Metabolic bone disease in HIV infection

Introduction HIV mainly replicates in CD4+ T lymphocytes and monocyte/macrophages causing severe immunological impairment. In addition to the immune system, HIV infection affects tissues and organs such as kidney, liver, the central nervous system, heart and bone showing a complex pathogenesis 1. The advent and widespread use of highly active antiretroviral therapy (HAART) in the last two decades has led to a marked improvement in the treatment of HIV disease even though viral infection cannot be eradicated because HAART does not completely eliminate the viral reservoirs 2. HAART has dramatically changed the course of HIV infection from a fatal infection to a chronic and relatively manageable disease. The increased life expectancy of HIV patients and the effects of HAART have changed the management of HIV infection. Nowadays medical treatment is no longer focused solely on HIV infection, opportunistic diseases and monitoring immune derangement, but also includes the control of metabolic, cardiovascular, liver, bone and kidney complications. In particular, bone alterations have been observed in the course of HIV disease representing a pivotal clinical problem in the management of HIV patients especially for a possible development of bone fractures 3. The major bone lesions detectable in HIV patients are related to bone demineralization (osteopenia/osteoporosis and osteomalacia) and osteonecrosis (4 for a review). This report will discuss the pathogenesis, diagnosis and treatment of major bone complications represented by bone demineralization diseases during HIV infection and HAART treatment. Osteopenia/osteoporosis in HIV-infected patients Bone alterations have been observed in the course of HIV disease since for nearly two decades (Table 1). In particular, reduced bone mineral density (BMD) is the most common bone lesion found in HIV-infected individuals 5,6. BMD is a parameter that predicts fracture risk, which in turn correlates with a shorter life expectancy 7. BMD is measured by the dual X-ray absorptiometry scan (DXA). According to the WHO Classification, BMD is commonly reported in terms of DXA T-score, which represents the number of standard deviations below the mean of a young, sex-matched control population. T-score values are considered normal above the limit of −1. Values between −1 and −2.5 indicate osteopenia (low bone mass) whereas a T-score value below −2.5 signifies osteoporosis 8,9. Osteoporosis is a systemic condition characterized by both quantitative and qualitative alterations that reduce bone strength 10.Table 1: Summary of HIV and HAART-related bone lesions.Several groups have used DXA to study BMD status during HIV infection. A meta-analysis of selected reports on bone loss in the whole HIV patient population (HAART treated plus naive) from 1994 to 2005 showed that these individuals had 6.4 fold increased odds of osteopenia and 3.7-fold increased odds of osteoporosis in comparison with uninfected individuals 11. The relation between antiretroviral treatment and osteopenia/osteoporosis has been noted in several studies 12–18 although other reports failed to find any influence of HAART on bone loss, disclosing no major differences between naive and HAART treated patients 19–23. A recent study on 492 patients belonging to the Aquitaine Cohort reported osteopenia in 50% and osteoporosis in 30% of HIV-positive cases but multivariate analysis did not show a significant correlation to bone loss and cumulative HAART or specific drug class 24. In spite of these opposing findings, a meta-analysis of selected cross-sectional studies demonstrated that the odds of osteoporosis were increased 2.4 times in HAART-treated patients compared with naïve individuals 11. In addition, the meta-analysis by Brown and Qaqish on 12 studies disclosed that patients treated with protease inhibitors have a higher prevalence of reduced BMD and the odds of osteoporosis in protease inhibitor-treated patients are 1.6 greater than in protease inhibitor-untreated individuals 11. The controversy over the role of antiretroviral compounds in BMD decrease could be explained by shortcomings in some studies. HAART typically combines nucleoside analogue reverse transcriptase inhibitors (NRTIs) with either HIV protease inhibitors or nonnucleoside reverse transcriptase inhibitors (NNRTIs), thus, the antiretroviral cocktail composition may differ within the same cohort with conceivably different effects on bone. In addition, some DXA studies analysed only the spine (mainly confined to trabecular bone), or hip (mostly cortical bone) or both bone sites. The choice of bone for DXA assay is not negligible; the