A higher prevalence of osteoporosis and osteopenia has been reported in NET patients, and for those younger than 50 presents with an increased risk of developing osteopenia or osteoporosis compared to the matched control group (odds ratio (OR) = 3.24, 95% CI: 1.36–7.73, p
= 0.008) [18
]. The exact mechanism of this increased risk is not entirely known. Besides bone metastasis, different factors could have an impact on bone health in GEP–NET patients, including hormone hypersecretion, specific micro-RNAs (miRNAs), nutritional status, vitamin D deficiency, quality of life, and aspects correlated to MEN1 (Figure 1
2.1. Hormone Hypersecretion
GEP–NETs can be associated with functional syndromes. The most frequent functional syndrome associated with NET is carcinoid syndrome (CS). Over 40 humoral substances have been identified as being potentially involved in the pathogenesis of carcinoid syndrome, but the main mediator is serotonin (5-HT) [35
]. 5-HT is also one of the key players in bone tissue dynamics [36
]. It has been demonstrated that osteoblasts, osteoclasts, and osteocytes express tryptophan hydroxylase 1 (TPH1), which synthetizes 5-HT, 5-HT transporters (SERT), and 5-HT receptors [37
]. Depending on the site of production, 5-HT can exert contrasting effects on bone density. When produced in the brain, 5-HT exerts a positive effect on bone mass by decreasing the inhibition of sympathetic nerves on osteoblasts and consequently enhancing bone formation. On the contrary, 5-HT secreted by the neuroendocrine cells of the gastrointestinal tract inhibits osteoblast proliferation by its binding to 5-HT1B
receptors expressed by pre-osteoblasts [39
]. The role played by gut-derived 5-HT in the regulation of bone metabolism has been further defined through a link between a member of the low-density lipoprotein receptor (Lrp) family, Lrp5, and the level of TPH1 in the neuroendocrine cells of the gastrointestinal tract [41
]. It has been demonstrated that Lrp5 knockout mice had increased TPH1 expression on neuroendocrine cells of the gastrointestinal tract, increased levels of circulating 5-HT, and decreased bone density [41
]. At the molecular level, the action of 5-HT on osteoblasts is mediated by the transcription factor forkhead box protein O1 (FOXO1). Under physiological conditions, FOXO1 interacts with both cAMP-responsive element–binding protein 1 (CREB) and activating transcription factor 4 (ATF4) in the nucleus of osteoblasts (Figure 2
]. The interaction with ATF4 promotes the transcriptional activity of FOXO1 and the expression of FOXO1-dependent genes, which mediate cell cycle arrest, whereas the interaction with CREB suppresses FOXO1 transcription and stimulates the expression of cyclin genes, which mediate cell proliferation. The balance of the opposing actions of these two transcription factors maintains normal osteoblast proliferation. In pathological conditions of high 5-HT levels, as observed in carcinoid syndrome, the association between FOXO1 and CREB is disrupted, favoring the formation of ATF4-FOXO1 heterodimers that resulted in an upregulation of the ATF4-mediated responses (Figure 2
). These events subsequently lead to suppression of cyclin genes and cell cycle arrest [42
The effect of 5-HT as a possible mediator in bone damage was also confirmed by several preclinical and clinical studies showing that selective serotonin reuptake inhibitors are associated with bone loss and an increase in fracture risk [41
]. However, clinical studies in NET patients did not confirm the alteration of bone metabolism in the condition of high levels of 5-HT. Two different studies demonstrated that bone mineral density (BMD), as well as markers of bone formation and markers of bone resorption, were not different between patients with high and low urinary 5-hydroxy-indoleacetic acid (5-HIAA) levels, the 5-HT metabolite (Table 1
]. Another study comparing 26 midgut NET with carcinoid syndrome and matched healthy controls showed that high circulating 5-HT levels in carcinoid syndrome were not associated with lower BMD, poorer bone structure, or lower levels of bone formation markers (Table 1
Besides carcinoid syndrome, functional pancreatic NET (pNET) have to be mentioned. The two most common functional pNET are gastrinoma and insulinoma, followed by VIPoma, glucagonoma, GRFoma, ACTHoma (leading to ectopic Cushing’s syndrome, ECS), tumors causing carcinoid syndrome or hypercalcemia (PTHrPomas), and somatostatinoma. In addition to these, there are also very rare functional pNET secreting renin, luteinizing hormone (LH), erythropoietin, glucagon-like peptide-1(GLP-1), insulin-like growth factor-2 (IGF-II), and cholecystokinin (CCKoma). Non-functional p-NET are not associated with specific symptoms but frequently secrete pancreatic polypeptide, chromogranin A, neuron-specific enolase, human chorionic gonadotrophin subunits, calcitonin, neurotensin, or other peptides [12
]. To our knowledge, there are no specific studies about functional and non-functional pNETs and bone health. Nevertheless, ECS caused by ACTH secretion and primary hyperparathyroidism (HPT) is already recognized as a cause of secondary osteoporosis [45
]. Gastrinoma will be discussed later within MEN1.
