SMAC/DIABLO: A Guardian Angel in Boosting Anticancer Drug-Induced Apoptosis

Apoptosis is an established hallmark of cancer. In normal conditions, apoptosis is strictly controlled; however, when it is not properly managed, it causes several complications, including cancer progression and drug resistance. SMAC/ Diablo (SMAC) is a mitochondrial protein that is released into the cytosol upon activation of BAX/BAK channels with apoptotic signals. SMAC protein interacts and neutralizes inhibitors of apoptosis (IAP) proteins and initiates the caspase cascade, which leads to apoptosis. SMAC is downregulated in several types of cancer, which led to the design of small-molecule inhibitors known as SMAC mimetics as new cancer therapeutics, and some of these molecules are in the clinical phase. It has also been shown that a combination of SMAC with standard anti-cancer drugs could be beneficial to drug-resistant cancer. Despite being a pro-apoptotic protein, it has been found that SMAC/Diablo is overexpressed in several types of cancers like lung, breast, bladder, cervix, pancreas, prostate, and colon, as well as in melanoma and glioma, and in cancer cells. Recently, we have reported that the overexpression of SMAC in cancers is essential for cell and tumor growth due to non-apoptotic regulation of phospholipid synthesis. The current review is focused on apoptotic and non-apoptotic functions of SMAC and its role in drug resistance.