Lactoferrin Is the Major Deoxyribonuclease of Human Milk

Lactoferrin as a Human Genome “Guardian”—An Overall Point of View

Structural abnormalities causing DNA modifications of the ethene and propanoadducts can lead to mutations and permanent damage to human genetic material. Such changes may cause premature aging and cell degeneration and death as well as severe impairment of tissue and organ function. This may lead to the development of various diseases, including cancer. In response to a damage, cells have developed defense mechanisms aimed at preventing disease and repairing damaged genetic material or diverting it into apoptosis. All of the mechanisms described above are part of the repertoire of action of Lactoferrin—an endogenous protein that contains iron in its structure, which gives it numerous antibacterial, antiviral, antifungal and anticancer properties. The aim of the article is to synthetically present the new and innovative role of lactoferrin in the protection of human genetic material against internal and external damage, described by the modulation mechanisms of the cell cycle at all its levels and the mechanisms of its repair.

SARS-CoV-2 and human milk: What is the evidence?

The novel coronavirus SARS-CoV-2 has emerged as one of the most compelling and concerning public health challenges of our time. To address the myriad issues generated by this pandemic, an interdisciplinary breadth of research, clinical and public health communities has rapidly engaged to collectively find answers and solutions. One area of active inquiry is understanding the mode(s) of SARS-CoV-2 transmission. Although respiratory droplets are a known mechanism of transmission, other mechanisms are likely. Of particular importance to global health is the possibility of vertical transmission from infected mothers to infants through breastfeeding or consumption of human milk. However, there is limited published literature related to vertical transmission of any human coronaviruses (including SARS-CoV-2) via human milk and/or breastfeeding. Results of the literature search reported here (finalized on 17 April 2020) revealed a single study providing some evidence of vertical transmission of human coronavirus 229E; a single study evaluating presence of SARS-CoV in human milk (it was negative); and no published data on MERS-CoV and human milk. We identified 13 studies reporting human milk tested for SARS-CoV-2; one study (a non-peer-reviewed preprint) detected the virus in one milk sample, and another study detected SARS-CoV-2 specific IgG in milk. Importantly, none of the studies on coronaviruses and human milk report validation of their collection and analytical methods for use in human milk. These reports are evaluated here, and their implications related to the possibility of vertical transmission of coronaviruses (in particular, SARS-CoV-2) during breastfeeding are discussed.

SARS-CoV-2 and human milk: what is the evidence?

The novel coronavirus SARS-CoV-2 has emerged as one of the most compelling public health challenges of our time. To address the myriad issues generated by this pandemic, an interdisciplinary breadth of research, clinical, and public health communities have rapidly engaged to find answers and solutions. One area of active inquiry is understanding the mode(s) of SARS-CoV-2 transmission. While respiratory droplets are a known mechanism of transmission, other mechanisms are possible. Of particular importance to global health is the possibility of vertical transmission from infected mothers to infants through breastfeeding or consumption of human milk. However, there is limited published literature related to vertical transmission of any human coronavirus (including SARS-CoV-2) via human milk and/or breastfeeding. There is a single study providing some evidence of vertical transmission of human coronavirus 229E, a single study evaluating presence of SARS-CoV in human milk (it was negative), and no published data on MERS-CoV and human milk. There are 9 case studies of human milk tested for SARS-CoV-2; none detected the virus. Importantly, none of the published studies on coronaviruses and human milk report validation of their analytical methods for use in human milk. These reports are evaluated here, and their implications related to the possibility of vertical transmission of coronaviruses (in particular, SARS-CoV-2) during breastfeeding are discussed.

Comparison of Trace Elements in High-Molecular-Mass Multiprotein Complex and in Female Milk from Which It Was Obtained

Background

Many biological processes are performed by different protein complexes. During the association of proteins and enzymes forming specific complexes, the latter can include ions of various metal ions, which may be important for their formation and biological function.

Objective of the Studies

However, to date in the literature there are no data on metal ions that are part of any protein complexes.

Methods

A very stable multiprotein complex (~1000±100 kDa) was separated from other proteins of nine samples of female milk by gel filtration on Sepharose 4B. The content of microelements in the stable multiprotein complex and milk was analyzed using two-jet plasma atomic emission spectrometry.

Results

The content of different elements in milk on average decreased in the order: Ca>P>Mg>Al≥Zn≥Fe>Cu >B (0.76–3500 μg/g of dry milk powder), while the content of some elements was very low (Sr>Mn>Cr>Ba>Pb>Ag>Ni>Cd, <0.03–0.5 μg/g). The content of eight elements in stable multiprotein complex was 1.2-9.6-fold higher than in milk and increased in the order: Ca≈Mg<P<Al<Fe<Pb<Ba<Cr<Cd<Zn, while content of SPC eight metals was 12.3-110-fold higher: Cu (12.3)>B (19.7)>Ag (28.7)>Ni (38)≥Sr (110).