ECS represents only 10% of all causes of Cushing’s, and among these pNET, represents only a small percentage of cases. However, ECS is the most frequent endocrine paraneoplastic syndrome reported in NET [47
]. It has well demonstrated that cortisol excess has a negative impact on bone health [48
]. Osteoporosis induced by glucocorticoid excess is due mainly to a direct effect on osteoblasts, osteocytes, and osteoclasts, which express glucocorticoid receptors (GR; Figure 3
]. In osteoblasts, cortisol excess induces a reduction of bone formation mediated by an upregulation of peroxisome proliferator-activated receptor (PPAR)-γ and an inhibition of the wnt pathway, which promotes, in turn, osteoblast apoptosis and differentiation of mesenchymal progenitors into adipocytes. These processes result in a low number of osteoblasts and decreasing bone formation [51
]. Sclerostin secreted by osteocytes induces osteocyte apoptosis and inhibits the wnt pathway. Moreover, the increase of the receptor activator for NF-κB ligand (RANKL)/osteoprotegerin (OPG) ratio, together with the increased macrophage colony-stimulating factor (M-CSF), stimulates osteoclastogenesis and bone resorption (Figure 3
In addition to this direct effect on bone, glucocorticoid excess may also have a negative impact on vitamin D levels [52
]. Different case reports describe the occurrence of osteoporosis in patients with GEP–NET and ECS (Table 1
]. Interestingly, a 34-year-old female patient with a history of multiple spontaneous rip fractures and osteoporosis, who had been diagnosed with NET of the appendix associated with ECS, reported an increase in BMD during the three years of follow-up after tumor resection [28
]. This study represents a potential reversibility of steroid-induced osteoporosis and, when possible, complete resection of the primary tumors should be considered in the management of such patients.
Similarly, hypersecretion of parathyroid hormone (PTH)-related protein determines, with different mechanisms, an imbalance between bone resorption (normal or increased, especially in the early phase) and bone formation (impaired, particularly in the chronic phase), with a consequent increased risk of fractures [53
MiRNAs are small noncoding single-stranded RNAs of 21–23 nucleotides long that regulate the expression of 1%–4% of human genes at a posttranscriptional level. Therefore, miRNAs are involved in many physiological and pathological conditions, including GEP–NET progression and metastatic spread [13
Several miRNAs are involved in the regulation of bone metabolism and homeostasis. However, in pathological conditions, impaired miRNA signaling could contribute to the onset of skeletal disorders, including osteoporosis [55
]. Among miRNAs that regulate osteoblast differentiation [56
], miRNA-210, miRNA-21, and mi-RNA 196a may play key roles in the regulation of bone metabolism in patients affected by GEP–NET (Figure 1
). It has been demonstrated that miRNA-210 enhances the differentiation of bone marrow mesenchymal stem cells into osteoblasts in addition to inducing the expression of vascular endothelial growth factors [58
]. Moreover, miRNA-210 is upregulated in GEP–NET and is associated with metastatic disease, but not with tumor grade (G) [59
]. Also, miRNA-21, which is overexpressed in pNET patients with high Ki67 proliferation index (>2) and presence of liver metastasis [60
], plays a crucial role in osteoblast differentiation, enhancing the expression of matrix metallopeptidases and promoting epithelial to mesenchymal transition [61
]. Finally, miRNA-196a regulates the osteogenic differentiation and proliferation of human adipose tissue-derived mesenchymal stem cells [62
]. It has been shown that high expression of miRNA-196a is significantly associated with pNET having aggressive biological behavior, characterized by advanced stage, lymph node metastases, higher mitotic count and higher Ki67 index (≥3%, corresponding to G2–G3 tumor), and worse clinical outcomes [63
However, the potential role of these or other miRNAs in the maintenance of bone metabolism in patients with GEP–NET has not been directly investigated. Therefore, specific studies are needed to better elucidate this topic.
