sphingosine 1-phosphate ( s1p ) is a biologically active lysophospholipid which is produced by the metabolism of sphingolipid catalyzed by sphingosine kinase .
s1p is involved in various cell activities including cell differentiation , apoptosis , proliferation , migration , and others .
the roles of s1p have recently drawn attention as s1p has been discovered to be a critical regulator of bone metabolism by many studies .
first , s1p is released from osteoclasts and facilitates the proliferation , migration , and survival of osteoblasts by working as a coupling factor between osteoblasts and osteoclasts.[2 - 4 ] second , s1p mediates bone destruction by inducing the formation of osteoclasts .
third , s1p is more abundant in circulating blood than bone marrow cavity.[6 - 8 ] when the difference of s1p levels between those two increases , osteoclast precursors are limitedly present in bone marrow cavity , promoting bone resorption.[9 - 12 ] however , the role of s1p in humans and the relative importance of three mechanisms mentioned in the above have not been clarified yet . in relation to this , the authors of this study have recently reported that bone resorption is promoted and bone mineral density ( bmd ) decreases as plasma s1p levels become higher in the human body .
these previous findings imply that s1p is not only important to humans but also , involved in two mechanisms facilitating bone resorption by directly promoting bone resorption and maintaining constant concentration difference between circulation system and bone marrow cavity .
in addition , the findings suggest that one of the bone resorption mechanisms is more important than the other and the bone formation - related mechanism .
this investigation is a preliminary study investigating if which mechanism is more significantly involved between two bone resorption mechanisms .
the study aims to identify the differences in s1p levels by collecting bone marrow in patients who underwent surgeries due to osteoporotic fractures and causes with other than osteoporotic fractures .
this study involved 16 patients who underwent hip surgery in the department of orthopedic surgery , asan medical center from july 2012 to september 2012 .
we excluded patients who took osteoporosis medications affecting bone metabolism for six months , and patients with diseases that develop secondary osteoporosis including hyperthyroidism and others .
osteoporotic hip fracture excluded high - impact fractures induced by car accident and others , and four subjects fell under this category ( table 1 ) .
the study was carried out after gaining the institutional review board ( irb ) approval of asan medical center and written consent form of all subjects .
the study investigated sex , age , body weight , and height of all subjects and identified their diagnosis at the time of operation .
bone marrow collected during surgery was stored in -70 deep freezer by dividing into 200 l .
bone marrow samples were centrifuged at 3,000 rpm for 5 minutes at 4 to separate bone marrow fluids for the measurement of s1p levels ( mol / l ) . centrifuged bone marrow fluids were kept at -70 before the measurement .
s1p competitive enzyme - linked immunosorbent assay ( elisa ) kit ( echelon biosciences inc . ,
salt lake city , ut , usa ) was used for the measurement of s1p levels .
mol / l , and inter- and intra - measurement coefficients of variation were 6.4% and 6.1% , respectively .
bone marrow s1p level of each patient was measured twice and the average value was used for analysis .
serum calcium ( mmol / l ) and phosphorus levels ( mmol / l ) were measured by applying cresolphthalein comlexone method and phosphomolybdate ultraviolet ( uv ) method , respectively , using toshiba 200fr autoanalyzer ( toshiba medical systems co. , ltd . ,
glomerular filtration rate ( gfr ) ( ml / min/1.73 m ) was obtained using the cockroft - gault formula .
bmd ( g / cm ) was measured in lumbar spine and proximal femur ( femoral neck , total hip ) using dual energy x - ray absorptiometry ( lunar prodigy advance , ge lunar , medison , wi , usa ) .
the differences of mean values between two groups were analyzed using an independent samples t - test , and the adjustment of confounding factors was performed with the analysis of covariance .
partial correlation analysis was performed to verify the association bewteen bone marrow s1p levels and bmds , and three analysis models were established for the adjustment of confounding factors .
age and sex were included in model 1 , body weight and height were added in model 2 , and serum calcium , phosphorus levels and gfr were added in model 3 .
