The role of lactoferrin in bone remodeling: evaluation of its potential in targeted delivery and treatment of metabolic bone diseases and orthopedic conditions

Abstract

Lactoferrin (Lf) is a multifunctional protein that is synthesized endogenously and has various biological roles including immunological regulation, antibacterial, antiviral, and anticancer properties. Recently, research has uncovered Lf's critical functions in bone remodeling, where it regulates the function of osteoblasts, chondrocytes, osteoclasts, and mesenchymal stem cells. The signaling pathways involved in Lf's signaling in osteoblasts include (low density lipoprotein receptor-related protein - 1 (LRP-1), transforming growth factor β (TGF-β), and insulin-like growth factor - 1 (IGF-1), which activate downstream pathways such as ERK, PI3K/Akt, and NF-κB. These pathways collectively stimulate osteoblast proliferation, differentiation, and mineralization while inhibiting osteoclast differentiation and activity. Additionally, Lf's inhibitory effect on nuclear factor kappa B (NF-κB) suppresses the formation and activity of osteoclasts directly. Lf also promotes chondroprogenitor proliferation and differentiation to chondrocytes by activating the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and phosphoinositide 3-kinase/protein kinase B(PI3K/Akt)signaling pathways while inhibiting the expression of matrix-degrading enzymes through the suppression of the NF-κB pathway. Lf's ability to stimulate osteoblast and chondrocyte activity and inhibit osteoclast function accelerates fracture repair, as demonstrated by its effectiveness in animal models of critical-sized long bone defects. Moreover, studies have indicated that Lf can rescue dysregulated bone remodeling in osteoporotic conditions by stimulating bone formation and suppressing bone resorption. These beneficial effects of Lf on bone health have led to its exploration in nutraceutical and pharmaceutical applications. However, due to the large size of Lf, small bioactive peptides are preferred for pharmaceutical applications. These peptides have been shown to promote bone fracture repair and reverse osteoporosis in animal studies, indicating their potential as therapeutic agents for bone-related diseases. Nonetheless, the active concentration of Lf in serum may not be sufficient at the site requiring bone regeneration, necessitating the development of various delivery strategies to enhance Lf's bioavailability and target its active concentration to the site requiring bone regeneration. This review provides a critical discussion of the issues mentioned above, providing insight into the roles of Lf in bone remodeling and the potential use of Lf as a therapeutic target for bone disorders.

Keywords: bone remodeling; fracture repair; lactoferrin; osteoporosis; signaling pathways.

Figure 1

The schematic diagram illustrates the various pathways involved in lactoferrin (Lf) signaling in osteoblasts. (A) Lfsignaling via LRP1 in osteoblasts leads to mitogenesis and differentiation through the RAS-MAPK pathway. Lf binds to LRP1, triggering the activation of ERK, which promotes osteoblast differentiation and bone formation. The activation of ERK also induces mitogenesis in osteoblasts. (B) Lf can also signal through the TGFb receptor II (TβRII) in osteoblasts, activating smad 2, 3. This pathway results in the upregulation of osteogenic genes and promotes osteoblast differentiation. (C) Lfsignaling via the IGF-1 receptor in osteoblasts activates the PI3K/Akt and mTOR pathway, promoting osteoblast differentiation and survival independently of LRP1. This pathway promotes the survival of osteoblasts by inhibiting apoptosis. Image is made using the Biorender Software.

Figure 2

The schematic diagram illustrates the regulation of osteoclast function by Lf. Lf inhibits osteoclastogenesis in two ways. Firstly, it lowers the RANKL/OPG ratio by acting on osteoblastic cells. Secondly, it directly inhibits osteoclastogenesis through its anti-inflammatory effect, which prevents downstream signaling following RANKL binding to RANK. Moreover, Lf’s ability to scavenge free radicals also inhibits the generation of ROS involved in osteoclastogenesis. Image is made using the Biorender Software.

Figure 3

Strategies to improve the delivery of Lf or Lf-derived peptides to the bone. Hydrogels have been used in preclinical models of critical-sized bone defects (mimicking non-unions) as sustained-release formulations to deliver high amounts of Lf to bones. In these models, Lf has been used for local delivery of Lf as implants at the site of the bone defect. This approach ensures Lf’s sustained release, which can stimulate bone regeneration and repair. In addition, systemic delivery of Lf through hydrogels has also been explored. In this case, hydrogels containing Lf are injected into the bloodstream, allowing for the controlled release of Lf over time. These approaches have shown promising results in promoting bone regeneration in critical-sized bone defects. Image is made using the Biorender Software.

Figure 4

The schematic diagram shows the effects of Lf in bone remodeling. The bone remodeling cycle comprises several stages, including quiescence, activation & resorption, reversal, and formation. In the activation & resorption stage, osteoclasts are recruited to the bone surface and resorb the old bone. In the reversal stage, osteoblasts are recruited to the bone surface to begin the process of new bone formation. The final stage involves the production of new bone matrix by osteoblasts, which undergoes mineralization. In the normal bone remodeling cycle, there is a balance between bone resorption and bone formation such that the amount of bone that is resorbed is replaced by an equal amount of new bone formation, resulting in the maintenance of a constant bone mass. In osteoporosis, there is an imbalance between bone resorption and formation because osteoclast activity is increased. In contrast, osteoblast activity is decreased, resulting in decreased bone mass and an increased risk of fractures. Lf acts at the activation & resorption phases by the mechanisms described in Figure 2 to inhibit bone resorption. Lf also acts at the reversal and formation stages to stimulate bone formation by the mechanisms described in Figure 1 . By these mechanisms, Lf corrects the remodeling cycle and restores bone mass. Osteioid, unmineralized bone matrix; osteon, mineralized bone matrix. Image is made using the Biorender Software.