human skeleton is composed of two different types of bone tissue: trabecular bone (comprising around 20% of the bone and mainly involved in the maintenance of mineral homeostasis) and cortical bone (80% and responsible for most support functions). Plainly, high bone turnover states, such as HIV-induced osteoporosis, involve trabecular bone (spine) earlier and to a greater extent compromising cortical bone (hip) only much later 25. Moreover, the effectiveness and duration of HAART treatment may also affect bone biology and hence the interaction between HAART and bone is noteworthy 26,27. Mechanisms of HIV-associated osteopenia/osteoporosis The pathogenesis of reduced BMD in HIV-infected patients is probably multifactorial. Osteopenia and osteoporosis are bone lesions mainly correlated to risk factors such as sex, age, low body weight, malnutrition, immobility, lifestyle factors (smoking, alcohol abuse), glucocorticoid, hypogonadism and lipodystrophy 28. The sum of traditional patient-related risk factors with HIV infection and HAART side effects can determine the onset of these bone lesions in HIV-infected patients. Bone cellular components Bone is a mineralized tissue composed of bone matrix and bone cells. Its homeostasis is mainly due to the tightly integrated contrasting activity of two major bone cell types: bone forming osteoblasts and bone resorbing osteoclasts. These cells are functionally connected and regulated by mediators such as hormones, vitamins and cytokines that strongly affect the skeletal biology throughout life 29. Osteoblasts arise from mesenchymal stem cells and determine the formation and structural organization of bone extracellular matrix and its mineralization 30. Mature osteoblasts synthesize several molecules involved either in bone formation or in regulating osteoclast activity such as type I collagen, osteocalcin, osteopontin, proteoglycans, receptor activator for nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG) 31. Notably, osteoblasts may also evolve to osteocytes when embedded in bone matrix, playing an important role in the control of architectural bone structure 32,33. Osteoclasts are members of the monocyte/macrophage lineage originating from multiple cellular fusions of their precursors 34 that proliferate and differentiate towards mature osteoclasts by means of macrophage colony-stimulating factor (M-CSF) and RANKL 35. M-CSF mainly induces precursor cell proliferation whereas RANKL plays a pivotal role in their differentiation, full functional activation and multiple cellular fusions of osteoclasts. Mature osteoclasts are able to resorb bone both by acid environment induction and secretion of lytic enzymes 36–38 such as cathepsin K and tartrate-resistant acid phosphatase (TRAP). The functional balance and cross-talk between osteoblasts and osteoclasts are crucial in determining bone mass (Fig. 1), which depends on the well tuned bone remodelling characterized by osteoclast bone resorption and osteoblast bone rebuilding phases 31. An imbalance of the osteoblast/osteoclast interaction due to pathological conditions such as infection, hormonal, immunological and metabolic disorders, impairs both bone mass and structure impairment resulting in increased bone fragility and fracture risk.Fig. 1: Flow chart indicating bone mass loss after HIV infection. The well-tuned regulation between the bone resorption by osteoclasts and bone rebuilding by osteoblasts determines the bone homeostasis (a). When HIV infection occurs, this balance is impaired by increase of osteoclast differentiation and activity associated to apoptosis activation and biological activity inhibition of osteoblasts (b). Hence, HIV infection is able to elicit a preferential bone resorption with subsequent bone mass loss.The role of HIV infection As avian, feline and murine retroviruses are known to infect osteoblasts and osteocytes 39–41, early studies focused on the hypothesis that human osteoblasts may be a permissive target for HIV infection decreasing BMD through a direct viral mechanism. Some reports showed that HIV-1 transmission can occur during bone transplantation 42 and HIV-positive PCR assay has been observed in bone graft 43. H9 cell line or peripheral blood mononuclear cells (PBMC) cocultivated with bone fragments from HIV-1-positive individuals displayed both a positive HIV RT activity and p24 detection in cell supernatants 44. However, it was not clear whether blood or bone marrow HIV-positive cells contamination could be excluded in these studies. Mellert et al.45 found that osteoblast-like cell lines were infected when challenged by HIV. Together, these data suggested that bone might be considered