Selective Role of Mevalonate Pathway in Regulating Perforin but Not FasL and TNFalpha Release in Human Natural Killer Cells

We have analyzed the effects of fluvastatin, an inhibitor of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase involved in mevalonate synthesis, on human NK cell-mediated anti-tumor cytolysis. Fluvastatin inhibited the activation of the small guanosin triphosphate binding protein (GTP) RhoA and the consequent actin redistribution induced by ligation of LFA1 involved in NK-tumor target cell adhesion. Also, fluvastatin reduced ganglioside M1 rafts formation triggered through the engagement of NK cell activating receptors as Fc gamma RIIIA (CD16), NKG2D and DNAM1. Cytolysis of tumor targets was inhibited up to 90% when NK cells were cultured with fluvastatin by affecting i) receptor-mediated increase of the intracellular free calcium concentration, ii) activation of akt1/PKB and iii) perforin and granzyme release. Fluvastatin displayed a stronger inhibiting effect on NKG2D, DNAM1, 2B4, NKp30, NKp44 and NKp46 than on CD16-mediated NK cell triggering. This was in line with the impairment of surface expression of all these receptors but not of CD16. Remarkably, fluvastatin did not affect the expression of the inhibiting receptors CD94, KIR2D and LAIR1. FasL release elicited by either NK-tumor cell interaction or CD16 or NKG2D engagement, as well as FasL-mediated killing, were not sensitive to fluvastatin. Moreover, TNF alpha secretion triggered in NK cells upon incubation with tumor target cells or engagement of NKG2D receptor was not impaired in fluvastatin-treated NK cells. Likewise, antibody dependent cellular cytotoxicity (ADCC) triggered through Fc gamma RIIIA engagement with the humanized monoclonal antibody rituximab or trastuzumab was only marginally affected in fluvastatin-treated NK cells. Altogether these findings suggest that interference with mevalonate synthesis impairs activation and assembly of cytoskeleton, degranulation and cytotoxic effect of perforins and granzyme but not FasL- and TNF alpha-mediated cytotoxicity.