongly associated with metastatic features. We therefore suggest SNCG protein expression in biopsy specimens may assist in distinguishing between potentially aggressive and potentially non-aggressive disease in prostate cancer screening. A large number of specimens need to be further explored to determine if SNCG protein expression is a predictive biomarker for evaluation of pre-surgery and survival of prostate cancer patients.