Iron excess upregulates SPNS2 mRNA levels but reduces sphingosine-1-phosphate export in human osteoblastic MG-63 cells.

We aimed to study the mechanisms involved in bone-related iron impairment by using the osteoblast-like MG-63 cell line. Our results indicate that iron impact the S1P/S1PR signalizing axis and suggest that iron can affect the S1P process and favor the occurrence of osteoporosis during chronic iron overload.

INTRODUCTION: Systemic iron excess favors the development of osteoporosis, especially during genetic hemochromatosis. The cellular mechanisms involved are still unclear despite numerous data supporting a direct effect of iron on bone biology. Therefore, the aim of this study was to characterize mechanisms involved in the iron-related osteoblast impairment.

METHODS: We studied, by using the MG-63 cell lines, the effect of iron excess on SPNS2 gene expression which was previously identified by us as potentially iron-regulated. Cell-type specificity was investigated with hepatoma HepG2 and enterocyte-like Caco-2 cell lines as well as in iron-overloaded mouse liver. The SPNS2-associated function was also investigated in MG-63 cells by fluxomic strategy which led us to determinate the S1P efflux in iron excess condition.

RESULTS: We showed in MG-63 cells that iron exposure strongly increased the mRNA level of the SPNS2 gene. This was not observed in HepG2, in Caco-2 cells, and in mouse livers. Fluxomic study performed concomitantly on MG-63 cells revealed an unexpected decrease in the cellular capacity to export S1P. Iron excess did not modulate SPHK1, SPHK2, SGPL1, or SGPP1 gene expression, but decreased COL1A1 and S1PR1 mRNA levels, suggesting a functional implication of low extracellular S1P concentration on the S1P/S1PR signalizing axis.

CONCLUSIONS: Our results indicate that iron impacts the S1P/S1PR signalizing axis in the MG-63 cell line and suggest that iron can affect the bone-associated S1P pathway and favor the occurrence of osteoporosis during chronic iron overload.