Bcl2L13 is a ceramide synthase inhibitor in glioblastoma

Therapy resistance is a major limitation to the successful treatment of cancer. Here, we identify Bcl2-like 13 (Bcl2L13), an atypical member of the Bcl-2 family, as a therapy susceptibility gene with elevated expression in solid and blood cancers, including glioblastoma (GBM). We demonstrate that mitochondria-associated Bcl2L13 inhibits apoptosis induced by a wide spectrum of chemo- and targeted therapies upstream of Bcl2-associated X protein activation and mitochondrial outer membrane permeabilization in vitro and promotes GBM tumor growth in vivo. Mechanistically, Bcl2L13 binds to proapoptotic ceramide synthases 2 (CerS2) and 6 (CerS6) via a unique C-terminal 250-aa sequence located between its Bcl-2 homology and membrane anchor domains and blocks homo- and heteromeric CerS2/6 complex formation and activity. Correspondingly, CerS2/6 activity and Bcl2L13 abundance are inversely correlated in GBM tumors. Thus, our genetic and functional studies identify Bcl2L13 as a regulator of therapy susceptibility and point to the Bcl2L13–CerS axis as a promising target to enhance responses of therapy-refractory cancers toward conventional and targeted regimens currently in clinical use.The sphingolipid ceramide has been widely shown to be an essential component of the mitochondrial phase of apoptosis progression. De novo synthesis of ceramides is catalyzed by six different ceramide synthases (CerS1–6), also referred to as “sphinganine N-acyl-transferases,” which generate ceramides with distinct fatty-acid chain lengths. Upon apoptosis induction, CerS activity is stimulated at mitochondria and mitochondria-associated membranes (MAMs), a distinct membrane compartment that links the endoplasmic reticulum to mitochondria (1). Ceramide production is necessary for Bcl2-associated X protein (Bax) to insert into the mitochondrial membranes, oligomerize, and subsequently form a pore resulting in mitochondrial outer membrane permeabilization (MOMP) and cytochrome c release (2, 3).Reflecting the important role of ceramide in regulating apoptosis and therapy susceptibility, a role for CerSs in the pathobiology of cancer is beginning to emerge. In breast cancer, CerS2 acts as a proapoptotic protein that increases chemosensitivity and inhibits tumor growth. Consequently, reduced expression of CerS2 has been shown to be a negative prognostic indicator in breast cancer patients (4). Similarly, CerS6 promotes therapy-induced apoptosis in colon cancer cells (5, 6), head and neck squamous cell carcinoma, and lung carcinomas (7, 8), and CerS1 acts as a proapoptotic factor in multiple cancer cell lines (9, 10).Although it has been well established that CerS-mediated ceramide synthesis is an integral part of mitochondria-controlled intrinsic apoptosis signaling and is an important factor regulating tumorigenesis, specific mechanisms of CerS regulation during apoptosis are not well understood. Here, we have identified and characterized the atypical Bcl-2 family protein Bcl2-like 13 (Bcl2L13) as a CerS inhibitor with elevated expression in glioblastoma (GBM) and other solid and systemic human cancers, and potent tumorigenicity in an orthotopic GBM tumor model. A series of yeast two-hybrid (Y2H), immunoprecipitation, and molecular analyses of intrinsic apoptosis signaling revealed that Bcl2L13 blocks apoptosis in response to conventional and targeted therapy upstream of Bax activation and MOMP, at least in part by inhibiting CerS2 and CerS6 activity. Thus, our genetic and functional studies revealed, for the first time to our knowledge, that CerS activity is under the control of Bcl-2 family proteins and that the Bcl2L13–CerS2/6 signaling axis may represent a novel target to sensitize cancer cells toward extant therapies.