p38α, the β-catenin chromatin associated kinase, as promising target in colorectal cancer stem cells for personalized therapy

Colorectal cancer (CRC) is the third most frequent malignancy, but the second cause of death for tumor in the western population. Only 14% of patients with advanced and metastatic disease survive five years from diagnosis. Recently, it has been shown that tumor relapse and chemoresistance depend on a small population of cells, called cancer stem cells (CSCs). Current evidence indicates that the Wnt cascade is the main driver in controlling CSC fate; the key player in this pathway is β-catenin, a cytoplasmic protein whose stability is regulated by the so-called “destruction complex”. During carcinogenesis, the increasing amount of β-catenin resulting from APC inactivation translocates into the nucleus, causing the transcriptional activation of several mitogenic genes, including c-Myc. c-Myc is one of the most important factors involved in CRC initiation and progression; indeed, it functions as a link connecting malignancy with stemness. During colorectal carcinogenesis, c-Myc is maintained upregulated through β-catenin-mediated transcriptional activation and ERK-mediated post-translational stabilization. Our data showed that p38α, a kinase involved in CRC metabolism and survival, contributes to both mechanisms. Previous reports in other tissues provided evidence that Wnt3a can activate p38, and the p38 pathway feeds into the canonical Wnt/β-catenin pathway at least at the level of GSK3β. Our findings also highlighted that CRC cells and colorectal cancer stem cells (CRC-SCs) have higher levels of activated p38 than their normal counterparts, and experiments using kinase-specific inhibitors revealed that these cells are “addicted” to p38 activity. Importantly, we found that p38α co-immunoprecipitates with β-catenin in both normal and cancer cells; however, these proteins are confined to the cytoplasm in colonocytes, while they significantly occupy discrete nuclear regions in CRC cells, CRC-SCs, and in vivo models. These data were further corroborated by the inhibitory effect of p38α blockade on several β-catenin-responsive genes (i.e. c-Myc, cyclin D1/2, survivin, and others). This functional interaction was further characterized by chromatin immunoprecipitation experiments, which demonstrated that p38α is a chromatin-associated β-catenin kinase required for the transcriptional induction of several Wnt target genes, including c-Myc. Additionally, we demonstrated that p38α, like ERK, stabilizes c-Myc protein levels by preventing its ubiquitination. The finding that the phenotypes arising after APC loss in the intestine are fully dependent on c-Myc target gene expression suggests that c-Myc inhibition may be a good target for chemoprevention in CRC. These considerations underline the relevance of molecular profiling and preclinical investigation in order to achieve more efficient and accurate therapies. Indeed, our study identifies p38α as a promising therapeutic target acting directly on c-Myc and CRC-SCs, which are thought to be responsible for tumor proliferation, metastatic dissemination, and chemoresistance.