Abstract 3618: SGKs survival signal via inhibition of pro-apoptotic Mixed Lineage Kinase 3 (MAP3K11) in cancer cells

Survival of cancer cells requires activation of pro-survival and inhibition of pro-apoptotic pathways in concert. It is utmost important to understand the underlying molecular pathways that regulate cancer cell survival and death, so that specific inhibitor could be identified and used to target cancer cell death pathways. Dexamethasone (DEX) is routinely administered to cancer patients, in order to reduce allergic responses. It is reported that DEX use in certain cancer patients could lead to early cancer relapse and detrimental. Earlier, we reported that insulin regulated PI3K-AKT pathway down regulated pro-apoptotic Mixed Lineage Kinase 3 (MLK3) and this down regulation of kinase activity, favored cancer cell survival. Since SGK family members are also regulated by PI3K via Glucocorticoid Receptor (GR) in response to DEX, and SGK is reported to have similar function like AKT, we hypothesized that SGKs might also regulate pro-apoptotic function of MLK3, similar to AKT but probably via a non-redundant pathway. Here we report that MLK3 is a direct target of SGK in cancer cells. Endogenous SGK physically associated with and phosphorylated MLK3 in HepG2 cells and this interaction was regulated by synthetic glucocorticoid, dexamethasone. JNK and MLK3 kinase activities were significantly inhibited either by over expression of SGK3 or by dexamethasone. The SGK3 and dexamethasone-mediated attenuation of MLK3, MKK7 and JNK kinase activities were blocked by PI3K inhibitor, LY294002, suggesting a link between PI3K-SGK and MLK3-JNK signaling pathways. SGK phosphorylated MLK3 both in vivo and in vitro. SGK1 and SGK2/3 phosphorylated MLK3 on Ser674 and Thr738, respectively. Furthermore, SGK3 inhibited MLK3-mediated cell death and JNK kinase activity in a manner, dependent on Thr738 phosphorylation of MLK3. Taken together, these data define the mechanisms of glucocorticoid-mediated inhibition of MLK3-JNK axis and cell death pathways. Furthermore, our data also identify MLK3 as an indirect target of glucocorticoids that might play a significant role in the pro-survival and anti-inflammatory actions of glucocorticoid in cancer cell. Therefore, it is conceivable that targetting MLK3 with its agonist, ceramide (by nano-ceramide particles) could possibly overcome DEX-induced cancer relapse. This work is supported by VA Grant #: BX000312 and NIH, R01 CA 176846 to AR; and VA Grant #: BX000571to BR. Citation Format: Gautam V. Sondarva, Velusamy Rangasamy, Navin Viswakarma, Sub