**Background:** Cancer cachexia (CCx) is a multifactorial wasting syndrome affecting ~35% of all cancer patients and up to 80–90% of those with gastric or pancreatic cancer, contributing to ~20% of cancer-related deaths. MicroRNAs (miRNAs) are small non-coding RNAs that regulate 30–80% of human gene expression and have been implicated in metabolic regulation and cancer, but their role in human CCx across multiple organs remains poorly understood. This study aimed to identify organ-specific miRNA signatures associated with CCx and explore their functional roles.
**Methods:** The study included 35 gastrointestinal cancer patients (pancreatic ductal adenocarcinoma or colorectal cancer) undergoing surgery at Klinikum rechts der Isar, Technical University of Munich. CCx was defined as ≥5% weight loss in the 6 months before surgery (modified Fearon criteria). Serum, liver, musculus rectus abdominis, visceral adipose tissue (VAT), and subcutaneous adipose tissue (SAT) biopsies were collected. A discovery miRNA array (169 miRNAs profiled, 158 captured) was performed on pooled serum samples from 4 cachectic and 4 non-cachectic cancer patients (2 men and 2 women per group). Ten candidate miRNAs were validated in individual serum and tissue samples from up to 23 CCx patients, 12 non-CCx cancer patients, and healthy controls (FREECE study, N=18 for serum; MOBB cohort for VAT controls; IPBS cohort for bariatric surgery weight loss comparison). In silico target prediction using MultiMiR (14 databases) identified putative target genes linked to skeletal muscle atrophy (GO:0014732) and triglyceride lipase activity (GO:0004806). Functional validation used anti-miR-27b-3p knock-down in C2C12 myoblast cells and anti-miR-375/anti-miR-424-5p knock-down in human visceral preadipocytes (PACs), with gene expression analysis by RT-qPCR. Statistical analyses used Mann–Whitney tests, Kruskal–Wallis tests, linear regression, and ROC analysis.
**Key Results:** In serum, miR-122-5p showed 2-fold downregulation (P=0.0396) and miR-194-5p showed 4.5-fold downregulation (P<0.0001) in CCx patients vs. healthy controls. Only miR-122-5p correlated with weight loss (Spearman r=−0.45, P=0.031) and distinguished CCx from non-CCx patients (AUC>0.70, P=0.0367). This association was CCx-specific, as no correlation was found in bariatric surgery patients with 26% weight loss. In muscle tissue, six miRNAs were differentially expressed; miR-27b-3p showed 0.5-fold lower expression in CCx vs. non-CCx cancer patients (P=0.0019), correlated negatively with weight loss (r=−0.408, P=0.0386), and distinguished groups with AUC>0.84 (P=0.0028). In silico prediction identified ACTN3, IL-15, MSTN, PPARGC1A, MYOG, and TRIM63 as miR-27b-3p targets in the muscle atrophy pathway. Anti-miR-27b-3p knock-down in C2C12 cells significantly upregulated Il-15 and Trim63 (both P=0.0286). Concordantly, IL-15 (P=0.0237) and TRIM63 (P=0.0442) were significantly upregulated in muscle tissue of CCx patients. In VAT, eight miRNAs were significantly lower in CCx vs. non-CCx cancer patients (P<0.05). miR-375 showed 3.3-fold lower expression (P=0.0096 vs. non-CCx; P=0.0003 vs. controls) and miR-424-5p showed 7.5-fold lower expression (P=0.0010 vs. non-CCx), both correlating with weight loss (P=0.0112 and P=0.0075, respectively) and distinguishing CCx with AUC>0.83. Anti-miR-424-5p knock-down in visceral PACs significantly upregulated LIPE (P=0.0095), PNPLA2 (P=0.0022), MGLL (P=0.0022), and LPL (P=0.0095). No significant changes were seen for miR-375 knock-down on these lipase genes. No miRNA expression differences were found in liver tissue, and only miR-142-5p was altered in SAT.
**Clinical Implications:** This study provides the first comprehensive multi-organ miRNA analysis in human cancer cachexia, identifying miR-122-5p as a potential serum biomarker for CCx that is independent of weight loss per se. The tissue-specific findings—miR-27b-3p in muscle atrophy via IL-15 and TRIM63 regulation, and miR-424-5p in adipose tissue lipolysis via LIPE, PNPLA2, MGLL, and LPL regulation—suggest these miRNAs may contribute mechanistically to the wasting syndrome. These miRNAs could serve as screening tools for early detection of CCx or as therapeutic targets. However, the cross-sectional design limits causal inference, and prospective studies with larger cohorts are needed to validate these findings and explore clinical utility.