Metformin lowers HMGA1a and RBM3 protein in breast cancer cells, and its effect on ESR1 splicing depends on cell density and exposure time

Metformin does not show a consistent, dose-dependent shift in full-length ESR1 transcript share at high seeding density in MCF-7 or TAMR1 cells, with values spanning 42–48% and 43–68%.

• Why it matters: The work addresses whether metformin’s reported ESR1 splicing effects are robust across culture conditions and identifies which splicing factor might mediate them.
• What they did: Cells were treated with 0.25–5 mM metformin at high or low seeding density, and ESR1 transcript isoforms were quantified by gel densitometry; HMGA1a and RBM3 proteins were measured by immunoblotting.
• The result: At high density, the full-length share was not reproducible across doses, while HMGA1a decreased in MCF-7 (and tended to decrease in TAMR1) and RBM3 decreased in TAMR1 at 24–48 h, implying HMGA1a is unlikely to carry the effect.

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