LDL induces Saos2 osteoblasts death via Akt pathways responsive to a neutral sphingomyelinase inhibitor

Abstract

Atherosclerosis is epidemiologically associated with postmenopausal osteoporosis (OP) presumably by common etiologic factors, reflecting a state of co-morbidity in aging. Osteoblasts make a significant facet of this co-morbidity state. Since oxidized low-density lipoprotein (oxLDL) is a major factor in generation of vascular wall pathology, we examined the ability of native LDL (nLDL) and oxLDL to induce Saos2 osteoblasts growth arrest. OxLDL induced Saos2 cell death with morphological features of apoptosis that was inhibited mainly by caspase-9 and partially by caspase-3 but not by caspase-8 inhibitors. nLDL, like oxLDL, has induced cell death, where 60% (P = 0.00033) and 30% (P = 0.075, ns) of the cell death, respectively, could be inhibited by scyphostatin (a neutral sphingomyelinase [nSMase] inhibitor). Upon similar condition, nLDL inhibited the phosphorylation of Akt and two of its downstream targets, fork head receptor (FKHR) and glycogen synthase kinase-3 (GSK3). This is a pathway that stimulates cell survival and proliferation. nLDL has also induced an increase in the proapoptotic Bcl-Xs and it has diminished the potential antiapoptotic Src kinase activity. At the 4 h time-point, upon a substantial decrease in nLDL-induced Akt phosphorylation, scyphostatin has inhibited the reduction in FKHR and GSK3 phosphorylation but inexplicably not that of Akt. Scyphostatin has also corrected the reduction in Src kinase activity. Taken together, the results indicate that nLDL has induced apoptosis in Saos2 osteoblasts by inactivation of the pathway downstream to Akt using nSMase, and by involvement of Src kinase. Inferring that caspase-9 was the main executioner (rather than caspase-8 and-3) in Saos2 cell death, indicates that the nSMase-induced release of ceramide, directly activated the intrinsic mitochondrial apoptotic pathway. With regard to the Akt inactivation by nLDL, Saos2 osteoblasts responded in an opposite fashion to the response reported by others, in macrophages.

Adrenocortical LDL receptor function negatively influences glucocorticoid output

“…The small molecule sunitinib is a potent multi-targeted receptor tyrosine kinase inhibitor (O'Farrell et al 2003, Sun et al 2003, suggesting that changes in intracellular signaling pathways are able to modulate the HSD3B2 transcription rate. Studies in respectively cultured mesangial cells (Jenkins et al 2000) and osteoblasts (Klein et al 2006) have indicated that exposure to native LDL -the normal substrate of the LDLR -increases the activity (phosphorylation state) of MAPK and tuberin and lowers Src protein levels and Akt activity. These combined findings suggest that, by changing the activity of major intracellular signaling pathways, enhanced adrenal uptake of native LDL from the plasma compartment might ultimately decrease the HSD3B2 expression in this tissue.…”

HDL3 reduces the association and modulates the metabolism of oxidized LDL by osteoblastic cells: A protection against cell death

“…The activation of this survival pathway by HDL leads also to the inhibition of the apoptosis of endothelial cells by inhibiting the mitochondrial apoptosis pathway [Nofer et al, 2001;von Eckardstein et al, 2005]. Moreover, Klein et al [2006] have also demonstrated that native LDL induce SaOS cell death by the inhibition of AKT, suggesting that by their ability to activate this pathway, HDL could counteract this effect.…”

Diet-derived fruit and vegetable metabolites suggest sex-specific mechanisms conferring protection against osteoporosis in humans

“…The current study suggests that greater FV intakes may modulate STS activity and promote a healthy bone phenotype. However, current data do not clarify if reduced inhibition of STS via lower deoxycholate in women with high FV intake directly leads to greater STS production and/or activity (Supplemental Table 8 61 . Lower threonate in OS women could relate to tissue mineralization and bone resorption 62,63 .…”