Regeneration and tissue maintenance are highly complex, regulated processes. Unfortunately, these processes run awry as the low-level molecular damage of aging increases over the years. Cells change their behavior, change the signals they produce, and one of the detrimental outcomes of these changes is fibrosis. This is the creation of scar-like collagen structures in place of the expected arrangement of cells and extracellular matrix. Since the fine details of that arrangement matter greatly to the correct function of organs, fibrosis is very harmful. It features prominently in the most common age-related diseases of the lungs, kidneys, liver, and heart, but can be found in other tissues as well.
With the explosion of interest in senescent cells as a cause of aging over the past few years, and a matching increase in funding for studies, research groups have been able to prove that the presence of lingering senescent cells is a significant cause of fibrosis. Senescent cells have an important transient role to play in wound healing and regeneration in general: in a perfect world some cells become senescent, their signals and their interaction with the immune system directs rapid and accurate reconstruction of tissue, and all of these temporary senescent cells then promptly self-destruct or are consumed by the immune cells called macrophages. Unfortunately this system starts to head downhill into disarray given a growing population of senescent cells that stick around for the long term. Their signals produce chronic inflammation, confuse the regulation of regeneration, and make matters worse in numerous other ways as well.
The open access paper noted here doesn't mention senescent cells at all, but it does focus on one of the proteins that both causes cells to become senescent and is also secreted by cells that have become senescent. The protein is called TGF-β1 and is one small part of the senescence-associated secretory phenotype, SASP, a range of molecules important to the short-term tasks carried out by senescent cells, but that cause disruption, damage, and ultimately organ failure when the number of lingering senescent cells grows large over the years. The authors of this paper show that TGF-β1 inhibition reduces fibrosis, a result that dovetails well with studies from the past few years that have demonstrated targeted removal of senescent cells to reduce fibrosis. All things consider, I think it should be taken as more evidence for the potential benefits of senolytic therapies that clear senescent cells from old tissues.
As a further matter of interest, note the comments on tissue stiffness in the paper. Where it occurs in blood vessels, this age-related change is a very important component of vascular aging and heart disease. Solid evidence for senescent cell removal to affect elasticity of tissues has so far only arrived for the lungs, and in mice, but the relevant mechanisms here are much the same in most tissue types. It is reasonable to be cautiously optimistic. If clearance of senescent cells does produce a significant reduction of vascular stiffening in humans, then that outcome is a very big deal. In that scenario, we should expect cardiovascular mortality to fall dramatically as senolytic therapies are deployed to the clinic.
Tissue fibrosis is a major cause of human morbidity and mortality worldwide. TGF-β1 signaling is a well-known driver of collagen expression and tissue accumulation important to wound repair. Exaggerated TGF-β1 signaling is also strongly implicated in numerous fibrotic diseases, including those involving liver, heart, and lung. For example, approximately 80% of the upregulated genes in lungs of patients with idiopathic pulmonary fibrosis are reported to be direct or indirect TGF-β1 target genes. Pathological collagen accumulation, and its promoting effects on tissue stiffness, are also strongly implicated in cancer progression. TGF-β1 signaling is both an initiator and a driver of tissue stiffness because accumulation of collagen and other matrix proteins promotes integrin-dependent latent TGF-β1 activation and further extracellular matrix deposition. Enhanced stiffness is thought to promote tumor cell β1 integrin activation, leading to more invasive tumor phenotypes and metastasis, consistent with the strong correlation of TGF-β1 signaling with poor cancer prognosis. For these and other reasons there has been much interest in TGF-β1 signaling as a therapeutic target.
Although attractive as a target, the critical roles of TGF-β1 in suppressing inflammation and epithelial proliferation give pause to the idea of global inhibition of TGF-β1 signaling. Indeed, systemic inhibition of TGF-β1 can lead to the development of squamous skin tumors and autoreactive immunity. In addition, chronic administration of several small-molecule inhibitors of TGF-β1 receptor (TβR) kinases has led to enhanced skin and colonic inflammation and abnormalities in cardiac valves. To minimize adverse consequences, an approach of blocking TGF-β1 activation in specific cell types using the unique pathway of αvβ6-dependent latent TGF-β1 activation has developed and is currently in clinical trial. But this integrin is primarily expressed in epithelia of lung, kidney, and skin. In an attempt to develop a more circumscribed inhibitor of TGF-β1 signaling centered on suppression of collagen accumulation, we undertook a high-throughput, image-based phenotypic screen of small molecules that could block TGF-β1-induced epithelial-mesenchymal transition (EMT) in vitro but not directly inhibit TβRI kinase itself.
We identified trihydroxyphenolic compounds as potent blockers of TGF-β1 responses. Remarkably, the functional effects of trihydroxyphenolics required the presence of active lysyl oxidase-like 2 (LOXL2), thereby limiting effects to fibroblasts or cancer cells, the major LOXL2 producers. This selectivity likely avoids the toxicities of long term general TGF-β1 inhibition in chronic disease processes such as fibrosis and cancer progression. Indeed, we have observed no adverse events in mice on the trihydroxyphenolic-rich diet for at least 6 months, including the absence of skin inflammation and discernible lesions in cardiac valves.
Mechanistic studies revealed that trihydroxyphenolics induce auto-oxidation of a LOXL2/3-specific lysine (K731) in a time-dependent reaction that irreversibly inhibits LOXL2 and converts the trihydrophenolic to a previously undescribed metabolite that directly inhibits TβRI kinase. Combined inhibition of LOXL2 and TβRI activities by trihydrophenolics resulted in potent blockade of pathological collagen accumulation in vivo without the toxicities associated with global inhibitors. These findings elucidate a therapeutic approach to attenuate fibrosis and the disease-promoting effects of tissue stiffness by specifically targeting TβRI kinase in LOXL2-expressing cells.