Moving galectin-3 closer to the goal line

Galectin-3 is a β-gelactosidase-binding lectin with a diverse array of functions in a variety of tissues and in recent years has become an increasingly promising target for treating cardiomyopathies and heart failure (8). Sixteen galectins have been identified in mammals, and galectin-3 is the sole member of the chimera subfamily due to its unique antideath domain and proline- and glycine-rich tandem repeats. While baseline expression levels of galectin-3 vary widely throughout the body, it is significantly induced almost universally by tissue damage. In the heart specifically, the protein is expressed at very low levels in the healthy myocardium but is rapidly upregulated in the early steps of tissue repair. In fact, galectin-3 is secreted in the circulation, and measurement of plasma galectin-3 for risk stratification is a class II recommendation in the American College of Cardiology/American Heart Association/Heart Failure Society of America guidelines for the management of heart failure (10).

However, a robust biomarker is not necessarily an efficacious drug target. Can the consistent upregulation in galectin-3 during cardiac tissue repair be leveraged as a therapeutic strategy to treat heart failure? To that end, much effort has been invested in discovering whether inhibition of galectin-3 can ameliorate cardiac remodeling or heart failure. As summarized recently (4), most literature points toward a protective effect when galectin-3 is pharmacologically inhibited or genetically disrupted. However, some reports have suggested there is no effect (3, 5) or possibly negative effects such as an accelerated hypertrophic response (2). Furthermore, the development of specific and potent inhibitors has been hampered by chemical challenges and a lack of specificity (8). Thus, galectin-3 is likely a therapeutic target but possibly not for every case where galectin-3 levels have been observed to increase. As heart failure represents a heterogenous collection of syndromes with both shared and unique molecular mechanisms, the ameliorative properties of galectin-3 inhibition likely depend on the specific pathophysiological fingerprint of the disease being treated. As part of the important ongoing discussion occurring in the literature to define the precise consequences of galectin-3 inhibition, in a recent article published in the American Journal of Physiology-Heart and Circulatory Physiology, Nguyen et al. (6) explored its role in a mammalian Ste20-like kinase 1 (Mst1) overexpression model of dilated cardiomyopathy (DCM).

Mst1 is a serine/threonine kinase that is a central component of the Hippo signaling pathway that regulates autophagy and apoptosis in the cardiomyocyte. Transgenic overexpression of Mst1 (Mst1 TG) results in a DCM phenotype (9), and Nguyen et al. (6) found a robust increase in galectin-3 mRNA and protein levels, ~40-fold over their nontransgenic littermates. Thus, they studied the role of galectin-3 in this model of DCM by genetic deletion and pharmacological inhibition using modified citrus pectin (MCP). Genetic deletion of galectin-3 was beneficial in the Mst-1 TG DCM model, showing reduced dilation and improved function by echocardiography and pressure catheter interrogation (Fig. 1). Furthermore, Mst1 overexpression increased fibrosis and expression of collagen and extracellular matrix genes, whereas galectin-3 deletion significantly blunted these effects at both 3 and 8 mo of age. There was even a benefit observed in heterozygous galectin-3 mice, although the improvement was attenuated compared with complete knockout of the protein. This is a significant finding since it provides hope for clinical utility, as any therapeutic approach would be unlikely to completely inhibit the protein. Interestingly, RNA seqencing showed that Mst1 overexpression resulted in extensive changes in gene expression that were mostly unaffected by galectin-3 deletion. The lack of effect here may be an indication of where galectin-3 sits in the disease pathway, not as a central node involved in transcriptional regulation but as a critical downstream mediator of specific elements of the disease pathway. Thus genetic deletion would benefit specific pathological mechanisms, like fibrosis, while not affecting the underlying transcriptional reprogramming.

Fig. 1.

Summary of findings from Nguyen et al. (6). The effects of mammalian Ste20-like kinase 1 overexpression (Mst1 OE) include increased expression of galectin-3 (Gal-3), chamber dilation, decreased ejection fraction (EF), increased fibrosis, and a disrupted gene expression profile. Genetic deletion of galectin-3 [Gal-3 knockout (KO)] reversed most of this phenotype, whereas pharmacological inhibition with modified citrus pectin (MCP) did not. DCM, dilated cardiomyopathy.

