Bone growth occurs through the proliferation of specialized cells of the cartilage tissue, known as chondrocytes, on either end of a bone. This process requires the synthesis of extracellular matrix (ECM) in the bone growth plates. A peptide hormone, known as C-type natriuretic peptide (CNP), plays a pivotal role by binding to its specific receptors present on the chondrocytes and triggering a signaling cascade in these cells. The CNP-bound receptor activates a molecule, cyclic guanosine monophosphate (cGMP), which in turn activates protein kinase G (PKG) that phosphorylates a membrane ion channel. This ion channel is responsible for hyperpolarizing the cell, i.e., causing an efflux of positive potassium ions out of the cell, thereby creating a net negative intracellular potential. This negative potential stimulates the opening of calcium channels, resulting in an influx of calcium ions into the chondrocytes via TRPM7 channels. Following this, calcium-dependent signaling molecules are turned on, which further drives ECM production and promotes bone growth. Of particular interest in this entire signaling pathway is an enzyme called phosphodiesterase 3 (PDE3), which converts cGMP to GMP. Inhibiting PDE3 is likely to increase the steady-state cGMP levels in the cell, thus driving the whole cascade of cellular events that can promote skeletal bone growth.
PDE3 inhibitors are already in clinical use for treating heart failure, thrombosis, asthma, and other conditions, but their potential in promoting bone outgrowth by enhancing CNP signaling remained unexplored. Now, a team of researchers led by Associate Professor Atsuhiko Ichimura from Ritsumeikan University, Professor Hiroshi Takeshima from Kyoto University, and graduate student Takaaki Kawabe from Kyoto University set out to investigate the role of PDE3 inhibitors in bone growth, both in vivo and ex vivo. This study was published in the British Journal of Pharmacology on June 02, 2025. Giving us more insights about this study, Dr. Ichimura says, "We previously reported that CNP stimulates autonomic Ca2+ influx mediated by TRPM7 channels in growth plate chondrocytes to facilitate ECM synthesis for bone growth. In this study, we attempted to stimulate CNP signaling using PDE inhibitors."
The research team conducted imaging studies along with biochemical and histological analyses of chondrocytes and in vivo using mouse models to understand the pharmacological effects of PDE3 inhibitors on bone growth. When cilostazol, a representative PDE3 inhibitor, was added to the growth medium of metatarsal bone cultures, the researchers observed a significant increase in bone outgrowth when compared to control cultures. "In cilostazol-treated bones, round and columnar chondrocyte zones were preferentially expanded," shares graduate student Takaaki Kawabe about the team's experimental findings.
To understand the in vivo effects of PDE3 inhibitors, three-week-old mice were treated with intraperitoneal injections of cilostazol for up to four weeks. Their body weight and body length (naso-anal length) were measured and compared with those of control treatments. During the four weeks of treatment, the body length of the cilostazol-treated mice significantly increased, demonstrating that the application of PDE3 inhibitors may potentially stimulate body growth in young mice. Immunochemical assessment of cGMP in cilostazol-treated cultured bones revealed that the cGMP content in these cells was almost 1.7 times higher than that in the control cultures. This underscores the ability of PDE3 inhibitors to selectively inhibit the conversion of cGMP into GMP in these cells, enhancing the CNP signaling pathway in these cells.
These findings demonstrate that PDE3 inhibitors activate CNP signaling, which drives ECM synthesis and bone growth. "Our research suggests that PDE3 inhibitors could be repurposed for treating conditions characterized by short stature, such as achondroplasia or idiopathic short stature," adds Dr. Ichimura and Professor Takeshima, highlighting the potential application of this study.
With further trials, PDE3 inhibitor drugs may find use in combination with current treatments for defective skeletal growth and lead to improved patient outcomes.
Caution: This research is based on preclinical experiments in mice. Self-administration of commercially available PDE3 inhibitors is strictly prohibited, as it is unlikely to promote bone growth and could cause serious side effects, such as hypotension and impaired blood clotting.
