Myeloablative and non-myeloablative conditioning media and cyclosporine treatment can create significant damages to patients by affecting electrolyte balance, through the alteration of the balance between intestinal absorption and renal excretion (42–44). Serum levels of calcium, magnesium, potassium, and phosphorus, in fact, tend to drop suddenly, thus creating a severe acid/base imbalance (44). Philibert et al. (43) observed that in 48 patients receiving HSCT from peripheral blood, 40% had serum potassium values less than 3.5 mmol/l, 33% had magnesium values less than 0.7 mmol/l, 43% had calcium values less than 2.14 mmol/l, and 35% had phosphorus values less than 0.6 mmol/l, immediately after HSCT. In total, 91% of patients had hypophosphatemia, while 49% had hypocalcemia after the transplantation. Crook et al. (44) reported severe hypophosphatemia in 77% of patients during the period of the conditioning regimen, or in the following week.

The hematopoiesis of transplanted cells may contribute to altering the phosphorus balance in patients undergoing HSCT, causing hypophosphatemia (45, 46); a diagnosis of severe hypophosphatemia after transplantation is directly associated with the proliferation of transplanted HSCs (44–48). Extensive phosphate consumption seen in HSCT patients is due to the need of transplanted HSCs of an energy source for the ex-novo proliferation of the hematopoietic cell compartment (44–48). Raanani et al. (45) observed a correlation between the release of the cytokines IL-6 and IL-8 and the severity of hypophosphatemia, defining the increase in these interleukins as a predictive index directly proportional to the severity of hypophosphatemia.

Hypomagnesemia observed in patients with HSCT may be one of the causes of altered electrolyte balance. Very low levels of magnesium cause a decrease in cAMP activity, which leads to prolonged opening of potassium channels, causing a decrease in serum levels of potassium itself (43, 49).

In addition, several studies (43, 44, 48, 50–53) have shown that intracellular magnesium depletion can compromise the functionality of parathyroid glands to secrete parathyroid hormone (PTH), through modulation of cAMP activity. Decreased PTH secretion, caused by hypomagnesemia, leads to hypocalcemia, and is associated with decreased active vitamin D (43, 49, 51, 52). Agus et al. observed that PTH-induced calcium release is severely impaired when the magnesium concentration is less than 0.8 mEq/l, and in severe hypomagnesemia there is resistance to PTH by target organs (49). It should be noted, however, that with magnesium replacement therapy, PTH serum levels are restored (42, 43, 52, 53).

Glucocorticoid therapy is the standard therapy against GVHD in allogeneic HSCT; these molecules alter PTH metabolism and calcium homeostasis. Among major side effects of glucocorticoids there are the suppression of osteocalcin (OCN) and alkaline phosphatase (ALP) expression, the induction of osteoclastogenesis and of bone resorption, and the impairment of osteoblastogenesis and osteoblastic activity, all causing severe bone loss and, consequently, osteoporosis (54). At the same time, glucocorticoids cause decreased intestinal calcium absorption and increased calciuria ( Figure 1 ), causing a severe decrease in serum calcium levels (55, 56) and, thus, inducing a strong stimulation of PTH production and leading to the establishment of secondary hyperparathyroidism (55, 57, 58). However, studies related to this topic show mixed results. There are studies in which no correlation has been found between glucocorticoids (even at high concentrations) and PTH increase (59–64), whereas some reports have observed that there is a dose-dependent PTH increase during treatment with glucocorticoids. A study using Northern blot analysis was conducted to determine whether PTH release in humans was directly stimulated by glucocorticoids (65). Dexamethasone was found to increase the amount of preproPTH mRNA in cells by directly acting at gene level in parathyroid cells, increasing PTH synthesis through stimulation of PTH gene transcription (65).

The reduction in intestinal calcium absorption and the sharp increase in PTH serum levels (66) could be also synergistically caused by the interference of post-transplant long-term glucocorticoid therapy with vitamin D metabolism, creating a non-competitive effect on the vitamin itself (67, 68). Indeed, after transplantation, vitamin D levels drop further in 90% of patients, a severe decrease that was mainly related to the intake of glucocorticoids, rather than myeloablative conditioning treatment. Physiologically, vitamin D stimulates intestinal absorption of calcium and consequently promotes a decrease in PTH production and release by the parathyroid glands. In HSCT patients, concomitant lack of exposure to the sun, due to their disease status, and long-term therapy with immunosuppressive glucocorticoids induce an overall vitamin D deficiency that affects the correct calcium and phosphate homeostasis and favors the occurrence of osteopenia and osteoporosis (66).