2.3. Nutritional Status
Bone health is strictly connected with nutritional status, which has been reported to be altered in 14% to 25% of NET patients [64
]. Particularly, patients with high-grade (G3) NET, progressive disease, and who are undergoing chemotherapy are at high risk of malnutrition (57.9%, 39.5%, and 42.3% respectively) [64
Nutritional status in GEP–NET patients is deeply affected by the excessive production of gastrointestinal hormones, peptides, and amines, which could cause malabsorption, diarrhea, steatorrhea, and altered gastrointestinal motility. Furthermore, surgical resection of GEP–NET could change the anatomy of the gastrointestinal tract, and therapy with somatostatin analogs (SSAs) could modify the function of the gastrointestinal tract by inhibiting the secretion of pancreatic enzymes and hormones, impairing normal absorptive function (Figure 1
]. It has been recently demonstrated that GEP–NET patients have a dietary pattern with significantly lower adherence to the Mediterranean diet compared to a health-matched control group, less frequently consuming vegetables, fruits, wine, fish/seafood, and nuts, and more frequently red/processed meats, butter, cream, margarine, and soda drinks [20
]. All these factors could affect the intake and absorption of vitamin D and calcium, which play key roles in the maintenance of bone health, with consequent bone loss.
The role of vitamin D is reported in a separate section of this manuscript.
The surgical approach or medical treatment represents the main therapeutic options reported in current guidelines for GEP–NET, depending on tumor location, grade, size, and symptoms. Surgical removal of the primary tumor is the preferred treatment where it is possible, but it can also be considered in metastatic disease, and this may have survival benefits in some sites.
Regarding pNET, enucleation, Whipple resection, or distal pancreatectomy/splenectomy are recommended in case of symptoms, intermediate-to-high grades (G2–G3), or measure greater than 2 cm. Conversely, if smaller than 2 cm, low-grade, and nonfunctioning, pNET can be monitored according to a “watch-and-wait” strategy [12
]. Regarding small intestinal NET, partial small bowel resections are usually performed for jejunal or proximal ileal tumors, whereas right hemicolectomy is indicated for tumors arising in or near the ileocecal valve [69
There are no data about a direct impact of surgical treatment on bone health in NET patients. Nevertheless, surgical treatment can influence nutritional status, causing diarrhea for several reasons. As previously discussed, the extent of the intestinal resection and the anatomical position of the resected region are the main factors with an impact on bowel transit time, vitamin B12 action, bile acid secretion, fat-soluble vitamins absorption and digestion [19
Concerning medical therapy of GEP–NET, SSAs are the treatment of choice in patients with low grade G1–G2 tumor, in functional syndrome, if surgical resection cannot be performed or in case of metastatic disease [12
]. The most common SSA side effects are steatorrhea, flatulence, nausea, and abdominal pain, all induced by inhibition of pancreatic digestive enzymes secretion and suppression of intestinal motility, regardless of dosage or formulation (octreotide or lanreotide) used [71
]. SSAs may reduce intestinal fluid secretion and also the secretion of pancreatic enzymes and bile acids, which diminish fat absorption. Thus, similarly to surgical treatment, a direct relationship between SSA treatment and bone has not been demonstrated. It could be possible that malabsorption of fat and fat-soluble vitamins may lead to reduced bone density due to vitamin D deficiency [19
]. Nevertheless, this hypothesis has been contradicted. The study from Motylewska et al. has not revealed any significant difference in 25-hydroxyvitamin D [25(OH)D] levels between patients with NET who were receiving or not SSAs, indicating that long-term SSA therapy does not affect serum vitamin D level. Furthermore, this study has shown that the average concentration of 25(OH)D did not depend on primary tumor localization. Thus, these results have not confirmed the hypothesis that patients with GEP–NETs may be a group of higher risk of vitamin D deficiency [29
Patients with metastatic pNET have a larger number of other treatment options, with cytoreductive effects. First-line SSAs can be followed with everolimus and sunitinib, and peptide receptor-radionuclide therapy (PRRT) with radiolabeled synthetic and stable SSA (90Yttrium DOTATOC/DOTATATE or 177Lutetium DOTATOC/DOTATATE) [69
Everolimus is an oral mTOR inhibitor. MTOR signaling has been shown to regulate osteoclastogenesis and osteoclast function [73
