Alternative isoforms of RNA polymerase III impact the non-coding RNA transcriptome, viability, proliferation and differentiation of prostate cancer cells

RNA polymerase III (pol III) synthesizes short noncoding RNA (ncRNA) exclusively and is unique in having alternative paralogues of one of its subunits, POLR3G and POLR3GL. Although most pol III target loci can be transcribed by either isoform, exceptions have been found. For example, depletion of POLR3G curtails the production of BC200 and snaR ncRNAs that are implicated in cancer progression. Furthermore, POLR3G may protect pol III against repression by MAF1, a key physiological regulator. Expression of POLR3G is promoted selectively by MYC, NANOG and OCT4A, master regulators of stem cell pluripotency, resulting in its preferential accumulation in undifferentiated cells. Indeed, differentiation of prostate cancer cells is suppressed by a positive feedback mechanism between POLR3G and NANOG, involving the control of NANOG mRNA degradation by ncRNAs. Specific knockdown of POLR3G inhibits proliferation and induces differentiation of prostate cancer cells, but this response is not seen following comparable depletion of its POLR3GL paralogue. ML-60218 is a cell-permeable small molecule pol III inhibitor that triggers the replacement of POLR3G with POLR3GL. Proliferation and viability of primary prostate cancer cells are suppressed by ML-60218, whereas differentiation is induced, effects that mimic POLR3G depletion. Transient exposure to ML-60218 reduced tumour initiating activity in a xenograft model. Untransformed prostate cells are much less sensitive to these treatments, raising the possibility of therapeutic benefit.