An Unanticipated Role for Sphingosine Kinase-2 in Bone and in the Anabolic Effect of Parathyroid Hormone

Abstract

Sphingosine-1-phosphate (S1P) is an anabolic clastokine. SPHK is the rate-limiting enzyme in S1P production and has two isoforms. To evaluate the roles of SPHK1 and SPHK2 in bone, we examined the skeletal phenotype of mice with selective deletion of SPHK1 in osteoclasts (SPHK1-Oc -/-) and mice in which the SPHK2 gene was deleted in all tissues (SPHK2 -/-). SPHK1-Oc -/- had normal bone mass. By contrast, SPHK2 -/- female mice had a 14% lower spinal BMD (p<0.01) and males a 22% lower BMD at the same site (p<0.001). SPHK2 -/- and control mice were subsequently treated either with daily PTH(1-34) or vehicle for 29 days. The response to PTH was significantly attenuated in the SPHK2 -/-mice. The mean femoral BV/TV increased by 24.8% in the PTH-treated female control animals vs 10.6% in the vehicle-treated female controls (p <0.01). In contrast, in the SPHK2 -/- female mice the difference in femoral trabecular BV/TV at the end of treatment was not significant (20.5 vs.13.3%, PTH vs. vehicle, p = NS). The anabolic response to PTH was significantly attenuated in the spine of male SPHK2 -/- mice (29.7% vs. 23.1%, PTH vs. vehicle, in controls, p <0.05; 26.9% vs. 19.5% PTH vs. vehicle in SPHK2 -/- mice, p = NS). The spine responded normally in the SPHK2 -/- female mice. Interestingly, suppression of Sost was blunted in the SPHK2 -/- mice when those animals were treated with an anabolic PTH regimen. We conclude that SPHK2 has an important role in mediating both normal bone remodeling and the anabolic response to PTH.

The Species Effect: Differential Sphingosine-1-Phosphate Responses in the Bone in Human Versus Mouse

“…Indeed, our data would support this with the increased expression of Sphk2 in murine osteoclasts upon differentiation. Surprisingly, Sphk2 −/− osteoclasts exhibited normal resorptive activity in vitro, despite global Sphk2 −/− mice showing reduced trabecular bone mass [23]. It could be argued that the in vivo phenotype is due to the importance of Sphk2 osteoblast function.…”

The Ying and Yang of Sphingosine-1-Phosphate Signalling within the Bone

“…Indeed, alterations in SPHK1 expression during osteoblast differentiation have been linked to the activation of S1PR1 signalling [ 23 ]. To the best of our knowledge, no studies have investigated the impact of intracellular osteoblastic SPHK1 and/or S1P signalling; however, it has been shown that the osteoclast-specific deletion of SPHK1 has no effect on bone mass in male or female mice [ 26 ]. Similarly, siRNA knockdown of SPHK1 in a murine macrophage cell line (RAW264) or in primary murine bone-marrow-derived macrophages had no effect on osteoclast maturation in response to RANKL [ 24 ].…”

“…The inhibition of SPHK2 in the murine macrophage cell line RAW264 resulted in a reduction in c-FOS expression and subsequent osteoclastogenesis [ 27 ], identifying intracellular SPHK2 signalling as essential during the initial stages of osteoclastogenesis. In contrast, SPHK2 −/− osteoclasts had normal resorptive activity in vitro, despite the observation that global SPHK2 −/− mice have reduced trabecular bone mass [ 26 ]. This finding led to the in vivo phenotype being attributed to alterations in osteoblast formation, which was demonstrated by a reduction in collagen 1 gene expression and a significant impairment of the anabolic response to PTH within these mice [ 26 ].…”

“…In contrast, SPHK2 −/− osteoclasts had normal resorptive activity in vitro, despite the observation that global SPHK2 −/− mice have reduced trabecular bone mass [ 26 ]. This finding led to the in vivo phenotype being attributed to alterations in osteoblast formation, which was demonstrated by a reduction in collagen 1 gene expression and a significant impairment of the anabolic response to PTH within these mice [ 26 ]. Crucially, the impact of SPHK2 on PTH signalling was not determined, making it difficult to draw any firm conclusions about the exact role of intracellular S1P signalling in this process.…”

Synthesis of PP2A-Activating PF-543 Derivatives and Investigation of Their Inhibitory Effects on Pancreatic Cancer Cells

“…The growth of cancer cells has been reported to be suppressed when both the SK isotypes are inhibited. These isotypes are known to act by being located in different parts of the cell [ 7 , 8 , 9 ]. S1P generation, according to the location of SK1 and SK2, plays an important role in inducing the survival of cancer cells in specific oncogenic signaling pathways [ 10 ].…”