Fibrosis is the formation of excessive fibrous connective tissue, creating scar-like tissue in affected organs. Fibrosis causes a stiffening of tissue, altering the way an organ functions. In dynamic organs such as the lung, fibrosis not only impairs oxygen exchange, but also inhibits the lungs from expanding and contracting during breathing. Fibroblasts are activated in diseases such as pulmonary fibrosis, cirrhosis of the liver, and after myocardial infarction (heart attack). Activated fibroblasts are called myofibroblasts. They become more contractile, migratory, and secrete pro-fibrotic matrix proteins, which form the fibrotic tissue and promote scarring.
Scientists often look for potential drug targets by comparing the entire genome between normal and diseased cells. However, many of the targets identified with this technique are ultimately not suitable for clinical development. Many genetic targets are correlated, but not causal for pathology. Even for those that are causal, many do not make it to clinical trials for reasons such as unavailability of drugs that inhibit those targets, insufficient knowledge about the target in order to design an inhibitor, or the target is unreachable by drug molecules.
To speed the transition from the lab to clinical trials, Drs. Andrew Haak and Daniel Tschumperlin of Mayo Clinic experimentally generated a list of druggable targets implicated in fibroblast activation. These are targets for which inhibitors have already been developed, much is known about the target, or the target is easily accessible by drug molecules. Fibroblast activation can be replicated in cell culture by treating fibroblasts with the protein transforming growth factor b, or TGFb. The scientists created a cell culture model of pulmonary fibrosis by activating fibroblasts with TGFb and used molecular biology techniques to inhibit the druggable targets previously identified. After target inhibition, they studied how affected the fibroblasts were by the inhibition of each particular target. They measured cell behavior such as contractility and common markers of activated fibroblasts, such as a smooth muscle actin and collagen. They found that STAT3 was a target that, when inactivated, hindered fibrotic activation, a primary mechanism of pulmonary fibrosis.
Haak tested the inhibition of STAT3 in a cell culture model that closer replicated human pulmonary fibrosis. He received cell samples from patients with the disease and tested the effects of blocking STAT3. He found that their treatment inhibited fibroblast activation, indicating that STAT3 may be a promising target for treatment of pulmonary fibrosis.
While much can be learned from cell culture models, such as the mechanisms behind cell behavior and disease, animal models are critical for pre-clinical testing of potential drugs. The scientists used a mouse model of pulmonary fibrosis, caused by treatment with bleomycin. Following injection with an inactivator of STAT3, the mice demonstrated reduced fibrosis, levels of fibrotic tissue, and markers for activated fibroblasts. This is first step toward the use of STAT3 inhibitors in pulmonary fibrosis.
The protein STAT3 has many previously known functions in the immune system. Because of this, concerns about side effects on the immune system must be addressed. Haak and Tschumperlin are now studying ways in which to selectively inhibit STAT3 function of activated fibroblasts in the lung, but not immune cells. If they are able to create such a selective drug, it could then be tested in clinical trials with patients suffering from pulmonary fibrosis.
The mechanism of action for one currently available drug, nintedanib, is not fully understood. When Haak measured the levels of activated STAT3, he found that it was reduced by nintedanib treatment, suggesting that nintedanib may fight pulmonary fibrosis in part by inactivating STAT3.
Since fibrosis is common to a number of diseases, the scientists decided to test out STAT3 inhibition in liver and heart fibroblasts. They found that common markers of activated fibroblasts were significantly reduced. This implies that STAT3 may not only be a druggable target of pulmonary fibrosis, but other fibrotic diseases, as well. Drugs that can be used in multiple ways are especially valuable, as they lower the expense and increase the speed of drug development and clinical testing. This allows life-changing treatments to be brought more rapidly and affordably to patients suffering from diseases such as pulmonary fibrosis, liver cirrhosis, or cardiac fibrosis.