Iron is an essential trace element that is required for the growth and survival of the body, and plays an important role in the whole life process of the body. The liver is the master storage place of iron and the master regulator of iron homeostasis. When the body is lack of iron, the development of the liver is slowed down, the structure of mitochondria and microsomes is abnormal, and the content of cytochrome P450 (CYP450) enzymes is reduced, which may even affect the biological metabolism process[25-26]. Any degree of iron deficiency can have an effect on health. Similarly, iron load can also damage health. Excessive iron can catalyze the production of reactive oxygen species through Fenton / Haber Weiss reaction. The accumulation of reactive oxygen species may lead to DNA damage, cell cycle arrest and mitochondrial dysfunction[28-29]. Increasing numbers of observational studies have shown that there is an association between VD and iron, and the specific mechanism has not been fully elaborated.
VD is mainly synthesized by ultraviolet rays, and environmental factors affect the seasonal fluctuation of VD concentration. The most classic role of VD is to regulate calcium and phosphorus metabolism and promote bone development. As research progresses, iron homeostasis also has a potential impact on bone formation, bone resorption and bone development. The serum 1,25-(OH)2D3 concentration reduced after treatment the dietary iron deficiency in rat, based on the effect of iron deficiency on bone metabolism. SD rats were injected with iron dextran (250mg/kg of body weight) every other day for 5 weeks, and iron loading caused bone injury. These studies revealed that iron deficiency or iron loading caused reduced VD activity levels, and this chain reaction could further exacerbate bone diseases. In addition, dietary red meat intake is considered to be an important factor in serum 25-(OH)D3. Portela stated that the red meat intake group had higher serum 25-(OH)D3 levels than the fish intake group. Because red meat with heme iron, dietary intake may promote the level of VD. Iron deficiency interferes with the transformation of VD in skin and intestine and metabolism in the body. These studies indicate that there is a certain correlation between iron and VD.
The liver and kidney are the main sites for the hydroxylation of VD, and the enzymes of VD hydroxylation are mediated by CYP450. CYP450 enzyme is a member of the heme-containing monooxygenase family. Mitochondrial P450 is located in the inner membrane and participated in the biosynthesis of bile acids, from cholesterol in the liver, steroid hormones in the adrenal and gonads, and metabolic activation of VD3 into the active form of 1,25-(OH)2D3 in the liver and kidney. The selective hydroxylation of VD3 by VD3 hydroxylase requires the cofactor NADH or NADPH as an electron donor. NADH or NADPH provides electrons to ferredoxin reductase, then donates it to ferredoxin, and then delivers them to CYP450 enzyme completes the selective hydroxylation of the substrate VD3 through a cascade of redox processes. Iron is the essential element of CYP450 function[37-38]. Iron deficiency may affect the function of these enzymes and may lead to VD deficiency. IDA has been demonstrated to reduce the activity of iron-containing enzymes in liver mitochondria, leading to impaired oxygen metabolism. In addition, the mitochondria of iron-deficient cells exhibit reduced cytochrome concentrations. These studies indicate that iron may play an important role in the hydroxylation process of VD, but the precise mechanism is not clear. This study proposed that iron may affect the hydroxylation level of VD through the influence of hydroxylase. In this study, an IDA model of SD rats was established with iron-deficient feed, focusing on the effects of different doses of iron on the serum 25-(OH)D3 and 1,25-(OH)2D3 levels in SD rats. Katsumata found that iron deficiency reduced the iron concentration in the kidney, further reducing the activity of 1α-hydroxylase, resulting in a decrease in serum 1,25-(OH)2D3 concentration in an iron-deficiency animal model. Heldenberg’s study on infants and young children with IDA, although supplemented with adequate VD3, the serum 25-(OH)D3 concentration of the children was still very low, and the serum 25-(OH)D3 concentration returned to normal after iron supplementation. In our experiment, iron deficiency led to the iron content of liver and kidney of SD rats decreased. The levels of serum 25-(OH)D3 and 1,25-(OH)2D3 in DFe group were significantly lower than those in C group. The levels of serum 25-(OH)D3 and 1,25-(OH)2D3 gradually increased with the increase in iron concentration. It is suggested that iron can promote the metabolic level of VD3 and convert it into an active form of 1,25-(OH)2D3 in the body. In this process, iron is used by the body and the level of VD3 activity increases.
