Caveolin-1-dependent and -independent uPAR signaling pathways contribute to ganglioside GT1b induced early apoptosis in A549 lung cancer cells.

Urokinase receptor interacts with α5β1-integrin and enhances cancer cell proliferation and metastasis. Activation of α5β1-integrin requires caveolin-1 and is regulated by uPAR, which upregulates persistently the activated ERK necessary for tumor growth. In this study, we show that the ganglioside GT1b induces proapoptotic signaling through two uPAR-ERK signaling pathways in A549 lung cancer cells. GT1b downregulated the expression of α5β1 integrin, caveolin-1, fibronectin, FAK, and ERK, whereas GT1b upregulated the expression of p53 and uPAR, suggesting GT1b mediated depletion of caveolin-1 in uPAR-expressing A549 cells also disrupts uPAR/integrin complexes, resulting in downregulation of fibronectin-α5β1-integrin-ERK signaling. Following p53 siRNA treatment, FAK and ERK expression was recovered, meaning the presence of reentry uPAR-FAK-ERK signaling pathway. These findings reveal that GT1b is involved in both caveolin-1-dependent uPAR-α5β1-integrin-ERK signaling and caveolin-1-independent uPAR-FAK-ERK signaling. These results suggest a novel function of GT1b as a dual regulator of ERK by modulating caveolin-1 and p53.