Cancer-associated adipocytes confer CDK4/6 inhibitor resistance in ER+ breast cancer through an IL-6/STAT3/SREBF2 axis coupled with cholesterol metabolism and cell cycle reprogramming.

Cancer-associated adipocytes (CAAs) within the tumor microenvironment (TME) critically regulate oncogenic progression. However, the mechanistic basis underlying CAAs-mediated CDK4/6 inhibitor (CDK4/6i) resistance in estrogen receptor-positive (ER+) breast cancer remains elusive. In this study, we revealed that CAAs supernatant demonstrated enhanced capacity to induce CDK4/6i resistance in ER+ breast cancer cells compared to NAs-derived conditioned medium. Through integrated RNA sequencing and cytokine microarray screening, we identified marked upregulation of IL-6 in both CAAs and their conditioned media. Mechanistically, CAAs-derived IL-6 activates the JAK-STAT3 axis, leading to transcriptional upregulation of SREBF2, which directly drives CDK4/6i resistance through HMGCR-mediated lipid metabolism and CDKN2C-mediated cell cycle progression (IL-6-STAT3-SREBF2-HMGCR/CDKN2C axis). Reciprocally, breast cancer cell-secreted exosomal miR-1246 promotes the transformation of normal adipocytes (NAs) to CAAs via PAX5-dependent regulation, and CAAs highly express UCHL1, which stabilizes KLF5 through K48-linked deubiquitination to activate NF-κB signaling, thereby augmenting IL-6 production (exosomal miR-1246-PAX5 and UCHL1-KLF5-NF-κB loop). Pharmacologic inhibition of HMGCR with simvastatin, alone or combined with IL-6 blockade, restored CDK4/6i sensitivity in vitro and in vivo, highlighting a clinically accessible strategy to overcome adipocyte-mediated resistance. Collectively, our findings establish that CAAs confer CDK4/6i resistance in ER+ breast cancer through the IL-6-driven SREBF2 activation axis, sustained by a reciprocal exosomal miR-1246/UCHL1-mediated feedback loop.