The function of DNA primase is to synthesize the small RNA primers for Okazaki fragments generated during discontinuous DNA replication. This primase is required because DNA polymerases cannot initiate RNA polymer synthesis and they can elongate polymers only using RNA polymers that have already been synthesized. Eukaryotic cells contain PRIM1 and PRIM2 subunits of nuclear DNA primase. PRIM1 is the smallest subunit of the heterotetrameric eukaryotic Pol alpha/primase complex. It alone carries the catalytic function of the enzyme and can elongate primers, whereas PRIM2 does not exert such enzymatic activity.
The methods in this study include: real-time quantitative PCR, cell proliferation and viability assays, DNA synthesis analysis, western blotting, wound-healing cell migration and invasion assay, animal experiments, immunohistochemistry, and chromatin immunoprecipitation assay.
Our results show that PRIM1 mRNA is highly expressed in human breast tumor tissues, particular in poorly differentiated tumor tissues. In the cell model, higher PRIM1 protein expression was detected in ER+ breast cancer cell lines (BT483, BT474, and MCF-7) than in normal breast cell lines (MCF-10A). Stable expression of PRIM1-siRNA in the ER+ BT-474 cells-xenograft tumors significantly reduced tumor volume in SCID mice (*p = 0.005). The anti-tumoral effects of inotilone isolated from Phellinus linteus was tested and had significant effects on the inhibition of PRIM1 protein expression in ER+ breast cancer cells. In vivo study was performed by administering inotilone (10 mg/kg, twice a week for 6 weeks), which resulted in significantly reduced BT-474-xenografted tumor growth volume compared with control (n = 5 per group, *p < 0.05).
This study provides evidences for the prognostic effects of PRIM1 with poor overall survival rate in the ER+ patients and will be valuable to test for therapeutic purpose.
Li-Ching Chen.
Has not received any funding.
All authors have declared no conflicts of interest.