Overview
Lactoferrin (LF) is an endogenous iron-containing glycoprotein from the transferrin family.
In mammals, it is present after birth in many exocrine secretions of the digestive, respiratory,
and reproductive tracts, including milk, saliva, bile, pancreatic juice, bronchial secretions,
seminal fluid, tears, and synovial fluid, among others
[Janssen RT, 1983; Masson PL, 1966, 1971; Saito K, 1992; Nikolaev AA, 1985].
Lactoferrin is also found in blood plasma, where it is released from neutrophils; in these cells
it is a major component of secondary secretory granules [Rado TA, 1984].
Molecular structure and iron binding
Lactoferrin is a species-specific glycoprotein, reflecting differences in molecular structure.
Chemical analysis [Metz-Boutigue MH, 1984] and cDNA cloning
[Powell MJ, 1990; Rey MW, 1990] indicate that human lactoferrin consists of a single
polypeptide chain of 692 amino acid residues with a molecular weight of approximately 80 kDa.
The protein contains intramolecular disulfide bonds and no free sulfhydryl groups.
X-ray structural analysis shows that the protein chain forms two globular lobes (N- and C-terminal),
connected by a helical segment [Baker EN, 1998]. Each lobe consists of two domains.
In each lobe, at the interface between domains, there is an iron-binding site. These sites differ
in iron release: one is “acid-labile” (iron is released at approximately pH 4.0), while the other is
“acid-stable” (iron is released only at approximately pH 2.0). Reported iron content in the “acid-stable”
site is 10–20% [Mazurier J, 1980; Iyer S, 1993].
Under physiological conditions, the lactoferrin molecule is typically only 10–20% saturated with iron,
which contributes to its antioxidant function. The N2 and C2 domains each contain an asparagine residue
to which an N-acetyl-lactosamine-type polysaccharide chain can be attached [Spik G, 1982].
Human lactoferrin and homology across species
The human lactoferrin gene is located on the short arm of chromosome 3 (region 3p21.3)
[Iijima H, 2005]. Considerable interest in lactoferrin stems from its diverse physiological
functions [Vorland LH, 1999; Tsuda H, 2002; Valenti P, 2004; Ward PP, 2005; Shimazaki K, 2005].
Bovine lactoferrin consists of 689 amino acid residues and shows approximately 69% homology with the
primary structure of human lactoferrin. The spatial structures are similar, although not identical
[Goodman RE, 1991; Pierce A, 1991; Moore SA, 1991].
Physiological functions
The molecular mechanisms underlying lactoferrin’s biological role are not fully understood. However, current evidence indicates that lactoferrin:
- participates in the metabolism and transport of iron and other variable-valence metal ions
[Harrington JP, 1992]; - can bind to the surface of bacteria, viruses, fungi, and protozoa
[Yu RH, 2000]; - binds to specific receptors on blood cells, liver, and vessels
[Suzuki YA, 2002]; - has been discussed in the context of RNase activity
[Kanyshkova TG, 2003]and transcription activation[Choi S-Y, 2005].
These properties contribute to lactoferrin’s key functional profile: antioxidant, antimicrobial, and immunomodulatory. Collectively, this supports its potential role as a detoxifying, anti-inflammatory, and anti-carcinogenic agent.
Antioxidant properties
Antioxidant effects of lactoferrin have been reported in multiple model systems
[Bemun S, 1997; Huang Sh-W, 1999; Steijns J, 2000]. One proposed mechanism involves binding of
Fe3+, reducing the pool of Fe2+ available to catalyze radical-generating reactions.
By decreasing available iron, the intensity of oxidative processes may decline.
The higher activity of apo-lactoferrin compared with lactoferrin saturated with Fe3+
supports the view that strong coordination binding of iron is central to antioxidant action
[Bemun S, 1997].
Another proposed mechanism is that Fe3+ bound to lactoferrin can be reduced to Fe2+ in the presence of free radicals, thereby neutralizing radical species. Because Fe2+ remains bound to lactoferrin, it may not catalyze further formation of reactive oxygen metabolites.
