Effect of β-sitosterol, a plant sterol, on growth, protein phosphatase 2A, and phospholipase D in LNCaP cells

Previous work from this laboratory suggests an activation of the sphingomyelin cycle as a mechanism for growth inhibition with the incorporation of beta-sitosterol (SIT) into human prostate cancer LNCaP cells. In the present study we examined two key enzymes that have been shown to play a role in the sphingomyelin cycle. Dietary sterols (SIT and cholesterol) were compared for their effect on LNCaP cell growth, phospholipase D (PLD) activity, and protein phosphatase 2A (PP 2A) activity and expression. PP 2A has been suggested as a direct in vitro target of ceramide action on cell growth and apoptosis. Ceramide also inhibits phorbol myristate acetate-stimulated PLD. SIT (16 mu M) increased PP 2A activity by 50% compared with cholesterol treatment in LNCaP prostate cells; however, SIT did not alter protein levels of PP 2A. There was an increase in PLD activity in the presence of phorbol myristate acetate in cells supplemented with 16 mu M SIT compared with those supplemented with cholesterol after five days of treatment. The present study suggests that the activation of PP 2A added support to the role of the activation of the sphingomyelin cycle by SIT treatment. However, the increase in PLD activity which was modest but significant, with SIT supplementation suggests that this pathway may be modulated by other mechanisms. This includes the incorporation of SIT into cell membranes that may alter fluidity and, thus, influence the activation of membrane-bound enzymes such as PLD.