Cancer initiation and progression is characterized by a series of deregulated mechanisms, by different phenomena such as immunosurveillance, evasion of growth suppressor factors, activation of invasion and metastasis, replicative immortality, induction of angiogenesis, and resistance to apoptosis, somatic mutations, alterations in the control of genomic, and replicative integrity (Hanahan and Weinberg, 2011, Hanahan, 2022). Colorectal cancer (CRC) is the third most common cancer with the highest incidence in the world, accounting for 10.2% and the second most common cause of mortality in men and women worldwide (9.4%) (Deo et al., 2022). Between 15% and 20% of CRCs are hereditary, such as hereditary nonpolyposis colorectal cancer (HNPCC) and familial adenomatous polyposis (FAP), while the rest are sporadic, with no personal history and only 25% of people who suffer from it have a family history of the disease (Migheli and Migliore, 2012).

In recent years, different therapeutic approaches have been used for cancer treatment, some more invasive than others. Surgical treatment is the most invasive and is used when the cancer is contained in solid tumors and is localized to one site. Other treatments include chemotherapy and radiotherapy; both destroy cancer cells and retard cell growth and can be combined with other treatments. New treatments aimed to potentiate antitumor immune responses have recently shown promising results (Darvin et al., 2018). Among them, immunotherapies that block immunosuppressive pathways have increased survival and significantly increased the overall objective response in patients (Abril-Rodriguez and Ribas, 2017, Reyes et al., 2020, Working, 2001). Immunomodulators, such as Immune checkpoint receptors and ligands blocking antibodies, regulate the action of the immune system by attenuating its activation, thereby allowing immune responses in patients (Bonnefoy et al., 2019). However, tumors can downregulate the activity of these immune checkpoint proteins as a mechanism of immune resistance (Dyck and Mills, 2017).

PD-1 in cancer cells (programmed death receptor 1) and its ligand PD-L1 found in antigen presenting cells (APC) and tumor cells are well-described immunomodulatory mediators. The blockade of the PD-1/PD-L1 axis has been a major breakthrough in anticancer therapy (Topalian et al., 2012). PD-1 blockade has been evaluated in multiple syngeneic tumor models, including colon cancer, melanoma, and breast cancer (Eng et al., 2019, Lichtenstern et al., 2020), which has helped to characterize the molecular pathways involved in its regulation. Similar results have been obtained using monoclonal antibodies against PD-L1, as this can block the suppression pathway (i.e., a PD-1/PD-L1 interaction) and result in upregulation of the immune response and inhibition of cancer progression (Brahmer et al., 2012). However, only a portion of cancer patients benefit from these therapies (Brahmer et al., 2012). Therefore, new treatments have been developed to improve its efficacy, including the combination of immunotherapy and epigenetic modulators (Coutzac et al., 2017).

Histone modification, such as acetylation and deacetylation, are critical molecular mechanisms to control gene expression. Histone deacetylase (HDAC) plays a role in removing the acetyl group from acetylating lysine residues in histone tails. Deregulation of these enzymes, either due to high or low activity of HDACs, plays an important role in the development and progression of cancer (Eng et al., 2019). Acetylation and deacetylation of non-histone proteins outside the chromatin environment is a critical post-translational modification involved in modulating multiple cellular processes, such as deacetylation of proteins involved in oncogenic pathways and those related to the immune system (Jung et al., 2020), including Signal Transducers and Activators of Transcription (STAT) (Aldana-Masangkay et al., 2011, Oszukowski and Pięta-Dolińska, 2011). HDACs have different functions and localizations in the nucleus and cytoplasm. HDACs are classified as zinc- and nicotinamide-dependent. The zinc-dependent group is subdivided into classes I, II, and IV. Nicotinamide-dependent compounds comprise class III sirtuins. Although most HDACs target histones, HDAC6 is located in the cytoplasm and interacts with and regulates the function of non-nuclear proteins (Yuan et al., 2005). In addition, HDAC6 controls several critical cellular functions closely related to oncogenic pathways, such as oncogenic cell transformation, cell cycle regulation, recruitment to gene promoter regions, and the migration and invasion of cancer cells (Jung et al., 2020). Similarly, it has been shown that HDAC6 can affect the gene expression of immunomodulatory pathways (Ruijter et al., 2003) and plays an essential role in maintaining immune tolerance since it favors recruitment to promoter regions of the transcription factor STAT3 (Lienlaf et al., 2016), helping in its transcriptional regulation.

The STAT3 transcription factor mediates the expression of various genes in response to specific cellular stimuli. It plays a critical role in multiple processes, is involved in the pathogenesis and development of malignant tumors, and the induction and maintenance of tumor immune tolerance. It is essential for the transduction of extracellular signals and is a nuclear transcription factor necessary for regulating genes involved in proliferation, survival, angiogenesis, tumor invasion, and promoting inflammatory mediators such as cytokines and chemokines (Cheng et al., 2014). Cytokines that activate the STAT3 pathway modulate the activation of this transcription factor by binding to membrane receptors, such as IL-10 and IL-6. Upon interaction, these receptors activate JAK protein kinases, which phosphorylate STAT3 and promote its dimerization and subsequent translocation to the nucleus, promoting an inflammatory microenvironment and immunomodulation. The functionality and localization of STAT3 are regulated by post-translational modifications, such as phosphorylation at tyrosine residue 705 and phosphorylation of serine 727 STAT3 activation. STAT3 acetylation at lysine 685 (Lys 685) is responsible for blocking STAT3 dimerization (Yu et al., 2009). These regulatory mechanisms are critical in CRC since it is known that when STAT3 translocate to the nucleus and activates the expression of STAT3 target genes, such as PD-L1, which promotes an immunosuppressive tumor microenvironment and, consequently, the survival of tumor cells (Lichtenstern et al., 2020). The above suggests that the HDAC6/STAT3 axis could modify the expression of different immunomodulators. Here we were able to demonstrate that this immunomodulatory regulation occurs in colorectal cancer cells, so it could occur in colorectal tumors. Which makes HDAC6 inhibition potentially an interesting therapeutic target for CRC.