Adverse Bone Effects Of Occupational Exposure To Lead

Lead(Pb) is one of metal element which has been considered as a environmental harmful pollutant of world wide concern.Lead can cause toxicity on several organs in human.Lead is a toxicant most easily accumulated and deposited over time in bone with a half-life of about 20 years.Bone is one target organ of lead and osteoporosis is the most familiar syndrome.The purposes of the present study are:1) to investigate the effects of occupational lead exposure on bone mineral density,bone metabolism,osteoporosis, lumbar vertebral fracture using the method of epidemiological investigation and the dose-response relationship between lead exposure and the toxicity effects on bone using the method of benchmark dose(BMD).2) to examine a possible relationship between lead nephropathy and its effects on the skeleton and the relationship betweenδ-aminolevulinic acid dehydratase(ALAD) gene polymorphism and lead toxicity on bone.and 3) to investigate the possible molecular mechanisms on osteoblasts using in vitro experiments of lead on the osteoblasts from calvarias of newborn SD rat.The target population comprised people who worked in a storage battery plant in Shanghai.The exposure group was consisted of the workers working in the workshop, who were occupationally exposed to lead for more than one year.The control group consisted of the office faculty,who were not exposed to lead at work.The exposure group included 298 subjects(male 254,female 44),and the control group included 81 subjects(male 56,female 25).The average length of lead works in the exposure group is 18.1 years(1～40 years).Two groups are comparable in age,weight,height,body mass index(BMI),smoking and drinking habits.We used the portable single-photon absorptiometry(SPA) to measure the bone mineral density of forearm radius in each subjects of study population.A socalled Z score recommended by WHO was used to define osteoporosis.Blood lead(BPb) and urinary lead(UPb) were measured as exposure biomarkers.Serum osteocalcin(BGP), serum alkaline phosphatase(ALP),serum bone specific alkaline phosphatase(BALP), urinary hydroxyproline(HYP),urinary N-acetyl-β-D-glucosaminidase(NAG), urinary albumin(ALB) and lumbar vertebral X-ray were also measured as effect biomarkers.We estimated benchmark dose(BMD) and the lower confidence limit of the benchmark dose(BMDL) for the adverse effects of bone and renal systems.We measured the ALAD genotype for each subject.Effects of lead on bone mineral density and bone metabolism:It was showed that BPb and UPb levels in the exposure group were significantly higher than those in the control group in males.The average BMD levels in the exposure group were lower than those in control group with no statistically significant differences in both genders. The values of BGP,HYP,ALP,BALP in the exposure group were higher than those in control group,and there were statistically significant differences in males for HYP, ALP,BALP.We defined the abnormal value using the 90%upper limits of each index of control group.The abnormal rates of ALPand BALP were significantly higher in exposure group of male and total population.After dividing the participants into several groups according to UPb and BPb levels,respectively,bone mineral density significantly decreased when BPb was higher than 30μg/dl;BGP significantly increased when BPb was higher than 20μg/dl;HYP,ALP,BALP significantly increased when BPb was higher than 10μg/dl;bone mineral density significantly decreased when UPb was higher than 5μg/g Cr;HYP significantly increased when UPb was higher than 5μg/g Cr;ALP,BALP significantly increased when UPb was higher than 10μg/g Cr;BGP significantly increased when UPb was higher than 20μg/g Cr.The prevalence of osteoporosis and the bone metabolism indexes were significantly increasing following the increase of BPb and UPb.After benchmark dose calculation for Z score and indexes of bone metabolisms,we could find ALP and BALP had the smallest values.The difference between male and female was the sequence between Z score and BGP.The difference of the values of BMDL between BPb and UPb was the sequence of BGP.Compared with the indexes of bone metabolism,Z score had higher BMDL values.Effects of lead on lumbar vertebral fracture:It was showed that the prevalence of lumbar vertebral fracture in exposure group was significantly higher than that in control group.The prevalence of lumbar vertebral fracture was significantly increasing following the increase of BPb and UPb in male and total population.The prevalence of lumbar vertebral fracture was significantly increasing following the decrease of bone mass in male and total population.Effects of lead on renal function and the associations with bone effects:It was showed that UNAG and UALB levels in the exposure group were significantly higher than those in the control group in males.The abnormal rates of UNAG and UALB were significantly higher in exposure group.UNAG significantly increased when BPb was higher than 10μg/dl;UALB significantly increased when BPb was higher than 20μg/dl;UNAG and UALB significantly increased when UPb was higher than 5μg/g Cr.The abnormal rates of UNAG and UALB were significantly increasing following the increase of BPb and UPb.After benchmark dose calculation for UNAG and UALB,the BMDL values of UNAG were smaller than UALB,and Z score was ranged between UNAG and UALB.We also found the renal dysfunction might develop earlier than osteoporosis.Osteoporosis caused by lead was related to renal dysfunction.With respect to renal dysfunction,osteoporosis was especially related to tubular damage but not to glomerular damage.Study on ALAD gene polymorphism:It has been proved that the susceptibility to lead toxicity is related toδ-ALAD gene polymorphism.Polymerase chain reaction-restriction fragment length polymorphism(PCR-RFLP) techniques were used in this study to analyseδ-ALAD genotype.The results revealed that the frequencies of ALAD_1-1 and ALAD_1-2 genotypes were 91.29%and 8.71%, respectively.No ALAD_2-2 homozygote was found.The frequencies of alleles of ALAD₁ and ALAD₂ were 95.65%and 4.35%,respectively.In the exposure group, BPb,UPb,BGP,HYP,ALP,BALP with ALAD_1-2 genotype were obviously higher than those of ALAD_1-1 genotype;bone mineral density with ALAD_1-2 genotype were significantly lower than that of ALAD_1-1 genotype.While in the control group,there was no remarkable difference between the two genotypes in blood lead,urinary lead and the indexes of bone metabolism.The absolute values of the linear regression coefficient of the indexes of bone metabolism with ALAD_1-2 genotype were larger than those of ALAD_1-1 genotype.In addition,the osteoblasts were obtained from calvarias of newborn SD rat and treated with lead acetate in order to understand the possible mechanisms on osteoblasts.In vitro experiments showed that lead acetate could decrease osteoblastic proliferation and the activity of ALP when the dose of lead acetate was higher than 50μmol/L.Lead acetate couldn't induce the expression of ODF while low level(1, 5μmol/L) lead acetate could up-regulate gene expression of OPG and VDR.High level lead acetate(＞50μmol/L) could repress the gene expression of OPG,VDR, TGF-βand IGF-I.In conclusion,high level of lead exposure could decrease bone mineral density, induce osteoporosis and affect bone metabolism.Occupational lead exposure could increase the prevalence of lumbar vertebral fracture.The occurrence of lumbar vertebral fracture might be associated with the decrease of bone mass.Osteoporosis might occur later than renal damage related to lead exposure.Osteoporosis caused by lead was related to renal dysfunction.With respect to renal dysfunction,osteoporosis was especially related to tubular damage but not to glomerular damage.ALAD gene, a susceptible gene to lead toxicity,may increase potential risk of lead toxicity at certain level of lead exposure.The degree of BPb and UPb level that is affected by ALAD gene polymorphism increases along with the rising level of lead exposure. ALAD_1-2 genotype carriers had a high risk of osteoporosis and fracture than ALAD_1-1 genotype.Low level lead could stimulate some cytokines expression in osteoblast and when the level increased,lead became more and more toxic.