p - values of less than 0.05 were considered to be statistically significant in all statistical analysis .
this study involved 16 patients who underwent hip surgery in the department of orthopedic surgery , asan medical center from july 2012 to september 2012 .
we excluded patients who took osteoporosis medications affecting bone metabolism for six months , and patients with diseases that develop secondary osteoporosis including hyperthyroidism and others .
osteoporotic hip fracture excluded high - impact fractures induced by car accident and others , and four subjects fell under this category ( table 1 ) .
the study was carried out after gaining the institutional review board ( irb ) approval of asan medical center and written consent form of all subjects .
the study investigated sex , age , body weight , and height of all subjects and identified their diagnosis at the time of operation .
bone marrow collected during surgery was stored in -70 deep freezer by dividing into 200 l .
bone marrow samples were centrifuged at 3,000 rpm for 5 minutes at 4 to separate bone marrow fluids for the measurement of s1p levels ( mol / l ) . centrifuged bone marrow fluids were kept at -70 before the measurement .
s1p competitive enzyme - linked immunosorbent assay ( elisa ) kit ( echelon biosciences inc .
, salt lake city , ut , usa ) was used for the measurement of s1p levels .
mol / l , and inter- and intra - measurement coefficients of variation were 6.4% and 6.1% , respectively .
bone marrow s1p level of each patient was measured twice and the average value was used for analysis .
serum calcium ( mmol / l ) and phosphorus levels ( mmol / l ) were measured by applying cresolphthalein comlexone method and phosphomolybdate ultraviolet ( uv ) method , respectively , using toshiba 200fr autoanalyzer ( toshiba medical systems co. , ltd . ,
glomerular filtration rate ( gfr ) ( ml / min/1.73 m ) was obtained using the cockroft - gault formula .
bmd ( g / cm ) was measured in lumbar spine and proximal femur ( femoral neck , total hip ) using dual energy x - ray absorptiometry ( lunar prodigy advance , ge lunar , medison , wi , usa ) .
the differences of mean values between two groups were analyzed using an independent samples t - test , and the adjustment of confounding factors was performed with the analysis of covariance .
partial correlation analysis was performed to verify the association bewteen bone marrow s1p levels and bmds , and three analysis models were established for the adjustment of confounding factors .
age and sex were included in model 1 , body weight and height were added in model 2 , and serum calcium , phosphorus levels and gfr were added in model 3 .
p - values of less than 0.05 were considered to be statistically significant in all statistical analysis .
three patients ( 75% ) were females among 4 patients with osteoporotic hip fracture ( table 1 ) . among 12 patients who underwent operation due to causes other than osteoporotic hip fracture , 7 patients were osteoarthritis , 4 patients were avascular necrosis ( avn ) of the femoral head , and a patient was hip dislocation .
there were a male ( 25.0% ) and three female ( 75.0% ) patients with osteoporotic hip fractures .
mean age was 64.99.8 years , showing younger mean age ( p=0.607 ) , and bmi was 25.13.9 kg / m , showing a higher tendency ( p=0.050 ) .
the average bone marrow s1p level of all subjects was found to be 3.401.10 mol / l ( range : 2.08 - 5.74
we compared bone marrow s1p levels of two groups according to the presence of osteoporotic hip fractures ( fig .
bone marrow s1p levels were 2.480.38 mol / l in patient group with osteoporotic hip fracture , and 3.701.09 mol / l in control group .
bone marrow s1p level was significantly low in patients with osteoporotic hip fracture ( p=0.047 ) ( fig .
bone marrow s1p levels were 2.311.94 mol / l in patient group with osteoporotic hip fracture and 3.761.10 mol / l in control group , showing significance ( p=0.025 ) ( fig .
when we analyzed the correlation between bone marrow s1p level and bmd , statistically significant correlation was not observed after the adjustment of several confounding factors ( =0.026 - 0.482 , p=0.171 - 0.944 ) ( table 2 ) .
