LAMP1 oversialylation results in excessive lysosomal exocytosis, promoting tumor invasion and drug resistance.

We have previously shown that the death receptor ligand TRAIL induces an increase in intracellular C16-ceramide in sensitive SW480 but not in resistant SW620 cells. Resistance in SW620 cells was overcome by exogenous ceramide leading us to propose that defective ceramide signaling contributes to TRAIL resistance. In this study we found that the increase in C16-ceramide in SW480 cells was inhibited by fumonisin B1, an inhibitor of ceramide synthases (CerS). Protein analysis revealed that TRAIL resistant SW620 cells expressed lower levels of ceramide synthase 6 (CerS6, also known as longevity assurance homologue 6), which prompted us to investigate the effect of CerS6 modulation on TRAIL phenotype. RNAi against CerS6 resulted in a specific and significant decrease of the C16-ceramide species, which was sufficient to inhibit TRAIL-induced apoptosis. In cells with decreased levels of CerS6, caspase-3 was activated but failed to translocate into the nucleus. CerS6 localized primarily to the perinuclear region, suggesting this enzyme may play a role in regulation of nuclear permeability. Moderate elevation in CerS6 expression was sufficient to reverse TRAIL resistance in SW620 cells. These results suggest that modulation of CerS6 expression may constitute a new therapeutic strategy to alter apoptotic susceptibility.

Purpose gp96 (grp94) is a key downstream chaperone in the ER to mediate unfolded protein response (UPR) and the pathogenesis of multiple myeloma (MM) is closely linked to dysregulated UPR. In this study, we aimed to determine the roles of gp96 in the initiation and progression of MM in vivo and in vitro. Experimental design We generated a mouse model with over-expression of XBP1s and conditional deletion of gp96 in B cell compartment simultaneously to identify the roles of gp96 in the development of MM in vivo. Using a shRNA system, we silenced gp96 in multiple human MM cells and examined the effect of gp96 knockdown on MM cells by cell proliferation, cell cycle analysis, apoptosis assay, immunohistochemistry and human myeloma xenograft model. The anti-cancer activity of gp96 selective inhibitor, WS13 was evaluated by apoptosis assay and MTT assay. Results Genetic deletion of gp96 in XBP1s-Tg mice attenuates multiple myeloma. Silencing of gp96 causes severe compromise in human MM cell growth through inhibiting Wnt-LRP-survivin pathway. We also confirmed that knockdown of gp96 decreased human MM growth in a murine xenograft model. The targeted gp96 inhibitor induced apoptosis and blocked MM cell growth, but did not induce apoptosis in pre-B leukemic cells. We have demonstrated that myeloma growth is dependent on gp96 both genetically and pharmacologically. Conclusions gp96 is essential for MM cell proliferation and survival, suggesting that gp96 is a novel therapeutic target for multiple myeloma.