Transforming growth factor-beta signaling: emerging stem cell target in metastatic breast cancer?

Area of Science:

• Oncology
• Molecular Biology
• Immunology

Background:

• Transforming growth factor-beta (TGFbeta) signaling plays a dual role in breast cancer, suppressing initial tumor formation but promoting metastasis in established cancers.
• In many human breast cancers, reduced TGFbeta receptor/Smad expression and elevated TGFbeta levels in the tumor microenvironment promote an invasive phenotype.
• TGFbeta signaling is implicated in maintaining the cancer stem cell pool and driving mesenchymal phenotypes in basal-like and HER2-positive breast cancers.

Purpose of the Study:

• To investigate the therapeutic potential of inhibiting TGFbeta signaling in breast cancer, particularly in basal-like and HER2-positive subtypes.
• To elucidate the mechanisms by which TGFbeta antagonists affect tumor progression, metastasis, and host anti-tumor immunity.
• To explore the differential effects of TGFbeta signaling in estrogen-dependent versus estrogen-independent breast cancers.

Main Methods:

• Utilized genetic mouse models of breast cancer, including those for triple-negative/basal-like subtypes.
• Administered TGFbeta-neutralizing antibodies and receptor kinase inhibitors to mouse models.
• Analyzed effects on tumor cell proliferation, apoptosis, metastasis (lung and bone), angiogenesis, anti-tumor immunity, and cellular phenotype (epithelioid vs. mesenchymal).
• Examined TGFbeta target gene expression patterns in human breast cancers and investigated the role of estrogen receptor status.

Main Results:

• In mouse models, TGFbeta antagonists significantly inhibited lung and bone metastasis in basal-like breast cancer without affecting tumor cell proliferation or apoptosis.
• TGFbeta inhibition led to de-repression of anti-tumor immunity, reduced angiogenesis, and reversed the mesenchymal, invasive phenotype of cancer cells.
• TGFbeta signaling appears to drive progression in estrogen-independent cancers and maintain the stem cell pool, while its role is suppressive in estrogen-dependent luminal cancers.
• Estrogen receptor-positive cells are unresponsive to TGFbeta due to silenced TGFBR2, and estrogen/anti-estrogen treatments modulate TGFbeta signaling.

Conclusions:

• Inhibition of TGFbeta signaling represents a promising anti-metastatic strategy for basal-like and HER2-positive breast cancers, potentially by promoting differentiation and reversing mesenchymal phenotypes.
• TGFbeta antagonists may enhance anti-tumor immunity and reduce angiogenesis, contributing to their anti-metastatic effects.
• The differential roles of TGFbeta in various breast cancer subtypes suggest that TGFbeta antagonists could antagonize anti-estrogen therapies in luminal cancers, necessitating careful patient selection for clinical trials.