this study identified that the bone marrow s1p levels were significantly lower in patients with osteoporotic hip fractures than in those without .
no studies have been performed to verify the association between the presence of osteoporosis and osteoporotic fracture by clinically measuring s1p levels in the bone marrow of humans .
the investigation is the first study to observe the positive effects on bone metabolism of high s1p levels in bone marrow , contradicting previous in vivo studies that suggested that s1p directly increases osteoclast differentiation and bone resorption . in a recent study
, the authors have verified that bmds decrease and bone resorption markers increase as plasma s1p levels become higher in all patients including men , and pre- and postmenopausal women .
in contrast , this investigation found the reduction in the prevalence of osteoporotic fracture as bone marrow s1p levels increase .
thus , the risk of fracture may increase as s1p levels rise in peripheral blood .
however , the risk of fracture may be reduced when bone marrow s1p level is higher . to sum up the above study results , s1p level is not directly involved in bone cells , instead bone resorption is accelerated as the concentration difference of s1p between peripheral blood and bone marrow cavity increases , supporting the hypothesis .
high plasma s1p concentration is maintained as s1p is consistently released by vascular endothelial cells , red blood cells ( rbcs ) , platelet , and others . on the other hand , concentration difference between plasma and tissue is formed by maintaining low s1p levels with constant breakdown catalyzed by intracellular s1p lyase or s1p phosphatase in each tissue including bone marrow . the s1p receptor type 2 ( s1pr2 ) expression of osteoclast precursors is dominant in peripheral blood with high s1p level , and it promotes the migration of osteoclast precursors to bone marrow by triggering the signaling pathways of g12/13-mediated rho activation . once osteoclast precursors migrate into bone marrow , they are differentiated to osteoclasts .
therefore , the migration of osteoclast precursors to bone surface is accelerated as the difference of s1p level between peripheral blood and bone marrow is higher , leading to higher risk of fracture due to increased bone resorption .
this study failed to demonstrate significant association between bone marrow s1p levels and bmd values .
however , the study involved a relatively small number of subjects , measuring bmds only in 14 patients .
taking this fact into consideration , the results could not be interpreted that s1p is not taking a crucial role in bone metabolism .
there is also limitation in generalizing the study results obtained from small sample size of 16 patients .
furthermore , the concentration difference between peripheral blood and bone marrow is more critical factor involved in osteoclast migration rather than s1p present in bone marrow .
however , the study was unable to measure s1p levels in the blood of same patient .
therefore , this investigation is considered as a preliminary study suggesting the conduction of clinical trials in the future .
this study verified that bone marrow s1p level was lower in patient group with osteoporotic hip fractures compared with control group .
these suggest that the difference of s1p levels between peripheral blood and bone marrow is critical on bone metabolism .
 backgroundsphingosine 1-phosphate ( s1p ) has been discovered to be a critical regulator of bone metabolism . 
 very recently , we found that higher circulating s1p levels were associated with higher rate of prevalent osteoporotic fracture in human.methodsthis was a cross - sectional study of 16 patients who underwent hip replacement surgeries . 
 bone marrow fluids were obtained during hip surgeries , and the s1p levels were measured using a competitive enzyme - linked immunosorbent assay ( elisa ) assay . bone mineral densities ( bmds ) at various skeletal sites were obtained using dual energy x - ray absorptiometry.resultsamong 16 patients , 4 patients were undergone operations due to hip fractures , and the others were done by any other causes . 
 bone marrow s1p levels were significantly lower in patients with hip fractures than in those without , before and after adjusting for confounding factors ( p=0.047 and 0.025 , respectively ) . 
 we failed to demonstrate significant associations between bone marrow s1p levels and any bmd values ( =0.026 - 0.482 , p=0.171 - 0.944).conclusionsin conjunction with our previous findings , these suggest that the effects of gradient between peripheral blood and bone marrow , but not s1p itself , may be the most critical on bone metabolism . 