Kinesin-like protein KIFC2 stabilizes CDK4 to accelerate growth and confer resistance in HR+/HER2- breast cancer.

Hormone receptor-positive and human epidermal growth factor receptor 2-negative breast cancer (HR+/HER2- BC) is the most common subtype, with a high risk of long-term recurrence and metastasis. Endocrine therapy (ET) combined with cyclin-dependent kinase 4/6 (CDK4/6) inhibitors is a standard treatment for advanced/metastatic HR+/HER2- BC, but resistance remains a major clinical challenge. We report that kinesin family member C2 (KIFC2) was amplified in approximately 50% of patients with HR+/HER2- BC, and its high expression was associated with poor disease outcome, increased tumor protein p53 (TP53) somatic mutation, and active pyrimidine metabolism. Functional assays revealed that depletion of KIFC2 suppressed growth and enhanced sensitivity of HR+/HER2- BC cells to tamoxifen and CDK4/6 inhibitors. Mechanistically, KIFC2 stabilized CDK4 by enhancing its interaction with ubiquitin-specific peptidase 9 X-linked (USP9X). Importantly, reexpression of CDK4 in KIFC2-depleted cells partially rescued the decreased growth and increased sensitivity to tamoxifen and CDK4/6 inhibitors caused by KIFC2 depletion. Clinically, high KIFC2 mRNA expression was negatively associated with the survival rate of patients with HR+/HER2- BC who received adjuvant ET alone or in combination with CDK4/6 inhibitors. Collectively, these findings identify an important role for KIFC2 in HR+/HER2- BC growth and therapeutic resistance, and support its potential as a therapeutic target and predictive biomarker.