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Osteoporosis is associated with both atherosclerosis and vascular calcification. No mechanism yet explains the parallel progression of these diseases. Here, we demonstrate that osteoclasts (OCL) depend on lipoproteins to modulate cellular cholesterol levels and that this controls OCL formation and survival. Removal of cholesterol in OCL via high-density lipoprotein or cyclodextrin treatment dose-dependently induced apoptosis, with actin disruption, nuclear condensation and caspase-3 activation. One mechanism linked to the induction of OCL apoptosis was the cell-type-specific failure to induce HMG-CoA reductase mRNA expression, suggesting an absence of feedback regulation of de novo cholesterol biosynthesis. Furthermore, cyclodextrin treatment substantially suppressed essential M-CSF and RANKL-induced survival signaling pathways via Akt, mTOR and S6K. Consistent with these findings, cholesterol delivery via lowdensity lipoprotein (LDL) significantly increased OCL viability. Interestingly, OCLs from the LDL receptor (LDLR)À/À mouse exhibited reduced size and lifespan in vitro. Remarkably, LDLR þ / þ OCL in lipoprotein-deficient medium phenocopied LDLRÀ/À OCL, while fusion and spreading of LDLRÀ/À OCL was rescued when cholesterol was chemically delivered during differentiation. With hyperlipidemia being associated with disease of the vascular system and bone, these findings provide novel insights into the selective lipoprotein and cholesterol dependency of the bone resorbing cell.

Thyroid hormones influence growth and differentiation of bone cells. In vivo and in vitro data indicate their importance for development and maintenance of the skeleton. Triiodothyronine (T3) inhibits proliferation and accelerates differentiation of osteoblasts. We studied the regulatory effect of T3 on markers of proliferation as well as on specific markers of the osteoblastic phenotype in cultured MC3T3-E1 cells at different time points. In parallel to the inhibitory effect on proliferation, T3 down-regulated histone H4 mRNA expression. Early genes (c-fos/c-jun) are highly expressed in proliferating cells and are down-regulated when the cells switch to differentiation. When MC3T3-E1 cells are cultured under serum-free conditions, basal c-fos/c-jun expressions are nearly undetectable. Under these conditions, c-fos/c-jun mRNAs can be stimulated by EGF, the effect of which is attenuated to about 46% by T3. In addition, T3 stimulated the expression at the mRNA and protein level of osteocalcin, a marker of mature osteoblasts and alkaline phosphatase activity. All these effects were more pronounced when cells were cultured for more than 6 days. These data indicate that T3 acts as a differentiation factor in osteoblasts by influencing the expression of cell cycle-regulated, of cell growth-regulated, and of phenotypic genes.

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