Cancer is the leading cause of mortality worldwide and has become a major concern with increasing age and changing lifestyle habits. The predominant cause of mortality by cancer is the occurrence of distant metastases and for most cancers, bone is a common site for metastasis. For example, it is estimated that ~70% of patients with advanced breast cancer and over 70% of patients with advanced prostate cancer develop bone metastases (BM). In addition, in patients with metastatic prostate cancer, the probability of BM is as high as 90% (1). BM not only increases the economic burden on healthcare systems and patients but also significantly reduces patient quality of life and survival rates. In addition, it frequently affects the spine, ribs, pelvis and femur, leading to a higher incidence of spinal cord compression, pathological fractures and intractable pain (2). In addition, complications related to BM are significant, typically occurring every 3–6 months on average. These complications reduce patient quality of life and mortality is frequently associated with unresolved skeletal complications. The prognosis of metastatic bone disease varies based on the primary cancer site, with patients with breast and prostate cancer often surviving for years, while patients with lung cancer typically survive only a few months (3). However, current clinical treatments for BM are limited, highlighting the need to evaluate new therapeutic targets.

For tumor metastasis, primary tumor cells must first undergo epithelial-mesenchymal transition (EMT) to invade surrounding tissues and enter the microvasculature of the blood or lymphatic system (4). Cancer cells in the bloodstream can spread to distant organs, settling in the metastatic microenvironment. They may become dormant or proliferate there, eventually forming secondary tumors (5). The occurrence of BM is associated with osteoblasts and osteoclasts (OCs) in the bone microenvironment (Fig. 1). BM can be categorized into osteolytic and osteogenic subtypes. In addition, NF-κB ligand receptor-activating factors (RANKL) are key mediators of osteoclastogenesis (6). Tumor cells in osteolytic BM can stimulate OCs and promote their differentiation by secreting cytokines such as TNF, RANKL, prostaglandins, leukemia inhibitory factor and IL-6, −8, −11, −15 and −17. Tumor cells can upregulate OC function by increasing the ratio of RANKL to osteoprotegerin (OPG) (7). OCs-mediated bone matrix degradation releases various cytokines and growth factors that promote tumor cell growth (7). Colony-stimulating factors (CSF) are a group of cytokines responsible for hemopoiesis, blood cell function regulation and maintaining homeostasis and overall immunity (8). CSF plays a crucial role in regulating blood cell function, given the instability and short lifespan of these cells (9). The CSFs are of a number of types and mainly include granulocyte-CSF (G-CSF), macrophage-CSF (M-CSF), granulocyte-macrophage CSF (GM-CSF) and multipotent CSF (multi-CSF) (10). In addition, CSF has profound effects on the development of granulocytes, macrophages and lymphocytes. The present article reviews the CSF family, its relationship to BM, the underlying mechanisms involved and the preclinical applications of CSF.

CSFs were first identified in an in vitro study on hematopoietic cells. In 1966, macrophage and granulocyte colony formation was discovered in hematopoietic cell studies, leading to the naming of CSFs in 1967 (9,11,12). Research has shown that CSFs are classified into four types: M-CSF (CSF1), GM-CSF (CSF2), G-CSF (CSF3) and multi-CSF (IL-3; Table I). CSF promotes the differentiation of hematopoietic stem cells into different blood cells, such as eosinophils, basic granulocytes and neutrophils (Fig. 2). Studies have concluded that CSF influences BM in various tumors (Table II). Although CSF has been extensively studied, it remains an area of active research. The physiological functions and effects of each CSF on cancer and BM are described in detail following.

CSFs play complex and diverse roles in developing and progressing BM in various types of cancer. They affect not only the hematopoietic system but also the skeletal microenvironment and the behavior of tumor cells through various mechanisms (Fig. 3). First, CSF promotes the bone metastatic potential of tumor cells. CSF increases the metastatic potential of cancer cells by promoting neoangiogenesis and enhancing their invasiveness (73). Studies have shown that G-CSF and GM-CSF can enhance the BM in tumor cells through multiple pathways. For example, G-CSF promotes tumor cell growth and metastasis by increasing angiogenesis and improving nutrient and oxygen availability to tumor cells (74). In addition, G-CSF can improve the tolerance of tumor cells to chemotherapeutic drugs, making it easier for them to survive and proliferate in the bone (75,76). GM-CSF enhances the pro-metastatic properties of the tumor microenvironment by promoting the generation and activation of TAMs, which are capable of secreting a variety of pro-angiogenic and pro-metastatic factors (for example, VEGF, MMPs and IL-10), further promoting BM of tumor cells (73).