an HIV reservoir where the limited blood flow and the particular anatomical structure may also induce a poor antiretroviral concentration to tackle the HIV infection 46. Moreover, the infection of osteoblasts may be closely related to the incomplete refilling of bone lacunae during bone remodelling with subsequent bone loss. In spite of these observations, further studies performed on human primary osteoblasts did not confirm the results obtained in osteoblast-like cell lines. The primary osteoblasts taken from HIV-positive individuals did not show viral DNA and RNA in PCR assays 47. In addition, another study disclosed the failure of HIV productive infection when cultures of primary osteoblasts were challenged with classical HIV laboratory strains 48. The lack of susceptibility may be partially explained by shortage of CD4 receptor and coreceptor proteins on osteoblast cell membrane 47,48. In addition, as observed on CD34+ hematopoietic progenitor cell membrane 49, conceivably the CD4/CXCR4 complexes might be not so sterically closed to constitute the trimeric complex with gp120 essential for HIV entry. In addition to the direct effect of HIV replication, the apoptosis process plays a pivotal role in HIV pathogenesis. The progressive loss of CD4+ T lymphocytes is also related to apoptosis activated by the interaction between HIV gp120 and the CD4 receptor 50,51. In addition, HIV-related apoptosis is a major mechanism involved in anaemia, thrombocytopenia and induction of neuronal cell death 52–54. A recent paper showed an increased rate of apoptosis in primary osteoblasts treated by gp120 or challenged with heat-inactivated HIV laboratory strains 48. Apoptosis activation occurs by a paracrin/autocrin mechanism due to TNF-α increase 48. This finding may suggest that part of the bone loss detected in HIV-infected patients may be related both to apoptosis and the decreased biological activity of osteoblasts. The inhibitory effect of HIV gp120 on osteoblast function was confirmed by Cotter et al.55 who found that gp120 (Fig. 2 and Table 2) reduces calcium deposition, alkaline phosphatase activity and bone specific Runt-related transcription factor 2 (RUNX-2) transcription factor expression after 24 h of treatment in primary osteoblast cultures. In agreement with these data, histomorphometric and analysis showed impairment in primary osteoblast functional activity and a decrease of in HIV-infected patients between HIV and osteoblast HIV gp120 determines factor expression increase in primary osteoblasts. TNF-α induces apoptosis activation in primary osteoblasts by In addition, gp120 determines a of several osteoblast activity an inhibition of biological function of these cells. The interaction between HIV and the osteoblast progenitor cells represented by the mesenchymal stem cells determines the inhibition of and proliferation by direct and HIV gp120 induces the inhibition of Runt-related transcription factor 2 (RUNX-2) and the activation of receptor with a preferential of mesenchymal cells differentiation from osteoblasts to HIV-related effects on bone are from bone marrow mesenchymal stem cells. Hence, some studies to whether mesenchymal stem cells and their differentiation towards osteoblasts are impaired by HIV infection (Fig. et showed that bone marrow mesenchymal stem cells could be infected to a low extent by HIV strains to of the these cells with subsequent inhibition of proliferation and differentiation from mesenchymal cells are also impaired by through the of TNF-α and the interaction between specific HIV proteins and mesenchymal cells towards osteoblasts was In particular, and gp120 viral proteins a of specific transcription factors involved in the differentiation and activity of osteoblasts. HIV gp120 (Fig. 2) is also able to the activation of receptor determining an differentiation from osteoblasts to reports have also analysed the influence of HIV on osteoclasts (Table RANKL and M-CSF are factors the proliferation and differentiation of osteoclast lineage cells. A significant increase in RANKL with an impairment of was in HIV-positive patients The RANKL increase correlated with high viral RNA indicating a direct relation between HIV infection status and RANKL Moreover, gp120 RANKL secretion (Fig. in primary T cells whereas the activation of RANKL in several cell such as primary T cells and cell line In RANKL HIV in and infected and a between HIV and RANKL of HIV-related of osteoclast HIV gp120 the of receptor activator for nuclear factor ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) in T lymphocytes and determining the increase of osteoclast differentiation and is a factor the proliferation and