Triple-negative breast cancer (TNBC), an aggressive breast cancer subtype characterised by the absence of oestrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression, remains highly dependent on platinum-based chemotherapy as a cornerstone of clinical management.

Despite the initial efficacy of such regimens, the emergence of platinum resistance frequently undermines treatment success, leading to poor disease control and significantly worsening patient survival outcomes.

A new study conducted by researchers at Sun Yat-sen University Sun Yat-sen Memorial Hospital, and published in SCIENCE CHINA Life Sciences, has now identified a critical molecular driver underlying this clinically challenging resistance.

To investigate the mechanisms of platinum resistance, the research team first established cisplatin-resistant TNBC cell line models (231-cisR and 468-cisR) by long-term exposure of parental sensitive cells to gradually increasing concentrations of cisplatin.

Comparative analysis between these resistant lines and their respective sensitive parental counterparts (231-pa and 468-pa) revealed that a circular RNA (circRNA) named circSCAP was significantly upregulated in platinum-resistant cells and tumour samples.

Further validation in clinical specimens confirmed that circSCAP expression was markedly elevated in platinum-resistant TNBC tissues relative to sensitive ones.

Subsequent functional characterisation demonstrated that circSCAP possesses inherent protein-coding potential: it contains a functional internal ribosome entry site (IRES) and a conserved open reading frame (ORF), which together enable the translation of circSCAP into a 129-amino-acid peptide, designated SCAP-129aa.

Immunoblotting and immunohistochemical analyses further confirmed that SCAP-129aa expression was consistently upregulated in platinum-resistant TNBC cell lines and patient-derived tumour tissues, mirroring the expression pattern of its parental circRNA.

A series of gain- and loss-of-function experiments validated that SCAP-129aa directly mediates platinum resistance.

In platinum-resistant 231-cisR and 468-cisR cells, targeted silencing of circSCAP (via shRNAs targeting its unique back-splice junction) significantly reduced SCAP-129aa expression and restored sensitivity to cisplatin, accompanied by a significant increase in cisplatin-induced apoptosis and DNA damage.

Conversely, overexpression of wild-type circSCAP (but not a translation-deficient mutant lacking the ATG start codon) in sensitive parental cells led to elevated SCAP-129aa levels and induced platinum resistance.

To elucidate the molecular mechanism by which SCAP-129aa drives resistance, co-immunoprecipitation (co-IP) coupled with mass spectrometry was performed to identify interacting proteins.

This approach revealed that SCAP-129aa directly binds to PIK3R2 (also known as P85β), a regulatory subunit of the PI3K/AKT signalling pathway.

Mapping of the interaction domain showed that SCAP-129aa specifically targets the SH2C domain of PIK3R2—a region critical for PIK3R2’s ubiquitination and subsequent proteasomal degradation.

Further experiments confirmed that SCAP-129aa binding inhibits PIK3R2 ubiquitination, thereby stabilising PIK3R2 protein levels.

Stabilised PIK3R2 in turn activates the PI3K/AKT pathway, enhancing cancer cells’ ability to withstand cisplatin-induced cellular stress and repair cisplatin-mediated DNA damage.

In vivo validation using orthotopic TNBC xenograft models in immunodeficient (NOD/SCID) mice further supported these findings.

Silencing circSCAP in platinum-resistant 231-cisR tumour xenografts significantly re-sensitised tumours to cisplatin treatment, resulting in reduced tumour growth rates.

Additionally, co-administration of cisplatin with a PIK3R2-specific inhibitor markedly enhanced antitumour efficacy in platinum-resistant TNBC xenografts, whereas no additional benefit was observed in xenografts derived from platinum-sensitive parental cells—confirming the specificity of the PIK3R2 pathway in mediating platinum resistance.

Clinical relevance was further established by analysing 73 TNBC patient tissue samples.

Immunohistochemical staining showed that high SCAP-129aa expression was significantly associated with worse overall survival (OS) in TNBC patients (hazard ratio = 5.912, log-rank P = 0.0004).

Moreover, elevated SCAP-129aa levels correlated with aggressive clinicopathological features, including increased lymph node and distant metastasis, advanced American Joint Committee on Cancer (AJCC) staging, higher Ki67 proliferation index, and a significantly higher rate of platinum resistance.

“Platinum resistance is a big barrier for TNBC,” said Qiang Liu.

“Our findings reveal a new circRNA-encoded protein-driven mechanism, offering targets to improve patient outcomes.”

Article: circSCAP-encoded SCAP-129aa mediates platinum resistance in triple-negative breast cancer via the PI3K/AKT pathway

Source: Science China Press