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).

FHL2 inhibits the activated osteoclast in a TRAF6-dependent manner

“…Macrophages/osteoclast precursors and osteoclasts were generated from bone marrow precursors as described (42). C2C12 and 293T cells were obtained from ATCC and were maintained as described.…”

“…Osteoclast-like cells were characterized by staining for tartrate-resistant acid phosphatase (TRAP) activity. The number of osteoclasts was counted as described previously (42). The characteristics of spread as compared to nonspread osteoclasts are illustrated in Figure 7B.…”

Osteoclasts: What Do They Do and How Do They Do It?

“…RANKL also promotes bone resorption by inducing the mature osteoclast to generate a complex composed of its receptor, TRAF6, and c-Src, which the cytokine specifically recruits to lipid rafts in the plasma membrane. 35 This event requires organization of fibrillar actin and is mediated via the phosphoinosotide-3-kinase (PI3-K)/AKT pathway.…”

Evaluation of bone alterations in the jaws of HIV-infected menopausal women

“…These drugs may increase the risk of bone fractures in people with HIV/AIDS. [4][5][6] The factors that may contribute to the reduction in bone density in HIV-infected women include menstrual dysfunction (hormonal imbalance), weight loss (alterations in nutritional status), decreased body mass, increased bone resorption, and Caucasian ethnicity. These risk factors are associated with an increase in the incidence of osteopenia and osteoporosis.…”

“…Furthermore, protease inhibitors are known osteopenic agents; however, a more consistent link between this group of drugs and bone loss remains to be proven. [4][5][6][25][26][27][28] Panoramic radiography can be considered an auxiliary tool for dentists in the detection of possible alterations and bone fractures in the jaw, especially in postmenopausal women. 13,14,29 Measurements of MI, PMI, and AD have already been used and have been validated in the scientific literature.…”