differentiation of the lineage and it is closely involved in the early phases of osteoclast The pivotal of M-CSF and its receptor in osteoclast differentiation was also confirmed by and bone alterations in in the or HIV infection of induces a significant increase in M-CSF and secretion which in turn further HIV infection of through the increase in and expression M-CSF osteoclast differentiation also the RANKL effect (Fig. In addition, et showed that bone marrow M-CSF a of compared with cells with This finding that M-CSF in As a receptor osteoclast differentiation by as a RANKL the of M-CSF during HIV infection impairs the balance between and osteoclasts (Table The role of HAART In the of HAART in the treatment of HIV infection led to a and decrease in HIV-related and HAART typically combines nucleoside analogue reverse transcriptase inhibitors (NRTIs) with either HIV protease inhibitors or nonnucleoside reverse transcriptase inhibitors results antiretroviral molecules and bone loss, several groups the possible bone of specific antiretroviral The role of The nucleoside (NRTIs) are antiretroviral molecules structure is a These compounds the of retroviruses by with the reverse transcriptase activity causing of the HIV DNA and are used in the major positive of these molecules in HIV clinical disclosed severe side effects such as and the DNA the involved in the of to and studies disclosed some differences in the induction of specific DNA The and are relatively inhibitors of than other nucleoside In the of or the of is the of with a decrease in does not to even though this condition is in individuals with and patients plus studies on patients demonstrated that is a relatively common in of individuals a whereas is in than of patients et analysed patients by and found an between and reduced BMD (Table These data suggest that by may osteopenia by a mechanism related to calcium loss as the bone to chronic This mainly affects the trabecular which represents the of bone and is a of calcium than cortical bone. 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The and studies performed on a number of patients with no of found the same of and in the and after 24 of treatment. The a over compared a treatment of and with a treatment of and in patients the and These results may be related to cohort as the patients in the studies did not show low is that the specific HAART and functional conditions of patients the interaction between and bone. The role of protease inhibitors inhibitors HIV by the viral protease a pivotal in the of the viral The viral obtained in the of protease cannot infect the target cells. and are the protease inhibitors used for antiretroviral The protease interaction has been in bone cell cultures (Table The effects of protease inhibitors on osteoclasts were osteoclast activity in and showed activity whereas and did studies demonstrated that and osteoclast activity through the of a to RANKL represented by of receptor associated factor RANKL to the of resulting in the activation of nuclear factor of activated cells and involved in the and differentiation of osteoclasts. A subsequent paper showed that had effects by This finding suggested a complex in the between protease inhibitors and the osteoclast inhibitors were also on the human mesenchymal stem cells to osteoblast These that and bone formation and calcium decreasing osteoblast activity A recent paper by et protease inhibitors on primary osteoblasts and found a decreased osteoblast activity of alkaline calcium and when and were these studies suggest that some protease inhibitors may determine bone loss by osteoclast resorption and the osteoblast rebuilding is noteworthy that some studies the possible between use of protease inhibitors and decreased is essential for the maintenance of a normal bone structure the bone The biological effects on bone remodelling are by a activation to in the by of in the whereas is mainly by The can determine through the of to bone The and involved in are and protease inhibitors are inhibitors of human but also and affect the to a activity (Table whereas no inhibition of is observed Some clinical studies in HIV-infected was observed the advent of HAART and a severe of was associated with infection and immune studies performed on a cohort of naive and HAART-treated patients demonstrated a high prevalence of a risk of These with the effects on regulation indicate that may be an mechanism of bone could be as a of bone impairment in HIV infection. This problem is not in the because the studies that have to bone impairment have used the DXA This is a to determine bone mineral but cannot between osteoporosis (low BMD and bone and (low BMD