CIP2A promotes osteoclastogenesis and postmenopausal bone loss by activating a novel HMGA1–c-Myc–PI3K/AKT axis

The human skeleton is not a static frame of bone; it is a living tissue in a constant state of renewal. This renewal relies on a delicate balance between two opposing forces. One set of cells, called osteoblasts, acts as the construction crew, laying down new bone to strengthen the structure. The other set, known as osteoclasts, serves as the demolition crew, breaking down old or damaged bone to make room for the new. In a healthy body, these two teams work in perfect sync, maintaining bone density and strength. However, when the demolition crew becomes too aggressive, breaking down bone faster than it can be rebuilt, the result is osteoporosis. This condition, which weakens bones and raises the risk of fractures, is a major health challenge for aging populations, particularly affecting women after menopause. While doctors have treatments to slow down this bone loss, they often come with side effects or do not address the root cause of why the demolition cells become overactive in the first place.

A team of researchers at Tongji Hospital in Wuhan, China, has now uncovered a specific molecular switch that drives this overactive demolition. They discovered that a protein called CIP2A, which was previously known mostly for its role in cancer, acts as a powerful accelerator for osteoclasts. The scientists found that during the process of bone breakdown, CIP2A levels rise significantly. It functions like a molecular scaffold, bringing together other proteins to stabilize a key regulator called HMGA1. This stabilized HMGA1 then moves into the cell's nucleus and turns on a gene for c-Myc, a master controller of cell activity. This c-Myc protein, in turn, activates a signaling pathway known as PI3K/AKT, which feeds back to keep c-Myc active, creating a self-reinforcing loop that keeps the osteoclasts working overtime. The researchers demonstrated that blocking CIP2A stops this entire chain reaction, effectively calming the demolition crew without harming the construction crew.

To reach these conclusions, the team started by observing bone marrow cells from mice in a laboratory dish. They watched how these cells transformed into osteoclasts when stimulated with a specific protein signal. They noticed that as the cells began to differentiate, the amount of CIP2A inside them increased steadily. To test if this protein was actually causing the change, they used genetic tools to either remove CIP2A or add extra amounts of it. When they removed the protein, the cells failed to form the proper structures needed to break down bone, and their ability to eat away at bone material dropped sharply. Conversely, when they added extra CIP2A, the cells became hyperactive, forming larger structures and eating through bone much faster. Crucially, they found that this protein only affected the demolition cells; it had no impact on the construction cells, suggesting a way to target bone loss without disrupting bone formation.

The researchers then moved to living mice to see if these findings held true in a complex body. They created a model of osteoporosis by removing the ovaries of female mice, a procedure that mimics the hormonal changes of menopause and leads to rapid bone loss. In these mice, they introduced a virus to lower the levels of CIP2A throughout the body. The result was a significant protection against bone loss. The mice with reduced CIP2A kept more of their spongy internal bone structure compared to untreated mice. To prove that this approach could work as a medicine, the team also tested a small molecule drug called TD52, which is designed to inhibit CIP2A. When they gave this drug to the osteoporotic mice, it successfully stopped the bone loss and reduced the number of active demolition cells. Importantly, the drug did not cause any visible damage to the heart, liver, or kidneys, indicating it was well-tolerated.

Digging deeper into the mechanics, the scientists wanted to understand exactly how CIP2A influenced the cells. They identified that CIP2A works by recruiting an enzyme called OTUD4. This enzyme acts as a protector, removing chemical tags that usually mark proteins for destruction. By recruiting OTUD4, CIP2A prevents a protein called HMGA1 from being broken down. Normally, HMGA1 would be quickly degraded, but with CIP2A present, it accumulates and moves into the nucleus. Once inside, HMGA1 binds to the DNA near the gene for c-Myc and boosts its production. The study revealed that c-Myc then activates the PI3K/AKT pathway, a major signaling route in the cell. In a twist that explains why the bone loss is so persistent, this pathway also helps to keep c-Myc levels high, creating a positive feedback loop that locks the cell into a state of high activity. When the researchers broke this loop by blocking c-Myc or the PI3K/AKT pathway, the bone loss was halted, confirming that this specific chain of events is the engine driving the disease.

The study also addressed whether this new target might have unintended consequences on other parts of the body. The researchers tested the effect of blocking CIP2A on the cells responsible for building bone. They found that neither removing the protein nor using the TD52 drug changed the behavior of these construction cells. The cells continued to build bone normally, and markers of bone formation in the blood remained unchanged. This specificity is a critical finding, as many current treatments for osteoporosis can inadvertently suppress bone formation along with bone breakdown. The ability to selectively quiet the demolition crew while leaving the construction crew untouched offers a promising new direction for therapy.

While the results are compelling, the researchers acknowledge that more work is needed before this approach can be used in human patients. The study was conducted in mice, and the long-term safety of the drug TD52 in humans has not yet been established. Additionally, the precise structural details of how these proteins fit together remain to be fully mapped. However, the discovery of this specific regulatory axis provides a clear roadmap for future drug development. By targeting CIP2A, scientists may be able to develop a new class of medicines that treat postmenopausal osteoporosis and other conditions involving excessive bone loss more effectively and safely than current options. The work transforms a protein known for its role in cancer into a key player in skeletal health, opening a new chapter in the understanding of how our bones are maintained and how they can be protected.