Emerging results suggest that ceramides with different fatty acid chain lengths might play distinct functions in the regulation of tumor growth and therapy. Here we report that de novo-generated C 18-and C 16-ceramides by ceramide synthases 1 and 6 (CerS1 and CerS6) play opposing proapoptotic and prosurvival roles, respectively, in human head and neck squamous cell carcinomas (HNSCCs). Unexpectedly, knockdown of CerS6/C 16-ceramide using small interfering RNA induced endoplasmic reticulum (ER)-stress-mediated apoptosis. Reconstitution of C 16-ceramide generation by induced expression of wild-type CerS6, but not its catalytically inactive mutant, protected cells from cell death induced by knockdown of CerS6. Moreover, using molecular tools coupled with analysis of sphingolipid metabolism showed that generation of C 16-ceramide, and not dihydro-C 16-ceramide, by induced expression of CerS6 rescued cells from ER stress and apoptosis. Mechanistically, regulation of ER-stress-induced apoptosis by CerS6/C 16-ceramide was linked to the activation of a specific arm, ATF6/CHOP, of the unfolded protein response pathway. Notably, while expression of CerS1/C 18-ceramide inhibited HNSCC xenograft growth, CerS6/C 16-ceramide significantly protected ER stress, leading to enhanced tumor development and growth in vivo, consistent with their pro-and antiapoptotic roles, respectively. Thus, these data reveal an unexpected and novel prosurvival role of CerS6/C 16-ceramide involved in the protection against ER-stress-induced apoptosis and induction of HNSCC tumor growth.—Senkal, CE, Ponnusamy, S., Bielawski, J., Hannun, YA, Ogretmen, B. Antiapoptotic roles of ceramide-synthase-6-generated C 16-ceramide via selective regulation of the ATF6/CHOP arm of ER-stress-response pathways.