Osteopenia and osteoporosis in HIV: pathogenesis and treatment

Hemin activation of innate cellular response blocks human immunodeficiency virus type-1-induced osteoclastogenesis

The normal skeletal developmental and homeostatic process termed osteoclastogenesis is exacerbated in numerous pathological conditions and causes excess bone loss. In cancer and HIV-1-infected patients, this disruption of homeostasis results in osteopenia and eventual osteoporesis. Counteracting the factors responsible for these metabolic disorders remains a challenge for preventing or minimizing this co-morbidity associated with these diseases. In this report, we demonstrate that a hemin-induced host protection mechanism not only suppresses HIV-1 associated osteoclastogenesis, but it also exhibits anti-osteoclastogenic activity for non-infected cells. Since the mode of action of hemin is both physiological and pharmacological through induction of heme oxygenase-1 (HO-1), an endogenous host protective response to an FDA-licensed therapeutic used to treat another disease, our study suggests an approach to developing novel, safe and effective therapeutic strategies for treating bone disorders, because hemin administration in humans has previously met required FDA safety standards.

Prevalence of causes of secondary osteoporosis and contribution to lower bone mineral density in HIV-infected patients

SUMMARY

Eighty-one percent of human immunodeficiency virus (HIV)-infected patients had one or more of seven evaluated causes of secondary osteoporosis, and this rate increases with age. The type and number of causes were associated with a lower bone mineral density (BMD), and with an increased rate of osteopenia/osteoporosis, regardless of age and body mass index.

INTRODUCTION

The objective of this study was to determine whether factors of secondary osteoporosis were associated with lower BMD in HIV.

METHODS

This was a cross-sectional study of 285 HIV-infected patients (25 % females) evaluating the impact of seven different factors of reduced BMD: hyperthyroidism, diabetes, chronic viral hepatitis, chronic kidney disease (CKD), hypovitaminosis D, secondary hyperparathyroidism, and hypogonadism. Dual-energy X-ray absorptiometry scan of the femoral neck was obtained at the clinical visit.

RESULTS

Mean age was 45.7 years; osteopenia and osteoporosis were diagnosed in 38 and 6 %, respectively. Overall, 230 patients (81 %) had secondary factors; 107 (38 %) had only 1 cause, 94 (33 %) had 2, and 28 (10 %) had 3 or more, predominantly vitamin D deficiency in 61 %, hepatitis C virus coinfection in 45 %, and secondary hyperparathyroidism in 27 %. The number of secondary factors was closely related to a lower BMD, which is statistically significant for patients having ≥2 causes (0.77 vs 0.73 g/cm(2), p = 0.02). The rate of osteopenia ranged from 36 % without any cause to 57 % with three or more, osteoporosis from 0 to 19 %, and Z-score <-2 SD from 0 to 27 %, respectively. In a multivariate linear regression, adjusting by age, body mass index, and HIV-related factors, the number of secondary factors was independently associated with a lower BMD (ß coefficient -0.134; p = 0.02), mainly due to patients with hepatitis C virus (HCV) coinfection, secondary hyperparathyroidism, and CKD.

CONCLUSIONS

A high prevalence of secondary causes of osteoporosis is observed in HIV-infected patients, and its type and cumulative number determine a lower BMD, after adjusting by age and body mass index.

HIV-1 protein induced modulation of primary human osteoblast differentiation and function via a Wnt/β-catenin-dependent mechanism

HIV infection is associated with metabolic bone disease resulting in bone demineralization and reduced bone mass. The molecular mechanisms driving this disease process have yet to be elucidated. Wnt/β-catenin signaling plays a key role in bone development and remodeling. We attempted to determine the effects of the HIV-1 protein, gp120, on Wnt/β-catenin signaling at an intracellular and transcriptional level in primary human osteoblasts (HOBs). This work, inclusive of experimental controls, was part of a greater project assessing the effects of a variety of different agents on Wnt/β-catenin signaling (BMC Musculoskelet Disord 2010;11:210).We examined the phenotypic effects of silencing and overexpressing the Wnt antagonist, Dickkopf-1 (Dkk1) in HOBs treated with gp120. HOBs exposed to gp120 displayed a significant reduction in alkaline phosphatase activity (ALP) activity and cell proliferation and increased cellular apoptosis over a 48 h time course. Immunocytochemistry demonstrated a significant reduction in intracytosolic and intranuclear β-catenin in response to HIV-1 protein exposure. These changes were associated with a reduction of TCF/LEF-mediated transcription, the transcriptional outcome of canonical Wnt β-catenin signaling. Silencing Dkk1 expression in HOBs exposed to gp120 resulted in increased ALP activity and cell proliferation, and decreased cellular apoptosis relative to scrambled control. Dkk1 overexpression exacerbated the inhibitory effect of gp120 on HOB function, with decreases in ALP activity and cell proliferation and increased cellular apoptosis relative to vector control. Wnt/β-catenin signaling plays a key regulatory role in HIV-associated bone loss, with Dkk1, aputative central mediator in this degenerative process.

