PLK1 and colorectal carcinoma: However, in CRC cells Stattic showed very marginal or no effect on the PLK1 mRNA expression level, while it strongly inhibited the PLK1 phosphorylation (Fig. 4B; Supplementary Fig. S6), suggesting that STAT3 acts primarily as an upstream signal to regulate PLK1 activity rather than PLK1 expression in CRC cells.