Philadelphia-negative myeloproliferative neoplasms (MPN) comprise a heterogeneous group of malignant diseases with abnormal hematopoietic stem cells, which classically include polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF) [1, 2]. Janus activated kinases 2 (JAK2) V617F mutation, which results in constitutive activation of the JAK tyrosine kinase and its downstream signaling pathways, has been found in most patients with MPN [3]. The constant activation of these signals leads to abnormal proliferation of myeloid cells [4]. It has been demonstrated that the cross-talk between hematopoietic cells and bone marrow (BM) microenvironment modulates leukemic stem cell proliferation, survival and migration, contributing to the progression of MPN [5, 6]. Paracrine feedback loop that directly stimulates mesenchymal stromal cells (MSC) from bone marrow (BM) (BM-MSC) to overproduce altered osteoblasts as well as inflammatory myelofibrotic cells [7, 8].
During the disease progression of MPNs, BM stroma responds abnormally to the hyperproliferative hematopoietic stem cells (HSCs) under the chronically inflammatory microenvironment [9]. The massive proliferation of fibroblasts in the BM stroma, new bone formation and the increased micro-vessel density leads to BM sclerosis, thereby causing hematopoietic failure [10, 11]. Studies have gradually revealed that tumor microenvironment (TME) plays a key role in fibrotic progression. One of the basic consist of TME is cancer-associated fibroblasts (CAFs), whose origin is really complicated [12]. α-smooth muscle actin (α-SMA), a fibrotic specific protein, plays an important composition in fibrogenesis, which has been widely used to evaluate CAF progression in recent years [13, 14]. Schmitt Graeff et. al. showed a positive correlation between the expression level of α-SMA and the degree of myelofibrosis [15]. Fibroblast activation protein-α (FAP) is an intermediate filament protein, usually expressed in mesenchymal cells, and the level of FAP might indicate the progression of CAF as well [16]. Stellate cell, smooth muscle cell, endothelial cell, epithelial cell or resident fibroblast could be transformed into CAFs via different mechanisms [17–19]. Transforming growth factor β1 (TGF-β1) is the main pro-fibrotic factor. Previous studies have validated that TGF-β1 is involved in tumorigenesis, tumor progression, metastasis, and closely related to various organs and tissues fibrosis [20, 21]. Studies have shown that TGF-β1 can promote the differentiation of fibroblasts into myofibroblasts (via SMAD2 and SMAD3 phosphorylation pathways) and the transition of epithelial cells into interstitial components (through AKT phosphorylation pathway), thereby increasing the expression of characteristic α-SMA and type I collagen, as well as causing contraction response of fibrous tissue [22, 23].
Protein post-translational modifications, such as acetylation or methylation, play an important role in epigenetic regulation [24]. Histone deacetylases (HDACs), as a class of potential anti-cancer targets, have attracted extensive attention. HDACs not only regulate the deacetylation of chromatin which affects gene transcription, but also regulate the deacetylation of non-histone proteins, some of which are involved in modulating cell cycle, differentiation and apoptosis [25]. It was reported that HDAC enzyme activity was increased in CD34 + cells from patients with PMF [26]. Additionally, it was found that HDAC8 expression of MSC derived from JAK2 positive PMF cases (PMF-MSC) was significantly higher than that of the control group. Furthermore, after treatment with the inhibitor of HDAC8, the supporting effect of PMF-MSC on malignant hematopoietic cells was decreased [27], suggesting that HDAC pathway also played a crucial role in bone marrow stroma. Similarly, it has been revealed that knockout of HDAC4 in lung fibroblasts can block TGF-β1-mediated AKT phosphorylation pathway during the progression of IPF [28]. A recent study also reports that HDAC5-mediated Smad7 silencing is crucial for fibroblast activation and the formation of hypertrophic scar [29].
However, whether HDACs are regulated during the progression of CAF remained unclear. In the present work, we investigated the expression characteristics and biological functions of HDACs in differentiation of BM-MSC to CAF and examined the regulatory effects of HDAC inhibitor (HDAC inhibitor) with chidamide in this process.