Anti-inflammatory and immunomodulatory effects
Lactoferrin can modulate the activity of phagocytic blood cells (monocytes and neutrophils)
and enhance the cytotoxic function of natural killer (NK) cells. It may also regulate lymphocyte-mediated
immunity [Baveye S, 2000; Legrand D, 2005; Kruzel ML, 2002; Artym J, 2004; Damiens E, 1998; Mincheva-Nilsson L, 1997].
Clinical observations support antibacterial and anti-inflammatory roles
[Vorland L, 1999; Zimecki M, 2001]. Lactoferrin has also been discussed as an acute-phase protein,
as serum levels may increase in inflammatory conditions and infections, including neonatal bacterial and viral infections,
rheumatoid arthritis, pulmonary tuberculosis, and acute pneumonia [Gutteberg TJ, 1984; Benini L, 1985; Baynes R, 1986].
The antioxidant, antimicrobial, and immunomodulatory properties of lactoferrin contribute to anti-inflammatory effects
across phases of inflammation [Conneely OM, 2001]. At sites of inflammation, lactoferrin may limit
oxygen-radical-mediated tissue damage via the mechanisms described above [Ward PP, 2002].
Malignant tumors and lactoferrin
Interest in lactoferrin’s immunomodulatory activity has motivated studies on its influence on malignant tumors. One proposal is that antitumor effects may be mediated primarily via immune modulation, such as increased IL-18 production in the gastrointestinal tract, activation of NK cells, and an increase in circulating CD8+ lymphocytes. This is consistent with reports that lactoferrin does not necessarily show a direct cytostatic effect on tumor cells in culture.
Study results (expand)
Experimental studies of human and bovine lactoferrin in animal models of transplantable tumors (including melanoma B-16, L5178, Y-ML25, and rectal carcinoma C26) report that subcutaneous or oral administration at doses of 1–5 mg/mouse can inhibit primary tumor growth and formation of distant metastases.
Intratumoral administration of bovine lactoferrin in orthotopic tumor models (squamous cell carcinoma and fibrosarcoma of the floor of the oral cavity) has been described as producing approximately 50–54% inhibition of tumor growth compared with control.
Mechanistic assumptions (expand)
Although many studies address lactoferrin’s anticarcinogenic action, the precise mechanisms remain uncertain. Proposed mechanisms include effects on xenobiotic metabolism enzymes (Phase I/II) and increased expression of proapoptotic factors leading to higher apoptosis.
Cytochrome P450 (IA2) and procarcinogen metabolism (expand)
One hypothesis suggests lactoferrin affects enzymes involved in procarcinogen metabolism. In studies using rat liver and rectal tumor models induced by diethylnitrosamine (DEN) and MeIQx, lactoferrin administration in combination with MeIQx was reported to normalize mRNA and protein levels of cytochrome P450 isoenzyme IA2 and reduce MeIQx-related adduct formation. Authors proposed that reduced metabolic activation of procarcinogens could contribute to anticarcinogenic effects in that model.
Increase in apoptosis (expand)
Another proposed mechanism is increased apoptosis. In a rat model of colon carcinogenesis induced by azoxymethane in combination with lactoferrin, increased expression of proapoptotic factors (including Fas and Bax) was reported in tissues of animals receiving lactoferrin compared with animals receiving carcinogen alone. Similar observations have been reported in vitro in various cell types.
Final conclusions
Multiple pathways may contribute to the anticarcinogenic and antitumor actions attributed to lactoferrin: antioxidant activity, activation of cytotoxic immune cells, antiangiogenic effects, induction of proapoptotic factors, and modulation of xenobiotic metabolism enzymes.
Considering lactoferrin’s role as a barrier protein in tissues and its behavior as an acute-phase protein, lactoferrin-based replacement therapy may be a promising approach for detoxification, reducing microbial load, and decreasing the intensity of inflammatory reactions.