**Author details**

NK cell cytotoxicity is mediated by the effector molecule, perforin, which creates pores in the phospholipid bilayer of host target cells leading to facilitation of granzymes entry resulting

The expression of perforin in NK cells is only induced by the direct binding of myeloid Elf1-like factor (MEF), a transcription factor that belongs to Ets family, to perforin 1 promoter at two sites. However, in cytotoxic T lymphocytes perforin 1 is regulated differentially at the transcriptional level [108]. Perforin 1 regulatory region has two enhancers that are responsive to IL-2R-activated signal transduction and bind to Stat5 [108]. IL2Rb or Stat5b knockout NK cells of mice showed significantly lower levels of perforin transcript which highlights the importance of these enhancers for perforin transcription [109]. One of the enhancers is responsive to IL-6 and IL-12 and, upon cytokine stimulation, can bind STAT1a and STAT4, respectively [110, 111]. On the other hand, granzymes are serine proteases present in cytolytic granules of NK cells and cytotoxic T cells. Granzyme B (GZMB) is the most comprehensively studied member of the granzymes family. It acts on target cell either through cleaving caspases or damaging mitochondria thus mediating cell death [112]. Transcriptional regulation of granzyme B in CD8+ T cells is mediated by binding of transcriptional factors as CREB1, RUNX1, and AP1 to granzyme B promoter, harboring a DNase-hypersensitive region, enhancing the transcription [113–115]. Granzyme B expression is epigenetically controlled by histone H3K9 acetylation in its promoter resulting in high gene expression levels [113]. Another regulator of granzyme B expression is NF-κB, where, the activation of NF-κB signaling pathway results in binding of NF-κB to an enhancer element

downstream to granzyme B transcriptional start site inducing its expression [116–118].

In a study conducted by our research group, a potential role of insulin-like growth factor-1 (IGF-1) was highlighted in modulating cytolytic potential of NK cells of HCC patients. miR-486-5p acts in a cell-specific manner, differentially modulating IGF-1 expression in NK cells and their target hepatocytes with a contemporary inhibitory impact on HCC progression [119]. Moreover, in another study, NK cells of HCC patients showed miR-182 overexpression compared to controls. NKG2D and NKG2A were upregulated and downregulated, respectively, in HCC NK cells. Upon forcing miR-182 expression in the HCC NK cells, upregulation of both receptors was observed. Finally, miR-182 was reported to induce NK cell cytotoxicity represented in perforin-1 upregulation and increase in cytolytic killing of

Our research team reported a novel role of miR-615-5p in NK cells activity of HCC patients. We have previously described miR-615-5p as potent tumor suppressor microRNA in HCC via repressing a pivotal mitogen in HCC, Insulin-like growth factor-II (IGF-II) as well as cellular proliferation, viability, and migration [121]. Recently, our team proved an opposing function for miR-615-5p in the NK cells of HCC patients. Forcing the expression of miR-615-5p repressed insulin-like growth factor-type 1 receptor (IGF-IR), attenuated NKs cytotoxicity, decreased CD56dim, increased CD56bright NK subsets, and reduced the cytotoxic markers NKG2D, TNF-α, and perforins. It also repressed NKG2D ligand (ULBP2) in Huh-7 cells [122].

**6.2. Recent approaches in fine-tuning of natural killer cell function**

in induction of apoptosis.

40 Natural Killer Cells

co-cultured Huh-7 cells [120].

Hend Mohamed El Tayebi

Address all correspondence to: hend.saber@guc.edu.eg

Genetic Pharmacology Research Group, Department of Pharmacology and Toxicology, Faculty of Pharmacy and Biotechnology, German University in Cairo, Cairo, Egypt
