A team of researchers led by Dr. Yihong Wan, PhD, of the University of Texas Southwestern Medical Center in Dallas, have published a research letter in Nature where they describe the microRNA molecule miR-34a as a key suppressor of osteoclast development and bone resorption with the potential to become a target for new therapies in osteoporosis and bone metastatic cancers.
MicroRNAs are short, single-stranded noncoding RNA molecules that act at a post-transcriptional level and play a critical role in the regulation of gene expression and cellular function. Several research efforts have been made to determine if some microRNAs, including miR-34a, are able to suppress primary tumors in humans.
Wan and colleagues studied several microRNAs related to cancers in a bone marrow ostoclastogeneis assay, observing that miR-34a blocked the development of osteoclasts, whereas the related microRNAs miR-34b and miR-34c did not. Additionally, miR-34a was downregulated by receptor of nuclear factor kappa-B ligand (RANKL) and rosiglitazone, an antidiabetic drug that works as an insulin sensitizer.
To further determine the relevance of miR-34a in bone resorption, miR-34a knockout (34a-KO) and heterozygous (34a-Het) mice were analyzed, showing that osteoclast diferentiation was enhanced in 34a-Het and 34a-KO cultures, whereas precursor proliferation and survival was unaffected, resulting in elevated numbers of serum carboxy-terminal telopeptides (CTX)-1 and osteoclasts. Deletion of miR-34a also decreased bone formation rate, osteoclast numbers and mineral apposition rate.
Wild-type mice were transplanted with either 34a-Het or 34a-KO mice bone marrow cells and only mice receiving the 34a-deficient cells exhibited higher CTX-1 levels. In addition, osteoclastic miR-34a conditional knockout mice (34a-Tie2-KO) had elevated osteoclast differentiation and bone resorption rates but showed unaltered bone formation, leading to decreased bone mass.
Overall, these results show that miR-34a deletion in the osteoclast lineage augments bone resorption, prompting researchers to investigate whether pharmacological administration of miR-34a nanoparticles could attenuate ovariectomy-induced osteoporosis, as well as bone metastasis of breast and skin cancers.
Using an ovariectomy (OVX) mouse model and a chitosan (CH) nanoparticle vehicle, researchers showed that OVX mice treated with miR-34a-CH had decreased CTX-1 and increased amino-terminal propeptide of type I procollagen levels, proving that OVX-induced bone loss was attenuated by miR-34a-CH tratment and that miR-34a overexpression is enough to impede osteoporosis, with the osteoclast proving to be the key site for miR-34a therapeutic benefit. The team also showed that bone metastasis in breast cancer and melanoma was reduced in osteoclastic miR-34a transgenic mice, with bone metastasis decreasing after miR-34a nanoparticle treatment, raising the hypothesis that systemic miR-34a administration can exert its anticancer effects in osteoclasts residing at the bone metastatic niche in the tumor microenvironment.
Mechanistically, the researchers identified transforming growth factor-beta-induced factor 2 (Tgif2) as an essential direct pro-osteoclastogenic miR-34a target, with Tgif2 deletion reducing bone resorption and abolishing miR-34a regulation. Even though miR-34a may also target genes other than Tgif2 in osteoclasts, the researchers’ genetic rescue ex vivo and in vivo showed that Tgif2 is the key miR-34a target, with other genes probably secondary or functionally irrelevant to osteoclastogenesis.
Dr. Wan noted that the roles of microRNAs in bone physiology are now beginning to be explored and understood. “There haven’t been many studies looking at microRNA in osteoporosis and ours is among the first showing both a genetic and pharmacological approach” she said.
This study can help pave the way for future miRNAs discoveries that are regulated by RANKL and control Tgif2 expression, as well as future clinical studies exploring the possible therapeutic roles of this miR34a-Tgif2 pathway in humans, openining a door for the development of a new generation of RNA-based osteoprotective medicine.
Alongside Dr. Wan, the team included Drs. Xian-Jin Xie and Tsubg-Cheng Chang and postdoctoral researchers Jing Krzeszinski, Wei Wei, HoangDinh Huynh, Zixue Jin and Xunde Wang. Lin He from UC Berkeley and Lingegowda Mangala, Gabriel Lopez-Berestein and Anil Sood from UT MD Anderson Cancer Center.
This research was funded by the Cancer Prevention Research Institute of Texas, the NIH, the Welch Foundation and the University of Texas Southwestern.