Diffuse GLUT1 Expression Is Associated with Reduced Cytoplasmic p-YAP Immunoreactivity in Endometrioid Endometrial Carcinoma

Background and Objectives: Metabolic reprogramming and dysregulation of Hippo signaling are recognized features of endometrial carcinogenesis; however, the relationship between regional GLUT1 expression patterns and Hippo pathway-related proteins remains incompletely understood. This exploratory study investigated these associations in endometrioid endometrial carcinoma and evaluated changes in YAP-related protein expression following GLUT1-targeting siRNA treatment in vitro. Materials and Methods: Formalin-fixed, paraffin-embedded tumor specimens from 50 patients with endometrioid endometrial carcinoma were retrospectively analyzed by immunohistochemistry for GLUT1, YAP, phosphorylated YAP (p-YAP, Ser127), MST1/2, and LATS1/2. GLUT1 expression was classified as mosaic, central, or diffuse within the sampled tumor tissue. Associations between GLUT1 patterns and Hippo pathway markers were assessed, including an exploratory analysis restricted to YAP-positive tumors. Complementary experiments were performed in Ishikawa cells using GLUT1-targeting siRNA, followed by semi-quantitative RT-PCR and Western blotting, each based on three independent biological experiments. Results: GLUT1 expression was mosaic in 12.0%, central in 56.0%, and diffuse in 32.0% of tumors. Total YAP expression was not significantly associated with GLUT1 distribution (p = 0.411), whereas diffuse GLUT1 expression showed a nominally significant association with absent cytoplasmic p-YAP immunoreactivity (odds ratio [OR] = 5.8, 95% confidence interval [CI]: 1.22–27.7; unadjusted p = 0.044). This association did not remain statistically significant after a focused two-test Benjamini–Hochberg false discovery rate correction (q = 0.088). The association was also not statistically significant in the YAP-positive subgroup (n = 41; OR = 4.50; p = 0.139). In Ishikawa cells, GLUT1-targeting siRNA significantly reduced GLUT1 mRNA expression (p = 0.0089), whereas GLUT1 protein expression showed a nonsignificant reduction (p = 0.1356). Total YAP and p-YAP levels decreased significantly in unadjusted paired analyses (p = 0.0149 and p = 0.0481, respectively). The normalized p-YAP/total YAP ratio also decreased from 0.803 ± 0.192 to 0.634 ± 0.136, although the difference was not statistically significant (p = 0.1488). Conclusions: Diffuse GLUT1 expression showed a nominally significant association with absent cytoplasmic p-YAP immunoreactivity in endometrioid endometrial carcinoma, but this association did not remain statistically significant after a focused BH-FDR correction. In vitro, GLUT1-targeting siRNA treatment was accompanied by changes in YAP-related protein expression without significant reductions in GLUT1 protein expression or the normalized p-YAP/total YAP ratio. These preliminary findings suggest a potential relationship between GLUT1 expression and Hippo-YAP signaling but do not establish altered YAP phosphorylation or a direct causal mechanism. Further validation is warranted.