Abstract 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-eIF2α-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-eIF2α-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. Citation Format: Rumeysa Ozyurt, Gamze Bildik Elcik, Weiqun Mao, Robert C. Bast, Zhen Lu. Dual inhibition of SIK2/3 induces ER Stress and ER-phagy, enhancing the cytotoxicity of autophagy blockade in ovarian cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 323.
Özyurt et al. (Fri,) studied this question.