Study on the Effect and Mechanism of Lead on Bone Formation

Lead is a heavy metal harm to the health of human. It may have widely effect on nervous system, blood system, digestive system and urogenital system. Lead toxicity remains a serious public health hazard . Lead exposure to children and teenagers would affect the growth of physical and intelligence development.There is a close relationship between bioavailability of calcium and growth of skeleton, so maybe lead interfere the growth of skeleton intimately association with the effect of lead on calcium bioavailability? The skeleton serves as the major reservoir for ingested lead, accounting for 75-90% of body’s total lead burden, and 99% of body’s total calcium deposition in bone. So, it’s very interesting to investigate the interaction between lead and calcium, and the potential mechanism of lead on bone formation.The experiments presented here included three parts : First, The study of lead exposure to the children 2-7 years from different areas in ChongQing city. The calcium in blood, the growth of children and the syndrom of deficient of calcium（rickets） was observed. Second, Ablactation Wistar rat fed with different content of calcium diet. The growth of rat, the content of calcium and lead in bone, the microstructure of femur, the production and expression of OC, PTH and PTHr1 in blood were detected. ⁴⁵CACl₂ tracing was to study the effects of lead on calcium. At the last, Primary OB from cranium of newborn SD rat and UMR-106 exposed to 0-200μmol/L lead acetate. The proliferation and differentiation, the appearance and the marker, like OC, ALP and collagen I of OB and UMR106 were observed. Meanwhile, the expression of bFGF, cbfal/mnx2, c-fos, c-jun and activity of ERK1/2 by block agent PD98059 and/or lead were studied. The level of Ca²⁺ intracellular was measured with Fluo-3/AM by laser scanning confocal microscope （LSCM）.The main results and conclusions were summarized as follows:1. There was a difference remarkable in the level of blood lead and hair lead in children between industrial and non-industrial area, the growth of children differed too. The height and weight of children in non-industrial area was 106.11 cm and 18.63 kg, which was 103.74cm and 17.57kg in industrial area. The blood lead was 65.30μg/L and 75.30μg/L in nonindustrial area and industrial area respectively. The level of blood lead in industrial area was higher than nonindustrial area, the likelihood of hair lead, but the content of blod calcium was opposite to it. The value of OC in non-industrial area and industrial area were 11.22μg/L and 21.20μg/L. The level of phosphous in blood of industrial area children was higher than non-industrial area, but the percentage of [Ca]×[P]≥3.5 was more in non-industrial than industrial area. Taken together indicated that the difference of lead exposure in children from different area was significant, and consistent with the growth and development of children. Lead inhibited the growth and development of children.2. The growth and development of rat was inhibited exposed to lead. When exposed to lead 40d, the increase of weight, length of body and tail as followed: the rat in normal diet remained increase of these parameters, lead treatment diminished the increase, low calcium diet and lead treatment reduced most, while high calcium diet could extenuation the reduction of mead. The content of calcium in bone was highest in contol group, the next is high calcium group, and the lead group lower than high calcium group but higher than low calcium treatment rats. Moreover, the correlation between lead and calcium in bone was negative linearity.3. The development of skeleton of rat was inhibited by lead. The length and diameter of femur was smallest with low calcium and lead treatment, second was lead treatment, and the high calcium and lead treatment was smaller than the control group. The bone trabecula was less and scatter with lead and/or low calcium treatment, the density of cortical bone was lower too, and the number and the integrety of bone trabecula increased with high calcium ingestion.4. Lead inhibited the ingestion and utilization of calcium in vivo and in vitro. ⁴⁵Ca²⁺ in the fecal of lead group was far higher than control group. The level of calcium in bone declined exposed to lead and/or low calcium, while high calcium could extenuation the decrease. The tendence of lead in bone showed as opposite to calcium, and a negative correlation observed between lead and calium in bone. Moreover, LSCM reported that [Ca²⁺]_i elevated with lead exposure in UMR 106 rapidly.5. The expression of bone-related proteins exposed to lead were studied. The level of OC in blood declined with lead treatment regardless of sexuality, and which declined more in low calcium and lead group, the level of OC in high calcium and lead group higher than low calcium and/or lead group. Howerever, the content of PTH in serum raised by lead and/or low calcium exposure, and high calcium could mitigate this tendence. The inverse correlation between OC and PTH was observed. PTHrl mRNA levels in blood paralleled with the PTH activity induced by lead.6. The ultrastructure of OB and UMR 106 showed the damage of lead. The extent of damage raised with the increased content of lead, the structure of mitochondrium and reticulum damaged severely, the impairment of membrane system only in 200μmol/L lead to UMR106 and 100μmol/L lead to OB were observed.7. The appearance and the expression of marker of OB and UMR106 altered by lead. The ability of adherence of UMR106 decreased, granulation and vacuolus in OB with 50-100μmol/L lead exposure viewed. The expression of the marker of bone formation just like ALP and collagen I reduced accompanied with the concentration of lead increased.The secretion of OC of OB and UMR106 reduced by lead. The level of OC in medium decreased with lead treatment in a dose-dependent manner, but lead exposure resulted in no alteration in OC content both in OB and UMR106. The expression of bFGF, cbfal/osf2, c-fos and c-jtm which participated in the modulation of OC synthesis and secretion reduced too in a dose-dependent manner, while exposed to lead.Exposure to lead （1-200μmol/L） resulted in concentration dependent activation of p-ERK 1/2, which was significantly blocked by the ERK1/2 inhibitor PD98059. But there was no alteration of content of total ERK 1/2 with or without lead exposure.In summary, lead hindered the normal growth and development of skeleton. Low calcium treatment aggravated the effect of lead, and additional suitable calcium ingestion could lessen this inhibition of lead. The toxicity of lead on bone maybe partly due to the disorder of absorption and bioavailability of calcium, and the effect of lead on the expression of OC directly or/and indirectly maybe contribute to it too, which by activating the ERK 1/2 signal transduction pathway possiblly.