Endogenous retrotransposon reverse transcriptase supports osteocyte function: implications for antiretroviral therapy-associated bone loss

BackgroundTransposable elements (TEs) such as LINE-1 and HERV-K are mobile retroviral elements that have biological functions in several tissues. In a prior study, we demonstrated the positive contribution of LINE-1 activation to bone homeostasis and microfracture repair. People living with HIV experience a high prevalence of low bone mineral density (BMD) and fragility fractures, particularly when treated with nucleoside reverse transcriptase inhibitor (NRTI)-based antiretroviral therapy (ART). The mechanisms underlying ART-associated bone loss remain unclear. Several NRTIs inhibit not only viral but also endogenous reverse transcriptases (RTs) encoded by LINE-1 and HERV-K. Therefore, we hypothesized that since endogenous RT activity supports bone function, its inhibition by NRTIs may contribute to bone loss and secondary osteoporosis, as frequently observed in patients receiving ART. MethodsWe quantified L1-Ta and HERV-K DNA content, used as a proxy for endogenous RT activity, in trans-iliac bone biopsies, skeletal muscle, and peripheral blood mononuclear cells (PBMCs) in a clinically well-characterized cohort of postmenopausal women stratified into healthy, osteopenic or osteoporotic. Bone status was assessed through BMD measurements and bone remodeling was evaluated by extensive serum analyses combined with expression of osteoblastic and osteocytic biomarkers. The ex vivo results were corroborated in vitro, where the functional impact of RT inhibition was tested in IDG-SW3 cells differentiating from osteoblasts into osteocytes using three NRTIs (tenofovir, lamivudine, abacavir). The cell cultures were analyzed for osteocyte maturation, mineralization, and responsiveness to 1,25-dihydroxyvitamin D and parathyroid hormone (PTH). FindingsL1-Ta and HERV-K DNA content was significantly higher in bone than in muscle or PBMCs and declined with age, specifically in bone. Both elements were markedly reduced in osteoporotic bone, but unchanged in matched non-skeletal control tissues. L1-Ta and HERV-K DNA content correlated positively with BMD at three skeletal sites and with the expression of osteocyte-specific genes, particularly SOST and MEPE. Treatment of IDG-SW3 cells with NRTIs selectively impaired osteocyte endocrine function. Induction of FGF23 by 1,25-dihydroxyvitamin D and PTH-mediated suppression of SOST were inhibited, and gene expression in osteocytes and mineralization were slightly affected. Abacavir had the strongest impact, almost completely abolishing vitamin D3 induction of FGF23 and PTH responsiveness. InterpretationThese findings identify endogenous TE-derived RT activity as a previously unrecognized regulator of osteocyte function and bone homeostasis. Reduced L1-Ta and HERV-K DNA contents is associated with impaired osteocyte function and low BMD, while pharmacological RT inhibition disrupts key endocrine pathways in osteocytes. NRTI-mediated inhibition of endogenous osteocyte RT activity may therefore contribute to ART-associated osteoporosis. Assessment of TE-derived RT activity may improve the identification of individuals at risk for skeletal deterioration. Preservation of endogenous osteocyte RT activity represents a potential therapeutic strategy for mitigating bone loss in HIV-infected patients undergoing long-term ART. FundingThis research was supported by KAUST BAS/1/1037-01-01, the South East Norway Health Authority and Oslo University Hospital, Ullevaal (52009/8029); The 6th EU Framework Program (LSHM-CT-2003-502941); Legat til Forskning, Lovisenberg Diaconal Hospital.