Conclusions

The analysis of the relative content of sixteen elements in human milk and oligomeric complexes of proteins was performed for the first time. Data on the content of metals indicate that during the formation of protein which associates the increase in the content of metal ions bound with proteins of the complex can occur. Such metal ions can be important for the formation and biological function of protein complexes.

Cow Milk Lactoferrin Possesses Several Catalytic Activities

Lactoferrin (LF) is a Fe³⁺-binding glycoprotein, that was first recognized in milk and then in other epithelial secretions and barrier body fluids to which many different functions have been attributed to LF including protection from iron-induced lipid peroxidation, immunomodulation, cell growth regulation, DNA and RNA binding, as well as transcriptional activation, еtс. The polyfunctional physiological role of LF is still unclear, but it has been suggested to be responsible for primary defense against microbial and viral infections. It was shown previously that human milk LF possesses several enzymatic activities: DNase, RNase, ATPase, phosphatase, and amylase. Analysis of human, cow, horse, buffalo and camel LF showed a highly conserved three-dimensional (3D) structure including only detail differences in the species. Recently, it was shown that similar to human cow LF possesses DNase and RNase activities. Using different methods here we have shown for the first time that LFs from the milk of seven cows of different breeds possess high peroxidase, protease, amylase, protease, and phosphatase activities. Protease activity of cow LFs was activated by Mg²⁺ and Ca²⁺ ions. In contrast to human LFs, ATPase activity was revealed only in three of seven cow LF preparations. The discovery that LF possesses these activities may contribute to understanding the multiple physiological functions of this extremely polyfunctional protein including its protective role against microbial and viral infections.

Human serum and milk albumins are metal-dependent DNases

It is known that that human serum albumin (HSA) and alpha-lactalbumin (LA) possess DNA-binding sites. Electrophoretically homogeneous HSA and LA containing no canonical enzymes were isolated from human sera and milk. Here we have analyzed for the first time the possibility of DNA hydrolysis by these proteins. It was shown that HSA possesses metal-dependent DNase activity, while LA cannot hydrolyze DNA. Several rigid criteria have been applied to show that DNase activity is an intrinsic property of HSA from human sera and milk. HSA preparations were inactive after their dialysis against EDTA or in the presence of EDTA, but were activated after addition of several external metal ions: Mn²⁺  > Mg²⁺  > Ca²⁺ . The best activation of HSA preparations was observed in the presence of two metal ions: Mg²⁺ +Ca²⁺  > Mn²⁺  + Ca²⁺  ≥ Mn²⁺  + Mg²⁺ . In contrast to DNases having only one pH optimum, HSA preparations demonstrated two well-pronounced optima at pH 5.7-5.9 and 6.9-7.1 as well as a weak optimum at pH 8.4-8.6. These results demonstrate the diversity of HSA in the DNA hydrolysis at various pHs and in activation by various metal cofactors. Possible reasons for the diversity of HSA preparations are discussed. © 2018 IUBMB Life, 70(6):501-510, 2018.

Recognition of specific and nonspecific DNA by human lactoferrin

The general principles of recognition of nucleic acids by proteins are among the most exciting problems of molecular biology. Human lactoferrin (LF) is a remarkable protein possessing many independent biological functions, including interaction with DNA. In human milk, LF is a major DNase featuring two DNA-binding sites with different affinities for DNA. The mechanism of DNA recognition by LF was studied here for the first time. Electrophoretic mobility shift assay and fluorescence measurements were used to probe for interactions of the high-affinity DNA-binding site of LF with a series of model-specific and nonspecific DNA ligands, and the structural determinants of DNA recognition by LF were characterized quantitatively. The minimal ligands for this binding site were orthophosphate (K(i)  = 5 mM), deoxyribose 5'-phosphate (K(i)  = 3 mM), and different dNMPs (K(i)  = 0.56-1.6 mM). LF interacted additionally with 9-12 nucleotides or nucleotide pairs of single- and double-stranded ribo- and deoxyribooligonucleotides of different lengths and sequences, mainly through weak additive contacts with internucleoside phosphate groups. Such nonspecific interactions of LF with noncognate single- and double-stranded d(pN)(10) provided ~6 to ~7.5 orders of magnitude of the enzyme affinity for any DNA. This corresponds to the Gibbs free energy of binding (ΔG(0)) of -8.5 to -10.0 kcal/mol. Formation of specific contacts between the LF and its cognate DNA results in an increase of the DNA affinity for the enzyme by approximately 1 order of magnitude (K(d)  = 10 nM; ΔG(0)  ≈ -11.1 kcal/mol). A general function for the LF affinity for nonspecific d(pN)(n) of different sequences and lengths was obtained, giving the K(d) values comparable with the experimentally measured ones. A thermodynamic model was constructed to describe the interactions of LF with DNA.

