Synthetic lethal interaction of combined CD26 and Bcl-xL inhibition is a powerful anticancer therapy against hepatocellular carcinoma

AimCD26 is a membrane glycoprotein that has multiple functions, including dipeptidyl peptidase IV activity. CD26 expression varies in different tumor types, and its role in tumor growth in hepatocellular carcinoma (HCC) remains unclear. MethodsCD26 expression levels were examined in resected HCC and surrounding non-cancerous lesions. The effect of CD26 knockdown on the cellular proliferation of HepG2 or Huh7 cells, both of which highly express CD26, was studied in vitro. ResultsCD26 mRNA expression levels were significantly increased in HCC compared with their surrounding non-cancerous lesions. We confirmed that various HCC cell lines, especially HepG2 and Huh7 cells, showed high expression levels of CD26. siRNA-mediated knockdown of CD26 suppressed hepatoma cell growth in vitro. CD26 knockdown induced cell cycle arrest through the upregulation of Cip/Kip family proteins, p21 in HepG2 cells and p27 in Huh7 cells. CD26 knockdown did not affect apoptosis, but it increased expressions of the pro-apoptotic proteins Bim and Bak and the anti-apoptotic protein Bcl-xL, suggesting an addiction of CD26 knockdown cells to Bcl-xL for survival. We thus treated CD26 knockdown cells with ABT-737, a Bcl-xL/-2/-w inhibitor, and observed that the synthetic lethal interaction of combined Bcl-xL and CD26 inhibition induced significant apoptosis and impaired cellular viability. ConclusionCD26 mRNA was overexpressed in HCC, and its inhibition suppressed cellular proliferation through cell cycle arrest. The combined use of CD26 knockdown with a Bcl-xL inhibitor further elicited substantial apoptosis and therefore may serve as a powerful anticancer combination therapy against HCC.