Molecular target therapy for multidrug-resistant small cell lung cancer

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Small cell lung cancer(SCLC) is highly malignant because it easily acquires multidrug resistance. We have found that CD9 and HER2 are upregulated on the cell surface of SCLC cells when they become multidrug-resistant. CD9 induced adhesion of SCLC cells to extracellular matrices(ECM) resulting in chemoresistance by augmenting anti-apoptotic signals via integrins. Inhibition of CD9 with blocking antibody or small interfering RNA induced apoptosis of chemoresistant SCLC cells. On the other hand, HER2 inhibitor lapatinib recovered the sensitivity of chemoresistant SCLC cells by inhibiting the function of drug-efflux pumps expressed on them. This inhibitory mechanism is found to include not only direct pump inhibition but also indirect mechanism through HER2 inactivation-mediated Src activation and caveolin-1 phosphorylation. These findings indicate the possibility of CD9 and HER2 as therapeutic target to overcome multidrug-resistance of SCLC.