Glycosylation Of The Tnfr1 Death Receptor Controls Cell Fate

Activation of the TNFR1 death receptor by TNFα can induce cell survival or cell death signaling, however, the mechanisms regulating this TNFR1 signaling switch are poorly understood. TNFα-induced apoptosis is reported to require higher-order clustering and internalization of activated TNFR1, whereas surface retention of the activated receptor has been shown to promote survival signaling mediated by NFκB and MAPKs. Studies from our group have identified a novel glycosylation-dependent mechanism that controls this signaling switch. Specifically, we have found that TNFR1 localization and activity are regulated by the addition of a distinct sugar, an α2-6 linked sialic acid, by the enzyme ST6Gal-I, a Golgi sialyltransferase whose expression is upregulated in many cancer types. The sialylation effect on TNFR1 was examined in epithelial and monocytic cancer cell lines with forced overexpression or knockdown of ST6Gal-I, as well as in primary monocytes obtained from mice with ST6Gal-I knock-in or knockout. Data from these models indicate that α2-6 sialylation of TNFR1 blocks TNFα-induced apoptosis, and the suggested mechanism underlying this inhibition of apoptosis is the sialylation-driven inhibition of TNFR1 oligomerization and internalization, as observed through immunoblotting, immunocytochemistry and flow cytometry. Considering sialylated TNFR1 is retained on the plasma membrane following activation, we hypothesize that ST6Gal-I functions to not only block TNFR1-mediated cell death but also divert the cell9s activity to favor survival. Supporting this hypothesis, cells with elevated levels of ST6Gal-I exhibit heightened expression and activation of many pro-survival factors, such as NFκB, MAPKs and AKT, in addition to decreased activation of caspase-3. Since levels of ST6Gal-I are elevated in multiple tumor cell populations, our group proposes that ST6Gal-I protects tumor cells against TNFα-induced apoptosis within the inflammatory tumor microenvironment. As preliminary support for this concept, peritoneal ascites fluid was collected from ovarian cancer (OC) patients, and the acellular, cytokine-rich fraction was incubated with OC cell lines with or without forced ST6Gal-I expression. Expression of ST6Gal-I conferred strong protection against cell death induced by soluble ascites. Based on these collective findings, we postulate that sialylation of TNFR1 by ST6Gal-I diverts the cellular response to TNFα from apoptosis to survival, providing a mechanism by which tumor cells can evade immune cell killing. Citation Format: Andrew Holdbrooks, Matthew J. Schultz, Zhongyu Liu, Daniel Bullard, Susan L. Bellis. Glycosylation of the TNFR1 death receptor controls cell fate. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3566.