HGA-Induced Oxidative Stress Impairs Autophagy via Lysosomal Dysfunction in Alkaptonuria

Alkaptonuria (AKU) is a rare metabolic disorder caused by homogentisate 1,2-dioxygenase deficiency, leading to systemic accumulation of homogentisic acid (HGA) and progressive tissue degeneration characterized by dark urine, ochronosis, and severe osteoarthropathy. Chronic exposure to HGA promotes oxidative stress, chondroptosis, secondary amyloidosis, and impaired autophagy, an essential process for maintaining chondrocyte homeostasis. This study investigated the mechanisms potentially involved in autophagy dysregulation in AKU using the human C20/A4 chondrocyte line treated with 0.1 mM HGA, an established in vitro model of the disease. The findings were then verified using chondrocyte cells and cartilage tissue obtained from AKU biopsies. HGA treatment induced a time-dependent increase in oxidative stress, evidenced by elevated ROS levels, 4-HNE accumulation, and overproduction of mitochondrial superoxide. Autophagy assessment showed an early increase in autophagy-related markers, with increased LC3 and p62 expression and enhanced lysosomal biogenesis (LAMP1). However, prolonged HGA exposure was associated with reduced LC3/LAMP1 colocalization, persistent p62 accumulation, altered acidic compartment staining, and accumulation of autophagy-related structures, supporting a dysregulation of the autophagy-lysosomal pathway. Live-cell imaging further supported a transition from functional autophagy to lysosomal failure under chronic oxidative stress. Overall, this study suggests that prolonged HGA exposure disrupts the interplay between oxidative stress and autophagic flux. The progressive collapse of these adaptive mechanisms may contribute to chondrocyte degeneration and to the pathogenesis of cartilage damage in AKU.