The Regulation Role of PBK in Cell Cycle and Apoptosis of Prostate Cancer

Results: After DU145/LNCaP transfection, the PBK expression levels were measured at 48 hours by RT-qPCR, compared to the blank control groups, the data showed the expression of PBK was significantly decreased in the groups of Si-PBK cell lines (DU145, the inhibition rate=84.78% P<0.01; LNCaP, the inhibition rate=94.79%, P<0.01); While the expression of PBK was significantly increased in the overexpression groups (DU145, fold=1.20, P<0.05; LNCaP, fold=1.35, P<0.05). The results of cell cycle analysis showed that after inhibition of the expression of PBK caused cell cycle arrest at the G0/G1 phase (LNCaP, P<0.05; DU145, P<0.01); On the contrary, a higher proportion of S, G2/M phase was shown in the over-expression-PBK DU145/LNCaP cell lines (LNCaP, P<0.05; DU145, P<0.01). Lastly, the cell apoptosis of DU145 and LNCaP cells was significantly increased after the inhibition of PBK expression (P<0.05) while an opposite result shown in the PBK overexpression cell lines (LNCaP, P<0.01; DU145, P<0.05).


Introduction
Prostate Cancer (PCa) is one of the most common cancers and has ranks first among cancers which endanger the health of men and have surpassed lung and colon cancers in the United States [1]. In recent years, due to the aging of the population, the westernization of eating habits, and the improvement of diagnosis and treatment, the incidence of prostate cancer has been increasing and ranked third in urological cancers in China [2]. The natural history and prognosis of PCa are diverse. Patients can be asymptomatic without any symptoms or high invasiveness. The Prostate Specific Antigen (PSA) is currently widely used as an indicator for PCa screening, but it is reported that about 70% of men with serum PSA>4 μg/L have no PCa. And screening at this PSA threshold inevitably leads to a higher false positive rate, resulting in unnecessary needle biopsy and puncture-related complications including hematuria, pain, infection, and sepsis [3].
Due to the poor specificity of PSA, a large amount of medical resources is wasted in the treatment of PCa. Therefore, searching for sensitive and highly specific molecular biological markers, as an indicator of prognosis of early PCa, has important scientific significance for individualized treatment of PCa and avoiding waste of medical resources. Our previous research used gene scanning technology to carry out gene detection in 5 cases of PCa patients in our hospital and compared with their paracancerous tissues. PBK was found to be highly expressed in PCa, and it is among the top 10 of differentially expressed genes [4].
PDZ binding-kinase/T-LAK cell-originated proteinkinase (PBK/ TOPK) is a new member of the Mitogen-Activated Protein Kinase (MAPK) family, composed of 322 amino acids, and is a newly discovered silk-threonine kinase [5]. In normal tissues, Gaudet et al. [6] reported that human PBK mRNA was detected in adult testis, placenta, myocardium and pancreas, with the most abundant placenta; and low levels of PBK expression in skeletal muscle, kidney, liver, and lung were detected [6]. It is up-regulated in many tumors and leads to poor prognosis of diseases such as lymphoma, leukemia, breast cancer, melanoma and colon cancer [7][8][9][10][11][12]. Due to the relevance between PBK and cell mitosis, we hypothesized that PBK promotes tumor cell proliferation and shortens cell cycle in prostate cancer. Meanwhile, the effect of PBK on cell cycle and apoptosis in prostate cancer has not been reported.

Construction of Pbk Overexpression and Control Cell Lines:
Divided into PBK high expression group (Hi-PBK), silencing group

Discussion
Prostate cancer is one of the common malignant tumors in men worldwide, which prognosis is highly variable. With the popularity of serum PSA screening, more and more prostate cancer patients can be diagnosed at an early stage and get good therapeutic effects after undergoing radical surgery with adjuvant medication. However, PSA screening lacks specificity for the diagnosis and prognosis. We still do not have an effective prognostic assessment of early prostate cancer patients, leading to a lack of targeted treatment: delay treatment time, or over-treatment which wastes a lot of medical resources. Thus, to explore the molecular mechanisms affecting the prognosis of prostate cancer, and find new molecular markers for prostate cancer progression, recurrence and metastasis, it is possible to assess the prognosis of patients with early prostate cancer, and even provide more effective individual treatment.
As an important mitotic kinase, PBK has been reported to have a close relationship with the cdk1/cyclin B1 complex and plays an important role in mitosis [8]. PBK expression and phosphorylation increase during cell mitosis [13]. In the mitosis phase, PBK is upregulated and directly binds to cdk1/cyclin B1 to form a complex.
Threonine 9 (T9) in PBK will be phosphorylated by cdk1/cyclin B1 complex. Then, PBK and cdk1 after phosphorylated by T9 / cyclin B1 combines more closely [14]. In fact, PBK phosphorylated accumulates on the mitotic spindle and affects spindle formation [15]. Studies using siRNA to knockdown PBK have shown that PBK can affect spindle formation [8]. When PBK is inhibited, the spindle (especially the central part) in mitosis becomes blurred, and the subsequent cytoplasmic division cannot be successfully completed. As a result, the cells will split out of the multinucleated cells. Therefore, PBK is involved in the regulation of proliferation and cycle changes of malignant tumor cells to promote the tumor cell transformation [5].

Conclusion
Finally, we first demonstrated that PBK transcription factors can shorten the tumor cell cycle and inhibit cell apoptosis in prostate cancer. PBK has been shown to play a role in promoting cancer in prostate cancer. In addition, study on the expression of PBK in the cell cycle reported that PBK increased in G2 to M phase, which promoted the expression of cell cycle-related genes to accelerate cell cycle progression, causing cell proliferation, repairing DNA damage and inhibiting apoptosis [8]. PBK may play a similar role in prostate cancer. In this study, we also found that inhibiting PBK expression, tumor cells will remine in G0/G1 phase. PBK has been shown to play an important role in the division of prostate cancer cells. This demonstrated that PBK played an important role in the division of prostate cancer cells. In conclusion, the detection of PBK may contribute to the early diagnosis and prognosis of prostate cancer.