PDE4DIP-Derived MMG8 Supports Proliferation, Migration, and Tumor Growth in Hepatocellular Carcinoma Models.

Background

PDE4DIP encodes a scaffold protein that has been implicated in compartmentalized signaling and cytoskeletal organization, but the role of its myomegalin variant 8 (MMG8) isoform in hepatocellular carcinoma (HCC) remains unclear. To address this gap, we examined PDE4DIP expression in public HCC datasets and investigated the functional role of MMG8 in HCC models.

Methods

PDE4DIP expression was analyzed in The Cancer Genome Atlas Liver Hepatocellular Carcinoma (TCGA-LIHC) cohort and two Gene Expression Omnibus (GEO) cohorts (GSE14520, GSE36376). MMG8 function was assessed in Huh7 cells using siRNA-mediated knockdown and in Hepa1-6 cells using lentiviral Clustered Regularly Interspaced Short Palindromic Repeats - CRISPR-associated protein 9 (CRISPR-Cas9)-mediated knockout. Cell proliferation in MMG8-KD Huh7 cells and MMG8-KO Hepa1-6 cells was assessed using Cell Counting Kit-8 (CCK-8) assays, while Huh7 cell migration was evaluated using Transwell assays. Tumor growth was assessed using a murine subcutaneous tumor model. Immunohistochemical staining for Ki67 and cleaved caspase-3 was employed to assess tumor cell proliferation and apoptosis-associated changes, respectively. Gene set enrichment analysis was performed in TCGA-LIHC tumors stratified based on PDE4DIP expression.

Results

PDE4DIP expression differed between tumor and non-tumor tissues across HCC cohorts, although the directionality of this difference was not uniform. MMG8 knockdown in Huh7 cells reduced proliferation and migratory activity. A single-cell-derived MMG8-KO Hepa1-6 clone exhibited reduced proliferation in vitro and formed smaller tumors in vivo, with lower Ki67 positivity but no significant difference in cleaved caspase-3 positivity between groups. In tumors from the TCGA-LIHC cohort, PDE4DIP expression was associated with distinct transcriptional programs. Specifically, PDE4DIP-high tumors presented with positive normalized enrichment score (NES) values for several metabolic pathways, whereas adhesion/extracellular matrix (ECM), cell cycle/proliferation, and translation/ribosome-related pathways exhibited negative NES values.

Conclusions

These findings support a functional contribution of MMG8 to proliferative, migratory, and tumor-growth phenotypes in the tested HCC models. Bulk gene-level PDE4DIP expression in human tumors was associated with context-dependent transcriptional states and should not be interpreted as a direct surrogate for MMG8 function.