Cyclic Adenosine Monophosphate (cAMP)‐Dependent Phosphodiesterase Inhibition Promotes Bone Anabolism Through CD8⁺ T Cell Wnt‐10b Production in Mice

Abstract

Cyclic adenosine monophosphate (cAMP)-dependent phosphodiesterase (PDE) inhibitors such as pentoxifylline (PTX) suppress cAMP degradation and promote cAMP-dependent signal transduction. PDE inhibitors increase bone formation and bone mass in preclinical models and are used clinically to treat psoriatic arthritis by targeting inflammatory mediators including activated T cells. T cell activation requires two signals: antigen-dependent CD3-activation, which stimulates cAMP production; and CD28 co-stimulation, which downregulates cAMP-signaling, through PDE activation. PDE-inhibitors consequently suppress T cell activation by disrupting CD28 co-stimulation. Interestingly, we have reported that when CD8 + T cells are activated in the absence of CD28 co-stimulation, they secrete Wnt-10b, a bone anabolic Wnt ligand that promotes bone formation. In the present study, we investigated whether the bone anabolic activity of the PDE-inhibitor PTX, has an immunocentric basis, involving Wnt-10b production by CD8 + T cells. When wildtype (WT) mice were administered PTX, biochemical markers of both bone resorption and formation were significantly increased, with net bone gain in the axial skeleton, as quantified by micro-computed tomography (μCT). By contrast, PTX increased only bone resorption in T cell knockout (KO) mice, causing net bone loss. Reconstituting T cell-deficient mice with WT, but not Wnt-10b knockout (KO) CD8 + T cells, rescued bone formation and prevented bone loss. To study the role of cAMP signaling in Wnt-10b expression, reverse-transcription polymerase chain reaction (RT-PCR) and luciferase-reporter assays were performed using primary T cells. PDE inhibitors intensified Wnt-10b promoter activity and messenger RNA (mRNA) accumulation in CD3 and CD28 activated CD8 + T cells. In contrast, inhibiting the cAMP pathway mediators protein kinase A (PKA) and cAMP response element-binding protein (CREB), suppressed Wnt-10b expression by T cells activated in the absence of CD28 co-stimulation. In conclusion, the data demonstrate a key role for Wnt-10b production by CD8 + T cells in the bone anabolic response to PDE-inhibitors and reveal competing T cell-independent pro-resorptive properties of PTX, which dominate under T cell-deficient conditions. Selective targeting of CD8 + T cells by PDE inhibitors may be a beneficial approach for promoting bone regeneration in osteoporotic conditions.

The role of WNT10B in physiology and disease: A 10-year update

“…Chen et al (2022) showed that high expression of the histone lysine methyltransferase NSD1 (nuclear receptor binding SET domain-containing protein 1) could predict the overall survival of breast cancer patients, although the subtype of breast cancer that was analyzed was not discussed. Then, they investigated the role of NSD1 in paclitaxel-mediated drug resistance using parental MCF-7 (ER + breast cancer) versus MCF-7 PR (paclitaxel-resistant) cell lines.…”

“…PTX, which activates cAMP signaling to downstream effectors PKA, had been recognized to increase bone formation, the mechanism of which was unknown. In a preclinical model, Roser-Page et al (2022) used PTX to investigate bone turnover to determine if the mechanism of action was through T cells secreting WNT10B. PTX induced WNT10B production in CD3- and CD28-activated T cells.…”

Regulation of intracellular cAMP levels in osteocytes by mechano-sensitive focal adhesion kinase via PDE8A

“…The inhibition of phosphodiesterases PDE2, PDE3 and PDE4 were found to increase osteoblast differentiation in vitro [40]. The administration of the non-specific PDE inhibitor pentoxifylline increased bone mass in mice through production of Wnt10b from CD8+ T cells [41]. Pentoxifylline also increased skeletal angiogenesis and reversed osteopenia in ovariectomized rats [42].…”