Calcium and mineral metabolism may be further altered due to renal failure as a common complication following HSCT, causing profound effects on morbidity and long-term survival of patients. Chronic kidney disease (CKD), a late-stage gradual loss of kidney function, is commonly diagnosed after HSCT. In observational studies, CKD has shown a variable incidence in HSCT patients, varying between 18-66%, due to the different parameters used for CKD definition and the differences in the duration of follow-ups (69–71). In general, renal dysfunctions are multifactorial complications that can be caused by pre-transplant underlying diseases and comorbidities, conditioning regimens, transplant complications and prophylaxis treatments (72).

Similarly, acute kidney injury (AKI) is a severe renal complication characterized by defects in glomerular filtration. It is diagnosed in HSCT recipients with variable incidence depending on the type of HSCT [from 20% to 73% (73)], and on the administration of myeloablative and non-myeloablative conditioning treatments (70, 74–77).

Pre-existing renal pathologies, such as glomerulonephritis, develop from hematological malignancies for which HSCT intervention is planned, therefore, the underlying hematological diseases already provoke renal lesions before HSCT, increasing in this way the risk of severe renal failure in the long term HSCT survival (78). Moreover, comorbidities such as diabetes, hypertension, and arteriosclerosis have been shown to further contribute to renal failure after transplant (72).

Pre-operative conditioning regimes may damage the kidneys. Several chemotherapeutics have been reported to induce renal failure, such as pre-renal azotemia and acute tubular necrosis, due to their nephrotoxicity, so a reduction of their dosage in HSCT recipients has been recommended (72, 79, 80).

Several studies reported in HSCT patients undergone total body irradiation the occurrence of renal defects, diagnosed several years after the radiation exposure, due to the late effects of radiotherapy on kidney physiology (69, 72, 81–83). Typical histopathological features of radiation-induced nephropathy are vascular, glomerular, and tubulo-interstitial damages. Radiations cause DNA double-strand breaks resulting in apoptosis or necrosis of renal endothelial, tubular and glomerular cells (84).

Moreover, both chemotherapeutics and radiation therapy have been associated with hepatic sinusoidal obstruction syndrome, due their cytotoxic effects on hepatic sinusoidal endothelial cells. This HSCT complication has been seen to induce systemic detrimental effects, including AKI (85).

It has been observed that transplanted cells have nephrotoxic effects per se. This is evident in the “marrow infusion syndrome”, a dysfunction that is caused by the cryopreservants, (i.e., DMSO) that are commonly used for long-term storage of the hematopoietic stem cells before their transplantation. Residues of those reagents can be infused together with the stem cells, leading to hemolytic effects that eventually hamper renal functionality.

Another severe transplant complication observed in both autologous and allogeneic HSCT recipients is the transplant-associated thrombotic microangiopathy (TA-TMA), an endothelial injury syndrome, with a still unclear etiology, which leads to microangiophatic hemolytic anemia, intravascular platelet activation, and formation of thrombi within the microcirculation, with end-organ damage from ischemia, particularly in the kidneys. Among possible causes of TA-TMA, there are HSCT treatments, such as radiation, chemotherapy, calcineurin inhibitors, and some HSCT-associated complications, including viral infections, GVHD, and hepatic sinusoidal obstruction syndrome (72, 86–88). The recognition of the early stages of TA-TMA is often difficult and, if not promptly diagnosed and treated, it commonly leads to permanent renal injury and, possibly, death.

Patients undergoing transplant are at higher risk of infection due to the administration of immune suppressants. Sepsis has been ascribed as a HSCT complication that can damage renal functionality due to renal hypoperfusion, tubulointerstitial injury, and capillary thrombosis (89, 90). Patients who require treatment in intensive care unit after transplant have higher risks of developing AKI (76). Adenovirus infections and the reactivation of the BK (Polyomavirus hominis 1) virus, which can occur in HSCT recipients, can cause tubulointerstitial nephritis and cystitis (70, 91, 92).

Even prophylaxis measurements used to prevent infections can provoke renal failure, as has been reported in patients treated with antifungals, such as amphotericin B (78) and with aminoglycoside antibiotics (93).

