Regulation of PSAT1 and PHGDH by m6A in endocrine-resistant breast cancer cells.

The N-6-methyladenosine (m6A) modification of mRNA regulates transcript abundance in endocrine therapy (ET)-resistant breast cancer (BCa) cells. We reported that m6A reader HNRNPA2B1 decreased miR-145p and miR-424-5p targeting PSAT1 and miR-34b-5p and miR-876-5p targeting PHGDH, thus stimulating the serine synthesis pathway (SSP) in ET-resistant BCa cells. Here we examined m6A regulation of PSAT1 and PHGDH. We report that siMETTL3 increased miR-145-5p, reducing PSAT1, and miR-34b-5p and miR-876-5p, reducing PHGDH, without affecting HNRNPA2B1 or NFkB and decreasing MYC, known to stimulate PSAT1 and PHGDH transcription. In contrast, the METTL3 inhibitor STM2457 increased METTL3, MYC, HNRNPA2B1, NFkB, PSAT1, PHGDH, and serine synthesis, and decreased the miRNAs. These data suggest that reducing METTL3 protein and inhibition of its catalytic activity have different effects on these targets. Selected results were verified in ET-resistant T47D and ZR-75-1 BCa cells. METTL3's stimulation of translation may play a role in these differences. Indeed, siMETTL3 had no effect on MYC, PHGDH, or PSAT1 pre-mRNA whereas STM2457 increased these pre-mRNAs. Overall, our data support a model for m6A regulation of PHGDH and PSAT1 indirectly through miRNAs that target PHGDH and PSAT1.