Palmitate attenuates osteoblast differentiation of fetal rat calvarial cells

Abstract

Aging is associated with the accumulation of ectopic lipid resulting in the inhibition of normal organ function, a phenomenon known as lipotoxicity. Within the bone marrow microenvironment, elevation in fatty acid levels may produce an increase in osteoclast activity and a decrease in osteoblast number and function, thus contributing to age-related osteoporosis. However, little is known about lipotoxic mechanisms in intramembraneous bone. Previously we reported that the long chain saturated fatty acid palmitate inhibited the expression of the osteogenic markers RUNX2 and osteocalcin in fetal rat calvarial cell (FRC) cultures. Moreover, the acetyl Co-A carboxylase inhibitor TOFA blocked the inhibitory effect of palmitate on expression of these two markers. In the current study we have extended these observations to show that palmitate inhibits spontaneous mineralized bone formation in FRC cultures in association with reduced mRNA expression of RUNX2, alkaline phosphatase, osteocalcin, and bone sialoprotein and reduced alkaline phosphatase activity. The effects of palmitate on osteogenic marker expression were inhibited by TOFA. Palmitate also inhibited the mRNA expression of fatty acid synthase and PPAR gamma in FRC cultures, and as with osteogenic markers, this effect was inhibited by TOFA. Palmitate had no effect on FRC cell proliferation or apoptosis, but inhibited BMP-7-induced alkaline phosphatase activity. We conclude that palmitate accumulation may lead to lipotoxic effects on osteoblast differentiation and mineralization and that increases in fatty acid oxidation may help to prevent these lipotoxic effects.

IGF-1 Receptor Insufficiency Leads to Age-Dependent Attenuation of Osteoblast Differentiation

“…These results are consistent with the current idea that fatty acids inhibit osteogenesis and that age-related declines in bone formation and increased risk for osteoporosis are due to replacement of bone forming cells with adipose tissue (31,32). Congruent with this idea is our previous observations that the long chain saturated fatty acid palmitate reduced mRNA expression of Runx2, AP, OCN, and BSP as well as the BMP-7-stimulated mineralized bone formation in fetal rat calvaria cell cultures (17,27). In the current study, we found that palmitate inhibited expression of osteoblast markers in general in cells from both wild-type and Igf1r ϩ/Ϫ male mice.…”

“…Studies from other laboratories have indicated that the inhibition of triglyceride accumulation may prevent the deleterious effects of fatty acids on bone cells (35,36). Moreover, the AMP kinase activator 5-aminoimidazole-4-carboxamide-1 beta-riboside prevented palmitate-induced apoptosis in fetal osteoblast cells (17). However, this effect did not appear to require increased fatty acid oxidation.…”

“…Female Igf1r ϩ/Ϫ mice exhibited an increased susceptibility to high fat diet-induced insulin resistance suggesting aberrant fatty acid metabolism in Igf1r ϩ/Ϫ mice (10). Lipotoxicity has been proposed as a contributing factor to osteoporosis (11) and cell culture studies have shown that fatty acids can decrease osteoblast survival and differentiation (12)(13)(14)(15)(16)(17). We postulate therefore that Igf1r ϩ/Ϫ mice may exhibit increased sensitivity to fatty acid-induced attenuation of osteoblast differentiation.…”

“…BMCs were collected by centrifugation of the femurs and cultured in ␣ Minimum Essential Medium with 15% fetal bovine serum, plus 100 U/mL of penicillin and 100 g/mL of streptomycin sulfate, at 37°C with 5% CO 2 as previously described (17,25). To study the effect of palmitate, BMC cultures cells were cultured in medium as described above plus 5mM ␤-glycerol phosphate and 100-g/mL ascorbic acid and in the absence or presence of palmitate (100M).…”

Therapeutic potentials and modulatory mechanisms of fatty acids in bone

“…Palmitate also promotes apoptosis of osteoblasts through both extrinsic and intrinsic pathways, and PA‐induced high expression of cytosolic cytochrome C could be disrupted by inhibition of c‐Jun N‐terminal kinase (JNK) . In foetal rat calvarial cell cultures, palmitate affects neither proliferation nor apoptosis of calvarial cells but represses BMP‐7‐induced osteoblastic differentiation by reducing the activity of transcription factor SMAD, and thus further abrogating expression of osteogenic markers Runx2, osteocalcin, alkaline phosphatase and bone sialoprotein . Interestingly, enhancing fatty acid oxidation could block all lipotoxic effects of palmitate suggested above, indicating that fatty acid oxidation might relieve the negative effects of palmitate on osteoblasts …”