The HIV protease inhibitor ritonavir blocks osteoclastogenesis and function by impairing RANKL-induced signaling

Abstract

Highly active antiretroviral therapy (HAART), which includes HIV protease inhibitors (PIs), has been associated with bone demineralization. To determine if this complication reflects accelerated resorptive activity, we studied the impact of two common HIV PIs, ritonavir and indinavir, on osteoclast formation and function. Surprisingly, we find that ritonavir, but not indinavir, inhibits osteoclast differentiation in a reversible manner and also abrogates bone resorption by disrupting the osteoclast cytoskeleton, without affecting cell number. Ritonavir given in vivo completely blunts parathyroid hormone-induced osteoclastogenesis in mice, which confirms that the drug is bone sparing. In keeping with its antiresorptive properties, ritonavir impairs receptor activator of nuclear factor κB ligand-induced (RANKL-induced) activation of NF-κB and Akt signaling pathways, both critical to osteoclast formation and function. In particular, ritonavir is found to inhibit RANKL-induced Akt signaling by disrupting the recruitment of TNF receptor-associated factor 6/c-Src complex to lipid rafts. Thus, ritonavir may represent a bone-sparing PI capable of preventing development of osteopenia in patients currently on HAART. IntroductionThroughout life, bone remodeling occurs by a finely orchestrated process of osteoclastic resorption and osteoblastic formation. When intact, this process ensures maintenance of skeletal integrity and calcium homeostasis. In all circumstances bone loss occurs when the activity of the resorptive cell exceeds that of its anabolic counterpart. For example, postmenopausal osteoporosis is caused by an absolute increase of osteoclasts and osteoblasts, with the former activity outpacing that of the latter (1). Senile (type II) osteoporosis, in contrast, is a low-turnover disease, but once again bone resorptive activity exceeds that of matrix deposition and calcification (1). Therefore, one approach to dissecting the cause of bone loss in a specific clinical circumstance is to examine the direct effects of various drugs on generation and activity of osteoclasts and osteoblasts.Osteoclasts are multinucleated cells generated by the fusion of mononuclear progenitors of the monocyte/macrophage family (2). The pathway involved in osteoclast differentiation and activation requires two key elements: receptor activator of nuclear factor κB (RANK), found in osteoclasts and their precursors, and RANK ligand (RANKL), produced by osteoblasts and stromal cells in the bone marrow (2, 3). In addition, M-CSF is required for survival and proliferation of osteoclast precursors. Ligation of RANKL to RANK on macrophages prompts selective intracellular signals that eventuate in the assumption of the osteoclast phenotype (4).

Bone mineral density remains stable in HAART‐treated HIV‐infected men over 2 years

“…34 Our cohort of men had been treated with HAART for an average of 52 months, approximately 20 months longer than cohorts from other cross-sectional analyses, consistent with this hypothesis. It is also possible that some component(s) of HAART contribute to the improving BMD in HIV-infected cohorts, as several protease inhibitors have been demonstrated to influence bone cell function in vitro [46][47][48] or in vivo . 9 A further possibility is that the insulin resistance induced by HAART 49 may indirectly confer skeletal benefit.…”

Bone mineral density is not reduced in HIV‐infected Caucasian men treated with highly active antiretroviral therapy

“…Data from in vitro and animal studies suggest that different antiretroviral agents may exert divergent effects on skeletal tissue. 31,32 Definitive demonstration of the putative skeletal effects of particular antiretroviral agents in studies of BMD in humans conducted to date has been impossible because of the large number of drug regimens used in the cohorts studied.…”

Bone Health and Human Immunodeficiency Virus Infection

“…In vitro, certain protease inhibitors have been found to impact bone homeostasis by inhibiting osteoclast and/or osteoblast differentiation, whereas other studies have noted an inhibition of 1-a-hydroxylase associated with protease inhibitors that can lead to a reduction in calcitriol. [26][27][28][29] Despite the identification of these mechanisms, the results of clinical trials evaluating protease inhibitors and BMD losses have been conflicting, and definitive associations have not been established. 17,30,31 Efavirenz, an NNRTI, has also been associated with BMD losses.…”