Moreover, patients receiving allotransplant are at high risk of developing GVHD that can cause systemic detrimental effects, including nephrotic syndrome, transplant-associated microangiopathy and glomerulonephritis, most likely due to the excessive release of pro-inflammatory cytokines such as TNF-α, IL-6, and TGF-β (72, 88, 94, 95). To prevent GVHD and treat chronic GVHD, patients receive immunosuppressants, including steroids and calcineurin inhibitors, which sometimes need to be taken for prolonged periods of time, causing renal complications (71, 96, 97).

An impairment of kidney activity that is a quite common primary or secondary complication in HSCT patients is at the base of an altered systemic mineral homeostasis, and, indirectly, of bone metabolism, that could be further worsened if it is associated with malabsorptive syndromes.

Bone homeostasis is the result of a finely regulated and correctly balanced activity of two different types of cells: osteoblasts, which form new mineralized bone, and osteoclasts, which reabsorb old and/or damaged bone tissue. Alterations in this homeostasis lead to the development of two main classes of bone diseases: those characterized by an excess of new bone deposition and mineralization, and those characterized by increased bone mass loss and/or reduced mineralization. Ultimately, both these two classes of disease generate a structurally impaired bone tissue, with altered mechanical properties, prone to deformities and/or fragility fractures.

Recent studies have reported that alteration of bone homeostasis, with loss of bone mass (osteopenia, osteoporosis), is a common side effect of the HSCTs (98–100). Osteopenia is diagnosed in 50% of cases 4-6 years post HSCT, while osteoporosis is diagnosed in 20% (98). It has been reported that the loss of bone mass occurs already few months after HSCT, with patients showing a reduction in Bone Mineral Density (BMD) primarily at femoral neck and lumbar spine (101, 102). Transplanted patients have an overall higher fracture risk compared to the general population (103, 104). Pathogenesis of accelerated bone loss in HSCT recipients is a multifactorial dysfunction that is not yet completely understood due to its complexity and to the contribution of many factors that concomitantly impact bone homeostasis ( Figure 1 ) (105–108). These multiple factors include the treatments that HSCT patients undergo before, during, and after transplantation (108). Underlying hematological diseases may also contribute to osteoporosis per se by inhibiting osteogenesis and promoting bone resorption, thus predisposing HSCT-patients to higher risks of BMD loss (109, 110). The age at transplantation appears to be a risk factor itself for bone mineral deficit in recipients of HSCT. Indeed, HSCT patients less than 10 years were showed to have, at an average age of 14 years after the transplantation, significantly lower Z-scores at lumbar spine and femur neck than HSCT patients transplanted at over 18 years and non-transplanted sibling controls (111).

Pre-transplant myeloablative treatments have been shown to negatively impact bone metabolism. Chemotherapeutic compounds, such as alkylating agents, methotrexate, and doxorubicin can deregulate bone remodeling by directly inhibiting osteoblast proliferation and osteoblastogenesis and, simultaneously, by stimulating osteoclast activity and bone resorption (112–116), or by indirectly damaging the reproductive system and sex hormone metabolism, which play a fundamental role in regulating skeletal modeling and life-long remodeling of bone tissue (117–120).

Total body irradiation, often coupled with chemotherapy as myeloablative conditioning treatment, dramatically reduces BMD by suppressing bone formation, enhancing bone resorption, and disrupting the inorganic components of the bone matrix (121–125). Moreover, irradiation has been associated with hypogonadism, hypopituitarism and growth hormone deficiency, thus leading to a decrease in trophic factors important for bone formation (105, 126, 127).

Chemotherapy- and/or radiotherapy-caused hypogonadism is one of the main causes of the occurrence of osteoporosis in HSCT patients. Indeed, under physiological conditions, sexual hormones are among the major regulators of bone metabolism in both females (estrogens and progesterone) and males (testosterone) (128).

