Dual inhibition of SIK2/3 induces ER Stress and ER-phagy, enhancing the cytotoxicity of autophagy blockade in ovarian cancer

Cancer cells rely heavily on protein quality control mechanisms to survive intrinsic stress from rapid proliferation and extrinsic stress from the tumor microenvironment. Among these mechanisms, endoplasmic reticulum (ER)-selective autophagy (ER-phagy) has emerged as a critical process that maintains ER homeostasis by removing damaged ER components and misfolded protein aggregates. When the protein-folding capacity of the ER is exceeded, misfolded proteins accumulate and trigger ER stress, activating the unfolded protein response (UPR) through three major branches: PERK-eIF2alpha-ATF4, IRE1-XBP1s, and ATF6. ER stress, in turn, can induce ER-phagy as an adaptive mechanism to alleviate proteotoxic stress. Salt-inducible kinases SIK2 and SIK3 (SIK2/3) are serine/threonine kinases that regulate cellular metabolism and stress responses. While SIK2 has been implicated in ER-associated degradation (ERAD) and in promoting ovarian cancer progression and survival, the roles of SIK2/3 in regulating ER stress and ER stress-mediated autophagy remain poorly understood in cancer. We hypothesized that inhibition of SIK2/3 induces ER stress, which subsequently activates ER-phagy to maintain proteostasis. Further, combining dual SIK2/3 inhibition with GRN-300 (a selective SIK2/3 inhibitor) and autophagy blockade using chloroquine (CQ, an autophagy inhibitor) should enhance proteotoxic stress and exert potent anti-tumor activity in ovarian cancer. Here we report that inhibition of SIK2/3 triggers ER stress and activates ER-phagy as an adaptive survival mechanism in ovarian cancer cells. Genetic or pharmacological inhibition of SIK2/3 activated all three UPR pathways (PERK-eIF2alpha-ATF4, IRE1-XBP1s, and ATF6), leading to accumulation of polyubiquitinated proteins and aggregates, as well as the induction of CHOP and apoptotic cell death. SIK2/3 inhibition also upregulated the ER-phagy receptor CCPG1 in an ATF4-dependent manner, enhancing autophagic flux. Notably, combination treatment with GRN-300 and CQ synergistically reduced cell viability (combination index, CI < 0.9), exacerbated proteotoxic stress, and triggered CHOP-dependent apoptosis in multiple ovarian cancer cell lines. In three ovarian cancer xenograft models (OVCAR8, OC316, and SKOv3), GRN-300 plus CQ markedly suppressed tumor growth, increased apoptotic markers, and significantly prolonged survival compared to either monotherapy. These findings reveal a previously unrecognized role of SIK2/3 inhibition in driving ER stress and CCPG1-mediated ER-phagy and provide strong rationale for combining GRN-300 with autophagy inhibition as a promising therapeutic strategy for ovarian cancer.