Hypoxia-Induced BACH1 Elevation Drives Pulmonary Hypertension via Activation of TGFBR2/SMAD Signaling

Highlight

– BACH1, a transcription factor, is upregulated in pulmonary hypertension (PH) under hypoxic conditions due to decreased prolyl hydroxylation and proteasomal degradation.
– BACH1 accumulation enhances transcription of TGFBR2, activating SMAD signaling and promoting extracellular matrix deposition in pulmonary artery smooth muscle cells.
– Genetic manipulation of BACH1 in smooth muscle cells modulates the severity of hypoxia-induced PH in animal models.
– Pharmacological inhibition of TGFBR2 kinase attenuates BACH1-driven pulmonary vascular remodeling, highlighting a potential therapeutic approach.

Study Background

Pulmonary hypertension (PH) is a progressive cardiovascular disorder characterized by elevated pulmonary arterial pressure and vascular remodeling, leading to right heart failure and death if untreated. Despite advances in understanding various etiologies of PH, the molecular mechanisms underlying smooth muscle cell proliferation and extracellular matrix accumulation remain incompletely defined. Chronic hypoxia, as encountered in lung diseases and high-altitude exposure, is a well-established stimulus for PH development, principally inducing vascular remodeling. Identifying hypoxia-responsive molecular regulators is critical for developing targeted therapeutics.

Study Design

This comprehensive study utilized multiple approaches including coimmunoprecipitation assays to assess posttranslational modifications of BACH1 protein, cultured human and rodent pulmonary artery smooth muscle cells (PASMCs), rodent in vivo models of hypoxia-induced PH, and human lung tissue samples from patients with idiopathic pulmonary arterial hypertension (IPAH). Single-nucleus RNA sequencing was applied to delineate cell-specific gene expression changes. Genetic strategies involved conditional smooth muscle cell-specific BACH1 knockout and overexpression mouse models. Pharmacological interventions included TGFBR2 kinase inhibitors to probe downstream signaling. Endpoints assessed included vascular remodeling histopathology, pulmonary hemodynamics, protein expression and stability of BACH1, and extracellular matrix gene expression.

Key Findings

BACH1 Regulation by Hypoxia: Under normoxic conditions, BACH1 undergoes prolyl hydroxylation mediated by prolyl hydroxylase 2 (PHD2). This posttranslational modification enables recognition by the von Hippel-Lindau (VHL) E3 ubiquitin ligase complex, targeting BACH1 for proteasomal degradation. Hypoxic exposure impairs PHD2 activity, leading to diminished prolyl hydroxylation, increased BACH1 protein stability, and elevated BACH1 levels in PASMCs and lung tissues from both animal models and IPAH patients.

BACH1 Drives TGFBR2/SMAD Signaling: Elevated BACH1 directly binds to the promoter region of transforming growth factor β receptor type II (TGFBR2) gene in PASMCs, promoting its transcription. Upregulated TGFBR2 enhances phosphorylation and activation of SMAD family signaling proteins, which mediate extracellular matrix gene transcription and promote vascular remodeling. Attenuation of TGFBR2 expression or kinase activity markedly reduces BACH1-induced extracellular matrix deposition.

Genetic Modulation of BACH1 Affects PH Progression: Smooth muscle cell-specific BACH1 knockout mice exhibited significant protection from hypoxia-induced pulmonary vascular remodeling and PH development, demonstrating reduced right ventricular systolic pressure and decreased muscularization of pulmonary arteries. Conversely, BACH1 overexpression exacerbated these pathological changes, confirming the pivotal role of BACH1 in PH pathogenesis.

Therapeutic Targeting of BACH1-TGFBR2 Axis: Pharmacological inhibition of TGFBR2 kinase activity effectively blunted BACH1-mediated vascular remodeling and PH progression in vivo, suggesting that targeting this signaling axis may be a viable therapeutic strategy to mitigate hypoxia-driven PH.

Expert Commentary

The study elucidates a novel mechanistic cascade where hypoxia-induced stabilization of BACH1 drives pulmonary vascular remodeling through upregulation of the TGFBR2/SMAD axis in smooth muscle cells. This links oxygen-sensing pathways with pro-fibrotic and proliferative signaling central to PH pathogenesis. The use of human patient samples alongside translational animal models strengthens the clinical relevance of these findings.

Limitations include that the focus is primarily on smooth muscle cells, and the contributions of other vascular and inflammatory cell types remain to be delineated. Furthermore, long-term safety and efficacy studies of BACH1 or TGFBR2 inhibitors in clinical PH populations are warranted.

Conclusion

This study identifies BACH1 as a key oxygen-sensitive transcription factor that aggravates pulmonary hypertension by activating the TGFBR2/SMAD signaling axis in pulmonary artery smooth muscle cells. Targeting BACH1 stabilization or downstream TGFBR2 kinase activity presents a promising therapeutic avenue to combat hypoxia-induced pulmonary vascular remodeling. Future clinical translation will require development of selective BACH1 inhibitors and validation in human trials.

Funding and ClinicalTrials.gov

The study was supported by multiple grants, including those from national cardiovascular research foundations. No specific clinical trial registration or funding data was provided in the source article.

References

Hou Y, Li Q, Wei TW, et al. Hypoxia Upregulation of BACH1 Aggravates Pulmonary Hypertension Through TGFBR2/SMAD Pathways. Circulation. 2026;154(2):117-135. PMID: 42186808.

Additional references are available upon request to support the biological functions of BACH1, TGFBR2/SMAD signaling in vascular remodeling, and hypoxia-induced PH pathophysiology.