BRCA1 is a multifunctional tumor suppressor that coordinates homologous recombination repair, chromatin remodeling, cell cycle control and apoptosis. In triple‑negative breast cancer (TNBC), pathogenic BRCA1 mutations are relatively uncommon; however, decreased BRCA1 protein expression driven by accelerated proteasomal degradation has been implicated in aggressive disease and altered responses to DNA‑damaging therapies. Because clinically available methods to restore BRCA1 protein stability are lacking, the investigators performed an in silico screen of FDA‑approved drugs to identify compounds that might modulate BRCA1 protein homeostasis in TNBC.
The study evaluated pemetrexed (Peme), identified through the in silico screen, using a suite of biochemical, cell biological and in vivo approaches. BRCA1 protein abundance, ubiquitination and turnover were assessed by immunoblotting, ubiquitin pull‑down assays and cycloheximide chase analyses. BRCA1 deletion mutants were used to probe domain contributions to the observed effects. Functional consequences were measured with apoptosis assays and BRCA1‑dependent signaling readouts. Radiosensitizing activity of pemetrexed was tested in vitro and validated in TNBC xenograft models.
Treatment with pemetrexed increased BRCA1 protein levels while BRCA1 mRNA levels remained unchanged, indicating that the effect is post‑transcriptional. Biochemical assays demonstrated suppression of ubiquitin‑mediated proteasomal degradation and reduced BRCA1 turnover, consistent with protein stabilization rather than transcriptional upregulation.
Mechanistic interrogation included in silico docking studies and experiments with BRCA1 deletion mutants. These analyses suggested that the BRCA1 C‑terminal domain may contribute to the pemetrexed‑mediated stabilization effect. Collectively, the data are consistent with a model in which pemetrexed interferes with ubiquitination or proteasomal targeting of BRCA1, thereby increasing steady‑state protein levels.
Stabilized BRCA1 enhanced apoptotic responses to treatment in TNBC cell models. Conversely, BRCA1 knockdown attenuated these apoptotic effects, linking the pemetrexed‑induced increase in BRCA1 protein to downstream functional consequences. BRCA1‑dependent signaling readouts used by the authors supported the conclusion that restored protein stability translated into measurable changes in cell fate after DNA damage.
Combined treatment with pemetrexed and ionizing radiation significantly potentiated cytotoxicity in vitro compared with radiation alone. The radiosensitizing effect observed in cell‑based assays was validated in TNBC xenograft experiments, where combined pemetrexed‑radiation treatment inhibited tumor growth in vivo. The authors attribute these effects to pemetrexed‑mediated BRCA1 stabilization, which altered responses to DNA damage and increased treatment‑induced apoptosis.
The study identifies pemetrexed as a BRCA1‑stabilizing agent that enhances the therapeutic response to radiation in TNBC models. Pharmacological restoration of BRCA1 stability is proposed as a potential strategy to overcome therapeutic resistance in tumors with BRCA1 deficiency or functional compromise. Because pemetrexed is an FDA‑approved multitargeted antifolate, repurposing it to modulate BRCA1 protein stability may offer a translational route to clinical evaluation.
The abstract summarizes experimental approaches and key findings but does not report detailed numerical results, dosing regimens, treatment schedules, statistical analyses or safety/tolerability data. Those specifics were not available in the source text provided and therefore are not recapitulated here. Further assessment of translational potential would require full experimental details, dose‑response relationships, and evaluation of therapeutic window and toxicity in clinical contexts.
Pemetrexed increases BRCA1 protein stability via post‑translational suppression of ubiquitin‑mediated proteasomal degradation, likely involving the BRCA1 C‑terminal domain. This stabilization enhances apoptosis and potentiates the cytotoxic effects of radiation in TNBC cell lines and xenografts, supporting the concept that pharmacologic restoration of BRCA1 stability could improve responses to DNA‑damaging therapies in BRCA1‑deficient or functionally compromised breast cancer.
Keywords: BRCA1, pemetrexed, protein stability, radiosensitization, triple‑negative breast cancer.