TM9SF1 drives the lipophagic flux via AMPK-ULK1 signaling to sustain metabolic fitness in HER2-positive breast cancer

Therapeutic resistance and recurrence in human epidermal growth factor receptor 2-positive breast Cancer (HER2 + BC) remain critical challenges that portend poor patient outcomes. Dysregulated Autophagy and lipid metabolism contribute to tumor progression, yet the crosstalk between these pathways is poorly understood. This study investigates the role of transmembrane 9 superfamily member 1 (TM9SF1) in lipophagy and lipid metabolic reprogramming in HER2 + BC under metabolic stress. Clinically, TM9SF1 was significantly upregulated in HER2 + BC tissues and correlated with poor prognosis. Functionally, its expression correlated with markers of enhanced Autophagy and lysosomal lipid catabolism, and it promoted tumor cell proliferation in vitro and in vivo. Conversely, TM9SF1 knockdown suppressed lipophagy under both basal and starvation conditions, inhibiting lipid droplet (LD) hydrolysis and the conversion of triglycerides to free fatty acids. This suppression was phenotypically characterized by LD accumulation, reduced autophagosomes and lipophagosomes, and altered enzymatic and lipidomic profiles. Mechanistically, TM9SF1 sustained lipophagy by promoting the phosphorylation of AMP-activated protein kinase at Thr172 and UNC-51-like kinase 1 at Ser555. Consequently, TM9SF1 was pivotal for lipid metabolic reprogramming, maintaining energy homeostasis and enhancing adaptation to nutrient deprivation through lipophagy. Overall, our findings identify TM9SF1 as a key HER2 + BC-associated regulator that drives lipophagy via the AMP-activated protein kinase-UNC-51-like kinase 1 pathway, facilitating LD turnover and free fatty acids utilization to sustain energy homeostasis in HER2 + BC. This work establishes a critical link between malignant phenotypes and metabolic resilience. Targeting this regulatory network represents a promising strategy to dismantle the metabolic scaffolds underlying HER2 + BC aggressiveness and therapeutic resistance.