Other studies have shown that iron loading also affects the levels of 25-(OH)D3 and 1,25-(OH)2D3. Napoli found that adults suffering from thalassemia had lower serum 25-(OH)D levels. Otto-Duessel observed that the lack of VD in iron overload disease was related to the accumulation of liver iron. Excessive iron accumulation in the kidney would affect VDR, which would aggravate iron deposition and cause damage to the proximal tubules of the kidney. In our study, the serum 25-(OH)D3 and 1,25-(OH)2D3 levels in the HFe group were not found to be inhibited. It possibly because of the IDA model built in the early stage that the iron consumption in the rat body took too long. Iron only satisfied the needs of pre-consumption and growth and development requirements, and could not accumulate too much in the body, and thus did not inhibit the serum 25-(OH)D3 and 1,25-(OH)2D3 levels. We will extend the intervention period in future experiments to observe the effect of iron overload on VD.
25-(OH)D3 is the major circulating form of VD, and the concentration of 25-(OH)D3 in serum is usually used to assess the status of VD. However, Worf stated that iron intervention in women with IDA did not significantly improve the serum 25-(OH)D3 status. After intravenous injection of iron, there was no effect on the level of 1,25-(OH)2D3 .On the one hand, it could be explained as the severity of IDA. On the other hand, this could be explained by the severe lack of VD in IDA women, and the level of hydroxylation is not sufficient to maintain the body's utilization, which leads to unsatisfactory improvement of VD status after iron intervention. Pandey showed in a small-scale population study that the serum 25-(OH)D3 level of the iron and VD supplementation group was preferable to that of the iron supplementation group alone. The level of serum 25-(OH)D3 increased after the intervention of iron by Min and Panpan in our country[49-50]. The research results are controversial, the specific mechanism of iron's effect on VD is still unclear, and further experiments are needed to verify it.
Our results show that compared with C group, iron-deficient rats weighed less after 10 weeks, and the growth rate of rats in the DFe group was slower than that of other groups. Our study found that iron deficiency reduced the iron concentration in the liver and kidney, and the SI concentration was increased accordingly. The SI concentration was decreased with increasing the intervention concentration of iron. The concentrations of TIBC, TF and Tfr increased in the DFe group. The down-regulation of CYP2R1 and CYP27A1 expression at the protein and gene levels can be explained by iron deficiency reducing the activity of CYP2R1 and CYP27A1. VD with hormonal properties is to maintain the stability of the body's hormone levels. The protein and gene expression levels of CYP27B1 were up-regulated, and the expression of VD catabolic enzyme CYP24A1 was down-regulated accordingly. In our study, the gene expression levels of CYP2R1 and CYP27A1 were the lowest in the DFe group, and the HFe group was 3 times higher than the DFe group. It is illustrated that the decrease of iron reserve may be a necessary condition for the control of these hydroxylase enzymes. Even in the case of mild iron deficiency, the enzyme function may change. This phenomenon suggested that under the condition of sufficient iron, the hydroxylation of VD is the use of this metal enzyme, which can effectively carry out hydroxylation reaction; when iron deficiency, hydroxylase can not rely on iron, which leads to the down-regulation of liver hydroxylase expression and the up-regulation of kidney hydroxylase expression. It is suggested that iron may play a role in 25 hydroxylation by regulating CYP2R1 and CYP27A1 enzymes, and 1α-hydroxylation by regulating CYP27B1 and CYP 24A1 enzymes, thus affecting the levels of 25-(OH)D3 and 1,25-(OH)2D3. Iron supplementation may improve VD deficiency. The mechanism of iron hydroxylation in VD is based on the regulation of VD hydroxylase.
The results showed that different doses of iron had different effects on serum 25-(OH)D3 and 1,25-(OH)2D3 in SD rats. Correlation analysis showed that the serum levels of 25-(OH)D3 and 1,25-(OH)2D3 were negatively correlated with TIBC, TF and Tfr indicators; there was no correlation with SI. We think that reasonable iron supplementation to increase the level of VD activity is a simple, safe, cheap and effective method. At the same time, accurate identification of VD hydroxylation cofactors improves the bioavailability of VD, and the participation of iron in the formation of CYP450 enzymes plays a key role in VD metabolism. In our study, iron-deficient feed was used to establish an IDA model, the method reduced the levels of SI and Hb in rats at a stable rate, with no trauma and reliable effects.
Our study takes the lead in investigate the systematic mechanism of iron on VD metabolism. When the body is deficient in iron, the activity of VD hydroxylase may be damaged, thereby reducing the level of VD metabolism. However, our research also had certain limitations, we only based on animal experiments, the digestibility and absorptivity of iron and VD metabolism in group C and group M were not studied. It is necessary to further explore the effect of iron on VD metabolism in the population, so that proper iron supplementation can promote the expression of VD active products in the VD deficiency patient population. To understand the potential benefits of VD deficiency, provide new ideas for VD deficiency diseases.