Histone acetylation and deacetylation are important epigenetic mechanisms that regulate gene expression and transcription. Histone deacetylase 6 (HDAC6) is a unique member of the HDAC family that not only participates in histone acetylation and deacetylation but also targets several nonhistone substrates, such as α-tubulin, cortactin, and heat shock protein 90 (HSP90), to regulate cell proliferation, metastasis, invasion, and mitosis in tumors. Furthermore, HDAC6 also upregulates several critical factors in the immune system, such as program death receptor-1 (PD-1) and program death receptor ligand-1 (PD-L1) receptor, which are the main targets for cancer immunotherapy. Several selective HDAC6 inhibitors are currently in clinical trials for cancer treatment and bring hope for patients with malignant tumors. A fuller understanding of HDAC6 as a critical regulator of many cellular pathways will help further the development of targeted anti-HDAC6 therapies. Here, we review the unique features of HDAC6 and its role in cancer, which make HDAC6 an appealing drug target.

Recent translational studies in osteosarcoma are discussed with the purpose to shed light on the new molecular therapeutic targets. The genetic aberrations of vascular endothelial growth factor (VEGF), mammalian target of rapamycin, Wnt signaling pathway, the inactivation of p53, Rb, WWOX genes, and amplification of APEX1, c-myc, RECQL4, RPL8, MDM2, VEGFA might be involved in the pathogenesis of osteosarcoma. The promising therapeutic targets for osteosarcoma patients include: integrin, ezrin, statin, NOTCH/HES1, matrix metalloproteinases (MMPs), m-calpain, and Src, which are involved in tumor cell invasion and metastasis; aldolase A, fructose-bisphosphate, sulfotransferase family 3A, member 1, BCL2-associated athanogene 3, heat shock protein 70 (HSP70), B-cell lymphoma 2-interacting mediator (BIM), polo-like kinase 1, hypoxia inducible factor 1, alpha subunit, minibrain-related kinase, Bcl-xl, caspase-3, midkine, high mobility group box 1 protein (HMGB1), and Beclin1, which are involved in tumor proliferation and apoptosis; met proto-oncogene (hepatocyte growth factor receptor), v-erb-b2 erythroblastic leukemia viral oncogene homolog 2, insulin-like growth factor (IGF)-1R, fms-related tyrosine kinase 4, platelet-derived growth factor receptor, beta polypeptide, IGF-I/II, and c-kit, which are involved in tumor growth; endosialin, VEGF, thrombin, and MMPs, which are involved in tumor angiogenesis; transforming growth factor-α/β, parathyroid hormone-like hormone, interleukin-6, interleukin-11, receptor activator of nuclear factor-κB ligand, nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1, and cathepsin, which are involved in osteoclast function; Myc, HSP90, p-Met, p-Akt, p-STAT3, and cyclin D1, which are transcriptional factors; p-GP, hydroxysteroid (17-beta) dehydrogenase 10, HMGB1, BIM, inorganic phosphate, Bcl-2, PARP, mdm2, p21, Bax, and mitogen-activated protein kinase 1, which are involved in drug sensitivity. Furthermore, microRNAs such as miR-215 are also therapeutic targets. These translational studies in osteosarcoma have identified new molecular targets for osteosarcoma.

Acral melanoma is a dismal subtype of melanoma occurring in glabrous acral skin, and has a higher incidence in East Asians. We perform single-cell RNA sequencing for 63,394 cells obtained from 5 acral and 3 cutaneous melanoma samples to investigate tumor heterogeneity and immune environment. We define 5 orthogonal functional cell clusters that are involved in TGF-beta signaling, Type I interferon, Wnt signaling, Cell cycle, and Cholesterol efflux signaling. Signatures of enriched TGF-beta, Type I interferon, and cholesterol efflux signaling are significantly associated with good prognosis of melanoma. Compared with cutaneous melanoma, acral melanoma samples have significantly severe immunosuppressive state including depletion of cytotoxic CD8+ T cells, enrichment of Treg cells, and exhausted CD8+ T cells. PD1 and TIM-3 have higher expression in the exhaustive CD8+ T cells of acral melanoma. Key findings are verified in two independent validation sets. This study contributes to our better understanding of acral melanoma.

Low-density lipoprotein receptor-related protein 1 (LRP1, also known as CD91), a multifunctional endocytic and cell signaling receptor, is widely expressed on the surface of multiple cell types such as hepatocytes, fibroblasts, neurons, astrocytes, macrophages, smooth muscle cells, and malignant cells. Emerging in vitro and in vivo evidence demonstrates that LRP1 is critically involved in many processes that drive tumorigenesis and tumor progression. For example, LRP1 not only promotes tumor cell migration and invasion by regulating matrix metalloproteinase (MMP)-2 and MMP-9 expression and functions but also inhibits cell apoptosis by regulating the insulin receptor, the serine/threonine protein kinase signaling pathway, and the expression of Caspase-3. LRP1-mediated phosphorylation of the extracellular signal-regulated kinase pathway and c-jun N-terminal kinase are also involved in tumor cell proliferation and invasion. In addition, LRP1 has been shown to be down-regulated by microRNA-205 and methylation of LRP1 CpG islands. Furthermore, a novel fusion gene, LRP1-SNRNP25, promotes osteosarcoma cell invasion and migration. Only by understanding the mechanisms of these effects can we develop novel diagnostic and therapeutic strategies for cancers mediated by LRP1.