Inhibitory Effects of Syringic Acid on Endometrial Cancer Cell Growth and Migration and Its Synergistic Suppression with Doxorubicin

<b>Background/Objectives</b>: Endometrial cancer (EC), a malignancy arising from the uterine lining, is a leading gynecological cancer in developed countries. Syringic acid (SA), a naturally occurring phenolic compound, possesses various bioactivities including antioxidant, anti-inflammatory, chemoprotective, and anti-angiogenic properties. This study aimed to investigate the effects of SA on the proliferation and migration of RL95-2 EC cells, its protective role in normal endometrial stromal cells (HESCs), and the underlying molecular mechanisms. Furthermore, the potential synergistic anticancer effects of SA in combination with chemotherapeutic agents against EC were evaluated. <b>Methods</b>: Cell viability was assessed using nuclear fluorescence staining, the MTT assay, and clonogenic survival assay. Cell migration was evaluated through wound closure and Transwell migration assays. Gene expression levels were analyzed by the RT-PCR method. <b>Results</b>: SA significantly inhibited the proliferation of RL95-2 EC cells, with an IC₅₀ value of 27.22 μM. Co-treatment with SA and the chemotherapeutic agent doxorubicin (Dox) demonstrated an additive inhibitory effect. Mechanistically, both SA and the SA-Dox combination induced apoptosis by upregulating the expression of <i>caspases-3</i>, <i>-8</i>, and <i>-9</i>, increasing the expression of pro-apoptotic genes (<i>Bax</i> and <i>Bad</i>), and downregulating anti-apoptotic genes (<i>Bcl-XL</i> and <i>Bcl-2</i>). Cell cycle analysis revealed the downregulation of <i>cyclin D</i> and the upregulation of tumor suppressors <i>p21</i> and <i>p27</i>, contributing to growth arrest. In addition, both SA and the combination treatment effectively suppressed cell migration by downregulating matrix metalloproteinases (<i>MMP</i>s) and <i>β-catenin</i>. SA treatment also induced the expression of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) and activated NF-κB signaling, leading to an elevated expression of inflammatory mediators such as <i>COX-2</i> and <i>iNOS</i>. Furthermore, SA promoted oxidative stress in RL95-2 cells by inhibiting the Nrf2 pathway and reducing the expression and activities of antioxidant enzymes including catalase, glutathione peroxidase, and superoxide dismutase, thereby enhancing reactive oxygen species (ROS) accumulation. In contrast, in lipopolysaccharide-stimulated HESC cells, SA attenuated inflammation and ROS generation, indicating its selective cytoprotective role in normal endometrial cells. <b>Conclusions</b>: SA may serve as a promising adjuvant candidate to enhance chemotherapeutic efficacy while protecting normal cells by mitigating inflammation and oxidative stress.