Initial Characterization of Osteoblast Differentiation and Loss of RUNX2 Stability in the Newly Established SK11 Human Embryonic Stem Cell-Derived Cell Line

Tuning Forkhead Box D3 overexpression to promote specific osteogenic differentiation of human embryonic stem cells while reducing pluripotency in a three-dimensional culture system

Clinical use of human embryonic stem cells (hESCs) in bone regeneration applications requires that their osteogenic differentiation be highly controllable as well as time- and cost-effective. The main goals of the current work were thus (a) to assess whether overexpression of pluripotency regulator Forkhead Box D3 (FOXD3) can enhance the osteogenic commitment of hESCs seeded in three-dimensional (3D) scaffolds and (b) to evaluate if the degree of FOXD3 overexpression regulates the strength and specificity of hESC osteogenic commitment. In conducting these studies, an interpenetrating hydrogel network consisting of poly(ethylene glycol) diacrylate and collagen I was utilized as a 3D culture platform. Expression of osteogenic, chondrogenic, pluripotency, and germ layer markers by encapsulated hESCs was measured after 2 weeks of culture in osteogenic medium in the presence or absence doxycycline-induced FOXD3 transgene expression. Towards the first goal, FOXD3 overexpression initiated 24 hr prior to hESC encapsulation, relative to unstimulated controls, resulted in upregulation of osteogenic markers and enhanced calcium deposition, without promoting off-target effects. However, when initiation of FOXD3 overexpression was increased from 24 to 48 hr prior to encapsulation, hESC osteogenic commitment was not further enhanced and off-target effects were noted. Specifically, relative to 24-hr prestimulation, initiation of FOXD3 overexpression 48 hr prior to encapsulation yielded increased expression of pluripotency markers while reducing mesodermal but increasing endodermal germ layer marker expression. Combined, the current results indicate that the controlled overexpression of FOXD3 warrants further investigation as a mechanism to guide enhanced hESC osteogenic commitment.

Estrogens and selective estrogen receptor modulators differentially antagonize Runx2 in ST2 mesenchymal progenitor cells

Estrogens attenuate bone turnover by inhibiting both osteoclasts and osteoblasts, in part through antagonizing Runx2. Apparently conflicting, stimulatory effects in osteoblast lineage cells, however, sway the balance between bone resorption and bone formation in favor of the latter. Consistent with this dualism, 17ß-estradiol (E2) both stimulates and inhibits Runx2 in a locus-specific manner, and here we provide evidence for such locus-specific regulation of Runx2 by E2 in vivo. We also demonstrate dual, negative and positive, regulation of Runx2-driven alkaline phosphatase (ALP) activity by increasing E2 concentrations in ST2 osteoblast progenitor cells. We further compared the effects of E2 to those of the Selective Estrogen Receptor Modulators (SERMs) raloxifene (ral) and lasofoxifene (las) and the phytoestrogen puerarin. We found that E2 at the physiological concentrations of 0.1-1 nM, as well as ral and las, but not puerarin, antagonize Runx2-driven ALP activity. At ≥10 nM, E2 and puerarin, but not ral or las, stimulate ALP relative to the activity measured at 0.1-1 nM. Contrasting the difference between E2 and SERMs in ST2 cells, they all shared a similar dose-response profile when inhibiting pre-osteoclast proliferation. That ral and las poorly mimic the locus- and concentration-dependent effects of E2 in mesenchymal progenitor cells may help explain their limited clinical efficacy.

Glucocorticoids Hijack Runx2 to Stimulate Wif1 for Suppression of Osteoblast Growth and Differentiation

Inhibition of Runx2 is one of many mechanisms that suppress bone formation in glucocorticoid (GC)-induced osteoporosis (GIO). We profiled mRNA expression in ST2/Rx2(dox) cells after treatment with doxycycline (dox; to induce Runx2) and/or the synthetic GC dexamethasone (dex). As expected, dex typically antagonized Runx2-driven transcription. Select genes, however, were synergistic stimulated and this was confirmed by RT-qPCR. Among the genes synergistically stimulated by GCs and Runx2 was Wnt inhibitory Factor 1 (Wif1), and Wif1 protein was readily detectable in medium conditioned by cultures co-treated with dox and dex, but neither alone. Cooperation between Runx2 and GCs in stimulating Wif1 was also observed in primary preosteoblast cultures. GCs strongly inhibited dox-driven alkaline phosphatase (ALP) activity in control ST2/Rx2(dox) cells, but not in cells in which Wif1 was silenced. Unlike its anti-mitogenic activity in committed osteoblasts, induction of Runx2 transiently increased the percentage of cells in S-phase and accelerated proliferation in the ST2 mesenchymal pluripotent cell culture model. Furthermore, like the inhibition of Runx2-driven ALP activity, dex antagonized the transient mitogenic effect of Runx2 in ST2/Rx2(dox) cultures, and this inhibition eased upon Wif1 silencing. Plausibly, homeostatic feedback loops that rely on Runx2 activation to compensate for bone loss in GIO are thwarted, exacerbating disease progression through stimulation of Wif1. J. Cell. Physiol. 232: 145-153, 2017. © 2016 Wiley Periodicals, Inc.

Estrogens antagonize RUNX2-mediated osteoblast-driven osteoclastogenesis through regulating RANKL membrane association

In addition to its thoroughly investigated role in bone formation, the osteoblast master transcription factor RUNX2 also promotes osteoclastogenesis and bone resorption. Here we demonstrate that 17β-estradiol (E2), strongly inhibits RUNX2-mediated osteoblast-driven osteoclastogenesis in co-cultures. Towards deciphering the underlying mechanism, we induced premature expression of RUNX2 in primary murine pre-osteoblasts, which resulted in robust differentiation of co-cultured splenocytes into mature osteoclasts. This was attributable to RUNX2-mediated increase in RANKL secretion, determined by ELISA, as well as to RUNX2-mediated increase in RANKL association with the osteoblast membrane, demonstrated using confocal fluorescence microscopy. The increased association with the osteoblast membrane was recapitulated by transiently expressed GFP-RANKL. E2 abolished the RUNX2-mediated increase in membrane-associated RANKL and GFP-RANKL, as well as the concomitant osteoclastogenesis. RUNX2-mediated RANKL cellular redistribution was attributable in part to a decrease in Opg expression, but E2 did not influence Opg expression either in the presence or absence of RUNX2. Diminution of RUNX2-mediated osteoclastogenesis by E2 occurred regardless of whether the pre-osteoclasts were derived from wild type or estrogen receptor alpha (ERα)-knockout mice, suggesting that activated ERα inhibited osteoblast-driven osteoclastogenesis by acting in osteoblasts, possibly targeting RUNX2. Indeed, microarray analysis demonstrated global attenuation of the RUNX2 response by E2, including abrogation of Pstpip2 expression, which likely plays a critical role in membrane trafficking. Finally, the selective ER modulators (SERMs) tamoxifen and raloxifene mimicked E2 in abrogating the stimulatory effect of osteoblastic RUNX2 on osteoclast differentiation in the co-culture assay. Thus, E2 antagonizes RUNX2-mediated RANKL trafficking and subsequent osteoclastogenesis. Targeting RUNX2 and/or downstream mechanisms that regulate RANKL trafficking may lead to the development of improved SERMs and possibly non-hormonal therapeutic approaches to high turnover bone disease.