Estrogens have a positive activity on bone formation, both by inhibiting osteoblast apoptosis and by increasing their lifespan (129). Consequently, the absence of these hormones in hypogonadal HSCT female patients promotes a bone metabolic framework oriented towards resorption. At the molecular level, estrogen absence leads to the non-activation of Src/Shc/ERK signaling and, consequently, the promotion of JNK, which, by modifying the activity of a series of transcription factors, including c-Jun/c-Fos, Elk-1, and CCAAT enhancer binding protein-β (C/EBPβ) (130, 131), promotes osteoblastic apoptosis. In addition, estrogen deficiency leads to a clear increase in NF-κB activity in osteoblastic cells (132), leading to a possible decrease in the expression of Fos-related antigen-1 (Fra-1), a transcription factor essential for bone matrix formation (131), contributing to a deficit in bone anabolism. At the same time, the estrogen deficiency leads to a fourfold increase in bone resorption activity (133, 134); the absence of estrogen causes a rapid decrease in bone mass due to the expansion of T-cells that produce high levels of TNFα in the bone microenvironment. TNFα, together with activation of macrophage colony-stimulating factor (M-CSF) and RANK, activates its p55 receptor, promoting osteoclastogenesis (135–138). Osteocytes respond to estrogen deficiency; the absence of estrogen increases osteocyte apoptosis, resulting in increased concentrations of RANKL, probably released by apoptotic osteocytes or as a result of cell-to-cell signaling between healthy and apoptotic osteocytes, which induces increased bone resorption through stimulation of the osteoclastic cell compartment (131, 139–142).

Testosterone deficiency is a known cause of bone loss and the onset of osteoporosis in men (143–153). Chronic testosterone deficiency, such as that occurring in hypogonadal HSCT male patients, could concur with the early-onset osteopenia/osteoporosis observed in these individuals.

Indeed, under physiological conditions, testosterone acts on bone metabolism in two different ways:

Testosterone deficiency results in a decreased differentiation and activity of osteoblasts, leading to decreased new bone formation. Orchiectomized rodents showed that testosterone depletion induces a significant increase of RANKL and IL-6 expression in bone marrow-derived stromal cells, which strongly promote osteoclast differentiation, with consequently increased bone resorption and bone mass loss (156–158). Moreover, testosterone deficiency causes an indirect deficiency of estradiol (159, 160), which leads to an enhanced bone loss through the increased production of several inflammatory cytokines (IL-1 and TNFα, IL-6), which stimulate osteoclastic differentiation and activity and promote osteoblastic apoptosis (161–165).

Moreover, since testosterone plays a predominant role in male muscle strength and physical performance, it is conceivable that the absence of this hormone in hypogonadal HSCT male patients can cause sarcopenia, leading to greater weakness and increasing the risk of falling and fracturing (140, 166).

In relation to what has been described above regarding the importance of sex hormones in maintaining bone homeostasis, the onset of hypogonadism in HSCT patients obviously contributes to the loss of bone mass and the onset of osteoporosis; a greater relationship was found between individuals suffering from HSCT-derived medium/severe hypogonadism and decreased BMD and osteoporosis (167–169).

Several studies have reported that one early side effect of HSCT is the failure of bone morphogenesis, due to a severe reduction of osteoblast progenitor cells in bone marrow, causing post-transplant BMD loss (106, 108, 170–172). Moreover, the immune response, following the transplant, seems to play a pivotal role in altering bone homeostasis by promoting osteoclast maturation and activity (105, 106, 108). After HSCT, indeed, it has been observed that the balance between RANKL and osteoprotegerin (OPG), expressed/released by bone marrow stromal cells and osteoblasts, is shifted towards RANKL expression, causing an excessive induction of osteoclast differentiation and activity, with increased bone resorption. Simultaneously, it has been seen that certain cytokines typical of the immune and inflammatory response, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-6 and TNF-α can stimulate osteoclastic activity (173–175).

HSCT-derived immune derangement has been found to trigger bone loss, most frequently in allogeneic HSCT recipients, compared to autologous HSCT, due to the increased synthesis of specific inhibitors of the canonical Wnt/Beta Catenin pathway, Dickkopf-1, and Sclerostin (176), principally as a consequence of GVHD and/or administration of GVHD prophylactic treatments. It has been observed that GVHD occurs in 40-70% of allogeneic HSCT recipients and that the severity of GVHD is positively correlated with the degree of BMD reduction (106, 173, 174, 177–179). Anti-GVDH treatments, consisting in the administration of glucocorticoids and calcineurin inhibitors, also seem to play a major role in HSCT-associated osteoporosis (107, 108). High-dose glucocorticoid therapy is usually the first-line approach against GVDH, causing a rapid decrease of BMD in HSCT-recipients, by inhibiting new bone formation and promoting bone resorption (107, 180). Glucocorticoids can shift the ratio between OPG and RANKL towards the RANKL expression, with a consequent boost in bone turnover, in a time- and dose-dependent manner (102, 180–182). Similar to glucocorticoids, immune-suppressive calcineurin inhibitors, such as cyclosporin A and tacrolimus, seem to promote bone resorption and osteoclast activity, while both interfere with osteogenesis and osteoblasts activity (104, 183).