Obesity‐induced MBD 2_v2 expression promotes tumor‐initiating triple‐negative breast cancer stem cells

Obesity is a risk factor for triple‐negative breast cancer ( TNBC ) incidence and poor outcomes, but the underlying molecular biology remains unknown. We previously identified in TNBC cell cultures that expression of epigenetic reader methyl‐CpG‐binding domain protein 2 ( MBD 2), specifically the alternative mRNA splicing variant MBD variant 2 ( MBD 2_v2), is dependent on reactive oxygen species ( ROS ) and is crucial for maintenance and expansion of cancer stem cell‐like cells ( CSC s). Because obesity is coupled with inflammation and ROS , we hypothesized that obesity can fuel an increase in MBD 2_v2 expression to promote the tumor‐initiating CSC phenotype in TNBC cells in vivo . Analysis of TNBC patient datasets revealed associations between high tumor MBD 2_v2 expression and high relapse rates and high body mass index ( BMI ). Stable gene knockdown/overexpression methods were applied to TNBC cell lines to elucidate that MBD 2_v2 expression is governed by ROS ‐dependent expression of serine‐ and arginine‐rich splicing factor 2 ( SRSF 2). We employed a diet‐induced obesity ( DIO ) mouse model that mimics human obesity to investigate whether obesity causes increased MBD 2_v2 expression and increased tumor initiation capacity in inoculated TNBC cell lines. MBD 2_v2 and SRSF 2 levels were increased in TNBC cell line‐derived tumors that formed more frequently in DIO mice relative to tumors in lean control mice. Stable MBD 2_v2 overexpression increased the CSC fraction in culture and increased TNBC cell line tumor initiation capacity in vivo . SRSF 2 knockdown resulted in decreased MBD 2_v2 expression, decreased CSC s in TNBC cell cultures, and hindered tumor formation in vivo . This report describes evidence to support the conclusion that MBD 2_v2 expression is induced by obesity and drives TNBC cell tumorigenicity, and thus provides molecular insights into support of the epidemiological evidence that obesity is a risk factor for TNBC . The majority of TNBC patients are obese and rising obesity rates threaten to further increase the burden of obesity‐linked cancers, which reinforces the relevance of this report.