with normal bone of osteopenia/osteoporosis in HIV-infected patients In clinical their HIV-infected such as the of and specific are not several have been to bone conditions during HIV disease and the HAART (Table on and monitoring of HIV-related bone and DXA analysis is a to determine BMD and will between cortical and trabecular two different that may to antiretroviral Hence, DXA be performed in as studies have demonstrated that it will fracture risk A meta-analysis that the risk of hip fracture increased for standard decrease in BMD the DXA are not and cannot the analysis but a major be to BMD in HIV-infected patients. The recent by the for the T-score with the value by WHO only for in data are it is that the same can be to over the of have major risk factor for individuals than diagnosis is the that the BMD with that of a and sex-matched population. However, the has no clear value for osteopenia and osteoporosis and the are patients with values than −1 are as low bone whereas a severe bone mass is by values than The may further analysis of BMD and a of this parameter in the of bone loss in HIV-infected can only be by bone that will of bone As bone are the diagnosis of is and is by DXA analysis with some blood Hence, DXA may the rate in this population as an number of patients may have been with a diagnosis be in to Osteoporosis is commonly treated with or whereas high of and spine are to be in patients with in the patients with marked and in patients with severe spine the in patients with a diagnosis of Bone biology can be by laboratory and calcium calcium and In addition, bone formation or alkaline and bone resorption or or can with and may be obtained by and with a low a decrease in the function of in the kidney and in and the of kidney function alterations may occur with normal and may an impairment of function for monitoring and during may on the side effects of on bone. et suggested that RANKL and may be in some cases of therapy as impairment of the is well in patients protease though data indicate that and protease with and the and may to a in the bone resorption The of these cytokines is not part of the of HIV-infected patients. The management of osteopenia/osteoporosis in the course of HIV infection may be on a in risk calcium and and (Table and in HIV-related bone can be to and to control body In addition, patients have to an of calcium and However, be to HIV-infected patients The limit for is but this is below the that has been even with the of A of can be suggested to The only to is and when calcium is can be A low calcium has been demonstrated to reduce BMD and to increase the hip fracture risk The calcium for is between and but it is to this in it is important for to be to HIV-infected patients. When and low calcium have been can be are to The for is well demonstrated for and and human studies have been on the treatment of HIV-induced osteoporosis with to the low number of individuals the results obtained have a limited even though showed that increased BMD with to results were obtained in two therapy with data are on fracture studies are to the effects of both on BMD and fracture risk in HIV-related bone disease. especially in the risk of fractures is high and this is The and of treatment be for the data or be considered to bone mineral density and decrease fracture The effects associated with and osteonecrosis but the relatively low risk of this last effect does not the use of A possible in the treatment of osteoporosis may be the use of the As RANKL induces osteoclast activation and its was noted both in HIV-positive and patients with osteopenia/osteoporosis A clinical study on for demonstrated an increase in BMD and a decrease of bone turnover This finding suggested its possible use in osteoporosis treatment even in HIV-positive compounds such as interaction receptor and bone are study interaction further is these can be in HIV-infected patients (Table in diagnosis and therapy on bone in HIV-infected Bone is a major clinical in the course of HIV infection The advent of HAART has led to a longer life expectancy and bone disease is to the bone loss. Some in individuals indicate that the of osteoporosis after of between and and the fracture risk for a is greater than 50% These data suggest that the number of HIV patients with bone disease and fractures can be to increase dramatically in the because these patients also have two other HIV and antiretroviral Hence, antiretroviral therapy be by the clinical management of bone disease to reduce the risk of osteoporosis and fractures in these patients. This was by of the of the study for for selected of the of and the that have no or that may constitute a dual or