Second, CSF indirectly affects the occurrence and development of BM by regulating cellular components and factor secretion in the bone marrow microenvironment. Bone is a common site of metastasis for a number of cancers due to the high expression of specific chemokines and growth factors in its microenvironment, such as stromal cell-derived factor 1, which promotes tumor metastasis (77). It is considered that before the primary tumor reaches the bone, it alters the bone microenvironment to promote cancer cell proliferation and colonization. In addition, tumor cells induce metastasis by forming ‘pre-metastatic niches’ in the bone, comprising clusters of bone marrow-derived cells, creating a favorable environment for the subsequent invasion and growth of tumor cells (78). Osteolytic lesions caused by OCs are significantly associated with BM. In osteolytic BM, OCs activating factors act on OCs to induce its formation. CSF and RANKL are important activators of OCs. In addition, osteoclastogenesis is primarily modulated by the interaction of CSF-1R, M-CSF, as well as RANK and RANKL (79). Therefore, M-CSF is essential for OC generation and increased OC activity promotes osteolytic BM. M-CSF is a key factor affecting OC generation and can influence OC precursor's survival and development. Similarly, M-CSF induces cytoskeletal rearrangement of OCs by activating c-Src and phosphocreatine 3-kinase (80). The M-CSF directly affects OCs by binding to M-CSFR, which attracts a signaling complex comprising phosphorylated dDNA-activated protein 12 and the non-receptor tyrosine kinase Syk. In addition, it also activates ERK/growth factor receptor-binding protein 2 and Akt/PI3K signaling to regulate the proliferation, differentiation and growth of OCs and their precursor cells (80). In addition, M-CSF can induce osteoclastogenesis by increasing the OCs precursor RANK expression via RANKL binding (81). In a number of cancer models, tumor cells have been observed to secrete cytokines that stimulate osteoblasts, such as PTHrP, VEGF-A and hepatocyte growth factor. This increases the expression of RANKL and M-CSF, stimulating osteoclastogenesis and altering the bone microenvironment, thereby promoting BM development. M-CSF affects osteoclastogenesis and influences other members of the CSF family on OCs (2–7). In 1993, it was reported that M-CSF, GM-CSF and IL-3 could promote osteoclastogenesis (82). In addition, GM-CSF is stimulated by increased levels of NF-kB, resulting in increased OC activity, which in turn leads to bone destruction and highly metastatic tumor growth (15).

Third, CSF also plays an important role in regulating the immune system, ultimately affecting BM. G-CSF inhibits T-cell activity and reduces the killing effect of the immune system on tumor cells, making it easier for tumor cells to grow and metastasize in the bone (83).

BM often has a poor prognosis and high mortality due to delayed diagnosis and limited treatment options. Bone is a common site of metastasis in breast cancer; however, BM in early-stage breast cancer is difficult to detect. As a result, BM in breast cancer is often diagnosed late, leading to delayed treatment. BM has been reported to occur in ~70% of patients with advanced breast cancer. The median overall survival following BM diagnosis was 40 months, indicating a high mortality rate (84). Similarly, BM has been reported in >70% of patients with advanced prostate cancer (1). Epidemiological data from the United States show that lung cancer now has a higher mortality rate than breast and prostate cancers, ranking as the leading cause of cancer-related death (85). Recently, nanoparticles have provided new directions in lung cancer treatment (86,87). Distant metastasis is the primary cause of death in patients with lung cancer. Bone is a common site of metastasis in advanced lung cancer and in advanced patients with lung cancer, 40% develop BM (88). The median survival of patients with lung cancer BM is often <6 months (89). Therefore, developing effective treatments for BM remains a significant challenge for clinicians and researchers.