Type 1 interferons suppress accelerated osteoclastogenesis and prevent loss of bone mass during systemic inflammatory responses to Pneumocystis lung infection

HIV infection causes loss of CD4(+) T cells and type 1 interferon (IFN)-producing and IFN-responsive dendritic cells, resulting in immunodeficiencies and susceptibility to opportunistic infections, such as Pneumocystis. Osteoporosis and bone marrow failure are additional unexplained complications in HIV-positive patients and patients with AIDS, respectively. We recently demonstrated that mice that lack lymphocytes and IFN a/b receptor (IFrag(-/-)) develop bone marrow failure after Pneumocystis lung infection, whereas lymphocyte-deficient, IFN α/β receptor-competent mice (RAG(-/-)) had normal hematopoiesis. Interestingly, infected IFrag(-/-) mice also exhibited bone fragility, suggesting loss of bone mass. We quantified bone changes and evaluated the potential connection between progressing bone fragility and bone marrow failure after Pneumocystis lung infection in IFrag(-/-) mice. We found that Pneumocystis infection accelerated osteoclastogenesis as bone marrow failure progressed. This finding was consistent with induction of osteoclastogenic factors, including receptor-activated nuclear factor-κB ligand and the proapoptotic factor tumor necrosis factor-related apoptosis-inducing ligand, in conjunction with their shared decoy receptor osteoprotegerin, in the bone marrow of infected IFrag(-/-) mice. Deregulation of this axis has also been observed in HIV-positive individuals. Biphosphonate treatment of IFrag(-/-) mice prevented bone loss and protected loss of hematopoietic precursor cells that maintained activity in vitro but did not prevent loss of mature neutrophils. Together, these data show that bone loss and bone marrow failure are partially linked, which suggests that the deregulation of the receptor-activated nuclear factor-κB ligand/osteoprotegerin/tumor necrosis factor-related apoptosis-inducing ligand axis may connect the two phenotypes in our model.

Endocrine complications of human immunodeficiency virus infection: hypogonadism, bone disease and tenofovir-related toxicity

Treatment with highly active antiretroviral therapy (HAART) has revolutionized care of patients with HIV infection. The cost of increased survival has been antiretroviral toxicity and increasing age-related co-morbidities that include significant metabolic issues. Hypogonadism was first described in the setting of advanced AIDS and can be primary or secondary. Data regarding treatment largely concern patients with wasting. Varied syndromes involving bone have been described in patients with HIV including osteonecrosis, low bone mineral density (BMD) and osteoporosis, and rarely osteomalacia. Low BMD leading to osteoporosis is the most common bone pathology and may be as a result of HIV infection, drug toxicity or co-morbidities. However, increasingly fragility fractures are reported in HIV-infected patients, suggesting bone demineralization in this population is of clinical relevance. Further research is required to understand its pathogenesis and determine effective management; however, initiation of antiretroviral therapy seems to accelerate (in the short-term) bone demineralization. One particular antiretroviral agent, tenofovir is widely used and is potentially implicated as having a greater role in long-term bone and renal dysfunction. As this population ages, screening for low BMD will become increasingly more important.

Bone disease in HIV infection: a practical review and recommendations for HIV care providers

Low bone mineral density (BMD) is prevalent in human immunodeficiency virus (HIV)-infected subjects. Initiation of antiretroviral therapy is associated with a 2%-6% decrease in BMD over the first 2 years, a decrease that is similar in magnitude to that sustained during the first 2 years of menopause. Recent studies have also described increased fracture rates in the HIV-infected population. The causes of low BMD in individuals with HIV infection appear to be multifactorial and likely represent a complex interaction between HIV infection, traditional osteoporosis risk factors, and antiretroviral-related factors. In this review, we make the point that HIV infection should be considered as a risk factor for bone disease. We recommend screening patients with fragility fractures, all HIV-infected post-menopausal women, and all HIV-infected men ⩾50 years of age. We also discuss the importance of considering secondary causes of osteoporosis. Finally, we discuss treatment of the more severe cases of bone disease, while outlining the caveats and gaps in our knowledge.

Is PPARγ a prospective player in HIV-1-associated bone disease?

Currently infection with the human immunodeficiency virus-1 (HIV-1) is in most instances a chronic disease that can be controlled by effective antiretroviral therapy (ART). However, chronic use of ART has been associated with a number of toxicities; including significant reductions in bone mineral density (BMD) and disorders of the fat metabolism. The peroxisome proliferator-activated receptor γ (PPARγ) transcription factor is vital for the development and maintenance of mature and developing adipocytes. Alterations in PPARγ expression have been implicated as a factor in the mechanism of HIV-1-associated lipodystrophy. Both reduced BMD and lipodystrophy have been well described as complications of HIV-1 infection and treatment, and a question remains as to their interdependence. Interestingly, both adipocytes and osteoblasts are derived from a common precursor cell type; the mesenchymal stem cell. The possibility that dysregulation of PPARγ (and the subsequent effect on both osteoblastogenesis and adipogenesis) is a contributory factor in the lipid- and bone-abnormalities observed in HIV-1 infection and treatment has also been investigated. This review deals with the hypothesis that dysregulation of PPARγ may underpin the bone abnormalities associated with HIV-1 infection, and treats the current knowledge and prospective developments, in our understanding of PPARγ involvement in HIV-1-associated bone disease.