Introduction

Much has been reported on the adverse effects of cigarette smoke on wound healing. There is also some evidence that bone heals more slowly in smokers than in nonsmokers [13]. While it is still unclear how smoking impairs bone healing, based on the review of the current research on nicotine, the following potential pathway may explain the mechanism behind the phenomenon.

Tumour necrosis factor alpha (TNF-α) plays important role in bone healing

In addition to being an important proinflammatory factor, TNF-α is also an essential part of the innate immune response to injury, which contributes to the normal regulatory processes of bone resorption. The role of TNF-α during fracture healing was examined in wild-type and TNF-α receptor (p55(−/−)/p75(−/−))-deficient mice [10]. Histomorphometric measurements of the cartilage and bone as well as apoptotic cell counts in hypertrophic cartilage were detected which showed that endochondral tissue resorption was delayed two to three weeks in the TNF-α receptor knockout mice, indicating TNF-α mediated chondrocyte apoptosis as well as the expression of proresorptive cytokines which control endochondral tissue remodelling by osteoclasts. Similar results have been proven by other research groups [1, 16]. Furthermore, the induction of osteoprogenitor cell recruitment or osteogenic cell activation of TNF-α in the context of intramembranous bone formation was found by the same research group [9]. Research on the effects of TNF-α on cartilage cells indicated that TNF-α may alter the expression of a complex array of genes within murine chondrocytes that contribute to the destruction of joint surfaces, independent of its actions on synovial and immune cells [5]. Furthermore, studies on diabetes models also have shown that TNF-α may increase the expression of resorptive factors in chondrocytes through a process that involves activation of FOXO1 and TNF-α dysregulation and leads to enhanced osteoclast formation and accelerated loss of cartilage [2]. In addition, TNF-α appears to regulate, in part, the expression of three key proteolytic enzymes, i.e. matrix metalloproteinase 9 (MMP9), MMP13 and MMP14, that are known to be crucial to the progression of vascularisation and turnover of mineralised cartilage [12, 14]. Therefore, TNF-α signalling in healing fractures appears to coordinate the expression of specific regulators of endothelial cell survival and metalloproteolytic enzymes and is essential in the transition and progression of the endochondral phase of fracture repair. Our study examined the potential role of TNF-α as a regulator of MMPs in the retinal neovascularisation process which demonstrated an increased production of MMPs stimulated by TNF-α, enhancing the production of specific members of the MMP family [15]. These results suggested that TNF-α was able to enhance the expression of the MMP family in a wide range of types of cells. Another study showed that TNF-α could up-regulate the expression of intercellular adhesion molecule1 (ICAM-1) in mesenchymal stem cells (MSCs) and enhance the cells' migration ability, and the p38 signalling pathway might be involved in the TNF-α-induced migration ability for a role in wound repair and regeneration [8]. Therefore, it seems that TNF-α was an essential cytokine for fracture healing and proper concentrations of TNF-α ensure the normal bone healing process.

Nicotine may inhibit the secretion of TNF-α

It has been widely accepted that nicotine is one of the main effective ingredients that contributes to the harmful effect of smoking, and is also an agonist of the nicotinic receptor. Recently, a new anti-inflammatory pathway called the “cholinergic anti-inflammatory pathway” was discovered [18]. The research showed that the α7 nicotinic acetylcholine receptor (α7 nAChR) was the most important therapeutic target for inflammation [22] which is mainly expressed on the membrane of immune cells, including monocytes, macrophages, T and B lymphocytes, dendritic cells [6], and fibroblast-like synoviocytes (FLS) [20]. In brief, α7nAChR exerts its effects on immune cells by combining nicotinic agonists (nicotine, acetylcholine, etc.) via the triggering of various signalling mechanisms that probably interact with each other to achieve the anti-inflammatory effect, among which the JaK2–STAT3 and NF-κB signalling pathways were included [7, 21]. More experiments have confirmed the anti-inflammatory effect of α7nAChR using different models of inflammation [11, 17, 19].

Viewpoints

Based on the above-mentioned literature review and analysis, it is reasonable to believe that the nicotine in tobacco may at least partly contribute to the delayed healing of fracture by inhibiting TNF-α expression through the activation of the cholinergic anti-inflammatory pathway. In detail, the nicotine released from smoking may combine with the α7 nAChR and activate the anti-inflammatory pathways; the JaK2–STAT3 signalling pathway will be activated and the NF-κB signalling pathway will be inhibited consequently. As a result, the production of TNF-α will be greatly reduced which may result in prolonged soft callus stage and less MSCs migration in the bone healing process (Fig. 1).

Fig. 1
figure 1

The diagrammatic description of the viewpoints in this paper. α7nAChR  α7 nicotinic acetylcholine receptor, TNF-α   tumour necrosis factor α

Full size image

However, we should note that many other elements in smoking may also contribute to the delayed bone healing process. It is well known that more than 4,700 chemical compounds, including 43 cancer-causing substances, have been isolated in cigarette smoke, and nicotine and carbon monoxide (CO) are the elements of cigarette smoke associated with impairment of the healing process [3]. It has been reported that CO may bind to haemoglobin in the pulmonary capillaries to form the stable compound carboxyhaemoglobin, resulting in reduced oxygen-carrying capacity of blood and tissue hypoxia which may lead to the impaired bone healing process as well [4].

Prospect

In this paper, based on the current literature review, a reasonable explanation for the mechanism of smoking delayed fracture healing is illustrated. From our point of view, although many elements (CO, etc.) may contribute to the smoking induced impairment of the bone healing process, the potential role of nicotine and the cholinergic anti-inflammatory pathway are of great importance. From another point of view, this will be helpful to accelerate fracture healing. An appropriate level of TNF-α may facilitate endochondral ossification, which will inevitably accelerate the fracture healing process. In-depth research (in vitro, in vivo experimental evidence) on this subject will be more constructive and meaningful.