KLRK1 and hepatocellular carcinoma: In addition, the expression of NK-activating receptors NKG2D, NKp30 and NKp44 on the surface of NK cells significantly decreased with HIF-1α expression (Fig. 2), which was implicated in the decline of an apoptotic effect of NK cells on HCC cells through a decrease in the cleavage of caspase-3, -7, -8, and -9 (Fig. 4).