CSFs are widely used in patients with cancer after chemotherapy to elevate critically low leukocyte levels (9). The clinical role of CSFs has been widely publicized. In 1990, with the availability of recombinant mouse CSF experimental, it was revealed that subcutaneous injections of G-CSF into mice increased blood leukocyte levels. High doses of G-CSF have been associated with erythrocyte suppression in the bone marrow (90). In addition, a 1987 study revealed that injection of recombinant GM-CSF in mice does not significantly increase blood leukocytes; however, it markedly increases the macrophage numbers and activity (91). The most significant increase in the blood of mice injected with recombinant multi-CSF was in eosinophils, followed by neutrophils and monocyte levels (92). Further research is needed to determine whether prolonged use of high-dose CSF has additional side effects. Transgenic mice overexpressing GM-CSF indicates blindness and various inflammatory lesions. In addition, a number of transgenic mice overexpressing GM-CSF died at 2–4 months due to macrophage activation, leading to muscle atrophy (93). In IL-3 overexpressing mice, there were increased progenitor cells in the spleen and peritoneum and decreased bone marrow progenitor cells. In addition, 80% of the mice succumbed within 5 weeks (94). Mice overexpressing G-CSF cause the proliferation of granulocytes and progenitor cells but does not cause severe tissue damage (95). The role of different cytokines in influencing the immunogenicity of tumor cells and the vaccination properties of murine tumor cells have been investigated in an animal experiment. Tumor cells expressing mouse GM-CSF have the most significant stimulatory effect on anti-tumor immunity following irradiation (96). GM-CSF is used in the treatment of melanoma (97).

The first clinical trials of CSF, published in 1988, demonstrated a significant increase in neutrophils when recombinant human G-CSF was administered to patients with cancer prior to chemotherapy (98). In addition, G-CSF-associated side effects are minimal the most common bring bone pain, which might be due to increased bone marrow cell counts (99). A phase II clinical trial at the Cancer Research Institute in Australia found that treatment with recombinant human GM-CSF in 21 patients with advanced cancer resulted in a 10-fold increase in leukocytosis. The trial also observed increases in circulating neutrophils, eosinophils, monocytes and lymphocytes. In addition, side effects, such as bone pain, myalgia, rash and hepatic dysfunction, are observed with high doses of rhGM-CSF (100). The safety and efficacy of GM-CSF in combination with Ipilimumab (Yervoy) for treating metastatic malignant melanoma is in a phase II clinical trial (NCT01363206). In another clinical trial (NCT02156388), G-CSF was administered in patients with advanced metastatic cancer following chemotherapy, which revealed increased neutrophil counts and activity and shorter duration and acute symptoms of neutropenia. This minimized the incidence of serious infections, reflecting improved efficacy and a longer half-life. The therapeutic effect of G-CSF with trastuzumab in metastatic breast cancer has also been studied in a randomized phase II clinical trial (NCT00169104).

CSF has been widely used in treating rheumatoid arthritis, coronary atherosclerosis and various inflammatory and autoimmune diseases (28). A number of preclinical trials have demonstrated that CSF is closely associated with mechanisms of BM in various types of cancer. However, more evidence is required to establish its efficacy in treating BM. It is hypothesized that targeted therapies against M-CSF or its signaling pathway may have clinical applications in BM treatment. The effectiveness of combining CSF with existing anti-bone metastatic drugs, such as bisphosphonates or RANKL inhibitors, in inhibiting BM progression warrants further investigation. Therefore, CSF might serve as a therapeutic target for treating BM in different cancers.

Cancer is the leading cause of death and the incidence of distant metastases significantly reduces survival in patients with cancer. Bone is a common site of metastasis in patients with cancer, often leading to serious complications such as pain and hypercalcemia. In addition, BM significantly reduces quality of life and imposes a substantial economic burden on patients and healthcare systems. In addition, the limited treatment options for BM contribute to significant psychological distress and anxiety in affected patients. A number of studies have indicated that changes in the bone microenvironment are closely related to BM, suggesting that modulating the bone microenvironment may help alleviate metastasis. In BM, cancer cells first undergo EMT, invade the blood and lymphatic vessels and migrate into bone tissue. Prior to entering bone tissue, cancer cells secrete cytokines that modify the bone microenvironment, facilitating their colonization and proliferation. For example, tumor cells can release factors that induce osteoblast formation during BM. This enhances the osteoblast-mediated bone formation while increasing the resorption of mineralized bone by OCs, which severely disrupts normal bone homeostasis (2). CSF has been shown to stimulate the differentiation and proliferation of hematopoietic cells. In addition, the relationship between CSF and BM has been extensively studied. Further, M-CSF is essential for promoting the differentiation of OC precursors. Drugs targeting OCs, such as bisphosphonates and the RANK ligand inhibitor denosumab, have been established to treat BM. It was also observed that administering OC inhibitors such as bisphosphonates, OPG and RANKL antagonists before tumor inoculation reduces the incidence of BM (101,102). However, the clinical application of CSF in treating BM remains underexplored. The treatment of BM remains a significant challenge for oncologists and further clinical trials are required to determine whether CSF could serve as a therapeutic target for BM.