It has to be said, today's research materials make for a fascinating read. A group of scientists, in 2021, a decade into the general acceptance of the importance of cellular senescence as a phenomenon, conducts a study of lung fibrosis in which they achieve a reversal of that fibrosis using a bcl-2 inhibitor, venetoclax, and then publish a paper that fails to mention cellular senescence even once.
Fibrosis is a dysfunction of tissue maintenance, producing scar-like collagen deposits that disrupt tissue function. There is a weight of evidence for fibrosis as a phenomenon to be driven by the presence of senescent cells, including the use of various senolytic therapies in animal studies to reverse fibrosis. Initial human trials for one of those senolytic therapies are ongoing, and one of those trials was an attempt to treat lung fibrosis.
Many of the senolytic therapies established in animal studies are bcl-2 inhibitors, such as navitoclax, a close relative of venetoclax. Inhibition of BCL2 family proteins has the effect of driving senescent cells into self-destruction, reducing their resistance of programmed cell death stimuli. This was the one of the first approaches to the selective destruction of senescent cells to be validated in the laboratory, and is widely studied and appreciated in the research community.
So it is to my eyes a little odd for a research group to run a study using a senolytic drug, targeting a well-known apoptosis-related pathway, on a condition that is generally acknowledged to involve senescent cells, and then focus on everything other than senescence as a possible mechanism. Not even a mention in the discussion section. The exploration of bcl-2 in macrophages in the lung is certainly interesting, as macrophages are likely involved in everything that touches upon tissue maintenance, including fibrosis, but without addressing cellular senescence in some way it is hard to take the conclusions at face value here.
Mice were given bleomycin for 12 days to establish lung fibrosis, and then treated daily until 21 days with ABT-199, whose medical form is known as Venetoclax, a medication approved by the FDA for use in several forms of leukemia. Control bleomycin mice had lung fibrosis with widespread collagen deposition. The bleomycin mice that received ABT-199 had normal lung architecture at 21 days and no collagen deposition.
Pulmonary fibrosis is a chronic disease showing aberrant remodeling of lung tissue. Idiopathic pulmonary fibrosis is the most common form of pulmonary fibrosis and has a high mortality rate within three to five years. Currently approved medications have limited efficacy. ABT-199 acts by inducing apoptosis, or programmed cell death, in monocyte-derived macrophages in the lung. Macrophages are large white blood cells that engulf and digest anything that does not have the surface proteins of healthy cells. Targets can include cancer cells, microbes, and cellular debris.
Researchers isolated macrophages from people with IPF. They found a marked increase in the macrophage mitochondrial protein Bcl-2 - a regulator of apoptosis - as compared to lung macrophages from people without IPF. Mitochondrial Bcl-2 was also elevated in lung macrophages from bleomycin-exposed mice that have lung fibrosis. Researchers found that mice with a conditional deletion of Bcl-2 in lung macrophages were protected from pulmonary fibrosis in the bleomycin model, and they were also protected from asbestos-induced lung fibrosis. These conditional deletion results set the stage for the experiments showing that the Bcl-2 inhibitor ABT-199 was able to reverse fibrosis in the mouse bleomycin model.
Fibrosis progression is associated with apoptosis resistance in lung macrophages; however, the mechanism by which apoptosis resistance occurs is poorly understood. Here, we found a marked increase in mitochondrial B-cell lymphoma-2 (Bcl-2) in lung macrophages from subjects with idiopathic pulmonary fibrosis (IPF). Similar findings were seen in bleomycin-injured wild-type (WT) mice, whereas Bcl-2 was markedly decreased in mice expressing a dominant-negative mitochondrial calcium uniporter (DN-MCU). Carnitine palmitoyltransferase 1a (Cpt1a), the rate-limiting enzyme for fatty acid β-oxidation, directly interacted with Bcl-2 by binding to its BH3 domain, which anchored Bcl-2 in the mitochondria to attenuate apoptosis. This interaction was dependent on Cpt1a activity.
Lung macrophages from IPF subjects had a direct correlation between CPT1A and Bcl-2, whereas the absence of binding induced apoptosis. The deletion of Bcl-2 in macrophages protected mice from developing pulmonary fibrosis. Moreover, mice had resolution of fibrosis when Bcl-2 was deleted or was inhibited with ABT-199 (venetoclax) after fibrosis was established. These observations implicate an interplay between macrophage fatty acid β-oxidation, apoptosis resistance, and dysregulated fibrotic remodeling.