In contrast to the genetic approach, pharmacological treatment with a carbohydrate pectin, MCP, did not improve the DCM phenotype in Mst1 TG mice (Fig. 1). Pectins like MCP bind to the carbohydrate recognition domain of galectin-3 to inhibit its function and are frequently observed to have beneficial effects (4). However, the pharmacological profile of MCP is currently unclear, with its mechanism of action and specificity still being explored. In fact, a recent study has suggested that MCP does not actually inhibit galectin-3 in the canonical carbohydrate-binding site, as previously hypothesized, and may not actually inhibit galectin-3 at all (7). Nguyen et al. (6) suggested two other possibilities for a lack of effect of MCP. First, it is unknown whether MCP is able to cross the cell membrane and may therefore only be effective at inhibiting extracellular galectin-3 while having no effect on the intracellular pool. This explanation would suggest that the benefits of genetic deletion arise from the loses of the intracellular pool of galectin-3. Second, it is unknown whether MCP can inhibit galectin-3 once it has formed pentamers, as it is likely to do during the extreme upregulation that occurs during disease (6). Due to these uncertainties around pharmacological inhibitors of galectin-3, the neutral outcomes of MCP do not necessarily directly contradict the results from the genetic deletion of galectin-3. Nonetheless, many studies have still reported a beneficial effect of MCP in a variety of experimental models of cardiovascular disease (3), although whether this might be due to targets other than galectin-3 remains unclear. It would be interesting in the future to see the effects of other carbohydrate-based inhibitors in this model of DCM, such as N-acetyllactosamine or TD139.

Thus, the present study by Nguyen et al. (6) provides both good news and bad as the field moves toward a galectin-3-based therapy for cardiomyopathy and heart failure: blockade of galectin-3 is a strategy likely to provide therapeutic benefit but perhaps not via MCP-derived compounds. That the benefit is likely to revolve around blocking fibrosis in the remodeling heart is better news still, as the sustained fibrotic response represents an undertargeted, sustained, pervasive, and damaging component of heart failure. Blockade or reversal of fibrosis through galectin-3 inhibition is being actively pursued clinically in other organ systems. TD139 is a thiodigalectoside derivative inhibitor of galectin-3 whose exact mechanism of action is still unclear, but structural analysis suggests an interaction with specific binding grooves in the carbohydrate recognition domain of galectin-3 (1). The company developing TD139, Galecto Biotech, has completed a phase 1b/IIa clinical trial in the setting of idiopathic pulmonary fibrosis and received $91 million in financing to pursue phase 2/3 clinical trials. Additional efforts are underway in other fibrosis-related diseases in the liver and eye as well as an existing interest in cancer.

Applications of galectin-3 inhibition in the heart are moving toward the clinical setting, albeit slowly. Heart failure is a discouragingly complex set of syndromes, and the basic science work indicates that galectin-3 inhibition may not be a one-size-fits-all approach. The study by Nguyen et al. (6) moves the field closer to the goal of understanding in what circumstances galectin-3 inhibition would be beneficial in cardiovascular disease and both its strengths (reversing fibrosis) and limitations (lack of broad transcriptional reversal of the disease). The time may be approaching where galectin-3 will take center stage as a therapeutic target in heart failure.

GRANTS

The laboratory of J. A. Kirk is supported by National Heart, Lung, and Blood Institute Grant R01-HL-136737. R. A. de Boer is supported by The Netherlands Heart Foundation CVON DOSIS Grant 2014-40, CVON SHE-PREDICTS-HF Grant 2017-21, and CVON RED-CVD Grant 2017-11 and the Innovational Research Incentives Scheme Program of The Netherlands Organization for Scientific Research NWO VIDI Grant 917.13.350.

DISCLOSURES

The University Medical Center Groningen, which employs R. A. De Boer, has received research grants and/or fees from AstraZeneca, Abbott, Bristol-Myers Squibb, Novartis, Roche, Trevena, and ThermoFisher. R. A. de Boer is a minority shareholder of scPharmaceuticals. R.A. de Boer received personal fees from MandalMed, AstraZeneca, Novartis, Servier, and Vifor. J. A. Kirk has no conflicts of interest, financial or otherwise, to disclose.

AUTHOR CONTRIBUTIONS

J.A.K. drafted manuscript; J.A.K. and R.A.d.B. edited and revised manuscript; J.A.K. approved final version of manuscript.