]. A study in an osteotropic breast cancer model demonstrated that everolimus had a bone-protective efficacy both in vitro and in vivo, by impairing osteoclastogenesis and preventing the bone loss [74
]. Moreover, everolimus has been associated with nausea, vomiting anorexia, stomatitis, and diarrhea—all factors indicating a malnourished status [71
]. Sunitinib is an oral multitarget tyrosine kinases inhibitor (TKI), which mainly binds the vascular endothelial growth factor (VEGF) receptors and the platelet-derived growth factor (PDGF) receptors. In the literature, data about this drug class and bone metabolism are very scarce. However, it has been associated with an increased risk of osteonecrosis of the jaw in patients with bone metastatic renal cell carcinoma who were treated with bisphosphonates [75
]. Sunitinib is also associated with diarrhea and possible other complications related to malnutrition and vitamin D malabsorption [71
]. Cabozantinib is a new TKI that is currently under investigation in metastatic pNET. In a recent clinical trial on patients with advanced renal cell carcinoma and bone metastases, cabozantinib has been associated with an improvement of survival parameters when compared with everolimus. Furthermore, cabozantinib determined an important change in bone biomarkers, including decreases in the bone formation marker procollagen type 1 N propeptide (P1NP) and the bone resorption marker C-terminal telopeptide of type 1 collagen (CTx), but the authors did not provide an explanation for this [76
PRRT is generally well-tolerated, but it is not completely free from side effects that can be acute or delayed. Acute side effects include nausea, vomiting, and headache, usually mild and self-limiting. Other acute adverse events are fatigue, abdominal pain, asthenia, and flushing [71
]. To the best of our knowledge, at present, there are no studies about 177Lu DOTATATE or 90Y DOTATOC and bone metabolism.
Recommended chemotherapy, as cytotoxic therapy, includes the following: the combination of cisplatin and etoposide, streptozocin (STZ) with 5-fluorouracil (5FU), temozolomide (TMZ) +/- capecitabine, oxaliplatin combinations with fluoropyrimidines (5-FU or capecitabine) and irinotecan-based therapy [77
]. To our knowledge, all of these therapeutic agents may be associated with malnourished status, but their potential effect on bone has not yet been explored.
In the clinical management of CS, a new drug has recently been introduced. Telotristat etiprate is an oral inhibitor of the enzyme tryptophan hydroxylase, which is the rate-limiting step in serotonin synthesis. The phase 3 TELESTAR study demonstrated that treatment with telotristat ethyl was generally well tolerated and was associated with significant reductions in bowel movement frequency and urinary 5-HIAA levels in patients with CS not adequately controlled by SSA therapy. However, this agent has no effect on tumor mass [78
]. Common side effects of telotristat include nausea, abdominal pain, and a low rate of depression. We may speculate that, by reducing diarrhea, telotristat could ameliorate the nutritional status of NEN patients, but further studies are needed [79
Therefore, GEP–NET treatment can have a role in determining disturbances of bone metabolism, but the kind of relationship between tumor, its treatment, and bone needs to be clarified.
2.5. Impact of Quality of Life
Quality of life is an important measure of patients’ perception of the burden of the disease and the impact of the different treatment modalities. Several studies have reported a worse HRQoL in GEP–NET patients compared to matched controls [80
]. Particularly, NET patients reported poorer physical function, sleep disorders, discomfort, and depression. Furthermore, patients with recurrent disease reported significantly higher anxiety, impaired overall physical function, impaired sleep, and significant fatigue compared to those with no current NET [82
]. It has been reported that in patients with depressive disorder, the duration of the disease negatively correlates with BMD and physical activity, which, in turn, significantly reduces the HRQoL [83
]. Moreover, low physical activity is associated with an increased risk of osteoporosis [84
]. In addition, anxiety has been associated with BMD in the lumbar spine and femoral neck [85
]. Therefore, depression, low physical activity, and anxiety worsen bone health. Although the impact of depression and low physical activity on bone health is not yet been investigated in GEP–NET, we speculate that these two factors may have an impact on bone metabolism in GEP–NET patients. However, further studies are needed to better establish this potential correlation.