Many cytoplasmic metabolic enzymes including all major glycolytic and mitochondrial enzymes are also found in the nucleus. The nuclear functions of these enzymes are mostly unknown and but are independent of their catalytic activity and include epigenetic regulation and DNA repair. Our lab has recently shown that GTP-producing cytosolic IMPDH2 in melanoma induces ribosome biogenesis, actomyosin contractility, and disease progression. However, a subset of melanoma cells displays both cytosolic and nuclear IMPDH2 expression. We thus investigated potential nuclear functions of IMPDH2, that were linked to transcriptional regulation in lower organisms.
In vitro, melanoma cells with spontaneously occurring high nuclear-to-cytoplasmic (N/C) IMPDH2 ratios formed smaller colonies. Consistent with this, forced overexpression of nuclear IMPDH2 reduced melanoma cell clonogenicity/migration and increased pigmentation and senescence. IMPDH2 ChIP-seq and RT-qPCR analyses demonstrated that nuclear IMPDH2 acts as a transcriptional repressor for the E2F target gene, CDK4, a critical regulator of cell cycle progression. Pathway enrichment analysis of nuclear IMPDH2 interactome proteins discovered by IMPDH2 Co-IP-coupled LC-MS revealed oncogene-induced cell senescence as the most significantly associated pathway. Direct LC-MS-based comparisons of fractionated cytosolic and nuclear IMPDH2 revealed posttranslational modifications (K195/K450 trimethylation, S416 phosphorylation, K208 ubiquitination) that were present in cytosolic fractions but completely absent in nuclear IMPDH2. Further, TRIM21, an E3 ubiquitin ligase, was found to interact with IMPDH2, suggesting a role for this enzyme in IMPDH2 degradation in melanoma.
These data indicate IMPDH2 has a dual and cell location-dependent role in melanoma. In the cytoplasm, IMPDH2 acts canonically as a metabolic oncoprotein to regulate cellular GTP and promote disease progression. In contrast, nuclear IMPDH2 acts as a tumour suppressor to repress transcription of E2f target genes such as CDK4 and induce senescence. Increasing IMPDH2’s N/C ratio in melanoma cells by targeting its post-translational modifications and/or promoting its cytoplasmic degradation is a promising therapeutic strategy.