Mapping the zinc-transporting system in mammary cells: molecular analysis reveals a phenotype-dependent zinc-transporting network during lactation

The mammary epithelial cell transitions from a non-secreting to a terminally differentiated, secreting cell during lactation. Zinc (Zn) is a key modulator of phenotypic transition as it regulates over 300 biological functions including transcription, translation, energy transformation, intracellular signaling, and apoptosis. In addition, Zn must be redirected from normal cellular functions into the secretory compartment, as many components of the secretory system are Zn-dependent and an extraordinary amount of Zn is secreted (1-3 mg Zn/day) into milk. Herein, we utilized a "systems biology" approach of genomic and proteomic profiling to explore mechanisms through which Zn is reallocated during phenotype transition in the lactating mammary gland from mice and cultured mammary cells. Nine Zn transporters play key roles in Zn redistribution within the network during lactation. Protein abundance of six Zip (Zip3, Zip5, Zip7, Zip8, Zip10, Zip11) and three ZnT (ZnT2, ZnT4, ZnT9) proteins was expanded >2-fold during lactation, which was not necessarily reflected by changes in mRNA expression. Our data suggest that Zip5, Zip8, and Zip10 may be key to Zn acquisition from maternal circulation, while multiple Zip proteins reuptake Zn from milk. Confocal microscopy of cultured mammary cells identified the Golgi apparatus (modulated in part by ZnT5, Zip7, and Zip11) and the late endosomal compartment (modulated in part by ZnT2 and Zip3) as key intracellular compartments through which Zn is reallocated during lactation. These results provide an important framework for understanding the "Zn-transporting network" through which mammary gland Zn pools are redistributed and secreted into milk.

Molecular evolution of the transferrin family and associated receptors

BACKGROUND

In vertebrates, serum transferrins are essential iron transporters that have bind and release Fe(III) in response to receptor binding and changes in pH. Some family members such as lactoferrin and melanotransferrin can also bind iron while others have lost this ability and have gained other functions, e.g., inhibitor of carbonic anhydrase (mammals), saxiphilin (frogs) and otolith matrix protein 1 (fish).

SCOPE OF REVIEW

This article provides an overview of the known transferrin family members and their associated receptors and interacting partners.

MAJOR CONCLUSIONS

The number of transferrin genes has proliferated as a result of multiple duplication events, and the resulting paralogs have developed a wide array of new functions. Some homologs in the most primitive metazoan groups resemble both serum and melanotransferrins, but the major yolk proteins show considerable divergence from the rest of the family. Among the transferrin receptors, the lack of TFR2 in birds and reptiles, and the lack of any TFR homologs among the insects draw attention to the differences in iron transport and regulation in those groups.

GENERAL SIGNIFICANCE

The transferrin family members are important because of their clinical significance, interesting biochemical properties, and evolutionary history. More work is needed to better understand the functions and evolution of the non-vertebrate family members. This article is part of a Special Issue entitled Molecular Mechanisms of Iron Transport and Disorders.

Characterization of lactoferrin receptor on human spermatozoa

Lactoferrin (LF) is abundant in human seminal plasma and on sperm surfaces. However, lactoferrin receptor (LFR) on human spermatozoa has not yet been reported. To study the expression, localization and characteristics of LFR on human spermatozoa, different experimental approaches were applied: LFR gene was amplified from a human testis cDNA library and recombinant LFR (rLFR) protein was produced in the expression vector Escherichia coli BL21 (DE3); human sperm membrane proteins were extracted and analysed via Western blot; the binding of LF to LFR was investigated by Far-Western blot, immunoprecipitation and autoradiography analysis and the localization of LFR on sperm surfaces was detected using immunofluorescence. LFR gene was amplified from a human testis cDNA library and the molecular weight of rLFR was 34kDa. The native LFR on human spermatozoa was a 136-kDa tetramer which was anchored to the sperm head and mid-piece through glycophosphatidylinositol. LF could bind to LFR competitively in vitro. As far as is known, this study has elucidated for the first time that LFR was expressed at the testis level, was anchored to the sperm membrane by glycophosphatidylinositol during spermatogenesis. LFR may play important roles through binding to and mediating LF.

Lactoferrin Expression by Bovine Ocular Surface Epithelia: A Primary Cell Culture Model to Study Lactoferrin Gene Promoter Activity

Tear lactoferrin, mainly secreted by the lachrymal glands, exerts a protective effect on the ocular surface, and an abnormal decrease of its production may lead to an increased risk of infection and pathological alterations of ocular surface epithelia. In this study we analyzed whether corneal and conjunctival epithelia could be an additional source of tear lactoferrin, and whether conjunctival epithelial cells in culture could be a suitable model system to address regulation of lactoferrin gene expression. Real-time PCR and Western immunoblotting showed that in bovines lactoferrin is indeed produced by these epithelia, and that the human lactoferrin promoter can direct the expression of a CAT reporter gene, thus indicating that these cells are a true source of lactoferrin, and may be used in vitro to study the regulation of lactoferrin expression.