Letter to the Editor: Is the cellular attribution of liraglutide action in diabetic cardiomyopathy conclusive?

Abstract

A recent study published in World Journal of Diabetes by Zhu et al reported that liraglutide ameliorates streptozotocin-induced diabetic cardiomyopathy in rats, accompanied by activation of the adenosine monophosphate-activated protein kinase-Parkin mitophagy pathway. While the reported cardioprotective phenotype is convincing, the conclusion that these effects arise from direct cardiomyocyte receptor signaling warrants cautious interpretation. The expression of functional glucagon-like peptide-1 receptors in ventricular cardiomyocytes remains controversial, and downstream signaling events alone do not definitively establish cell-type specific receptor engagement. In addition, primary cardiomyocyte cultures may contain non-myocyte populations, which introduces uncertainty regarding the cellular origin of the observed molecular signals. This correspondence highlights the need for careful interpretation of cellular attribution and outlines experimental strategies that could help resolve this issue. Clarifying the cellular targets of liraglutide action is essential for accurate mechanistic understanding and translational application in diabetic heart disease.

Key Words: Diabetic cardiomyopathy; Liraglutide; Glucagon-like peptide-1 receptor; Cardiomyocyte; Endothelial cells; Cell-type specificity

Core Tip: The cellular targets mediating the cardioprotective effects of liraglutide in diabetic cardiomyopathy remain incompletely defined. While recent studies report beneficial cardiac phenotypes, the attribution of these effects to direct cardiomyocyte receptor signaling is not yet conclusively established. This correspondence highlights uncertainties related to cell-type specificity and potential non-myocyte contributions, and emphasizes the need for rigorous cell-type resolved validation. Clarifying the cellular basis of liraglutide action is critical for accurate mechanistic interpretation and future translational applications.

TO THE EDITOR

We read with interest the recent article published in World Journal of Diabetes by Zhu et al[1] reporting that liraglutide ameliorates streptozotocin-induced diabetic cardiomyopathy (DCM) in rats. Using combined in vivo and in vitro approaches, the authors demonstrate reduced myocardial fibrosis, attenuated cardiomyocyte hypertrophy, and improved cardiac function, accompanied by activation of the adenosine monophosphate-activated protein kinase (AMPK)-Parkin mitophagy pathway[1]. These phenotypic observations are convincing and add to the accumulating evidence that glucagon-like peptide-1 receptor (GLP-1R) agonists exert cardioprotective effects in experimental models of diabetic heart disease.

However, the mechanistic attribution of these benefits to direct activation of cardiomyocyte GLP-1R warrants more cautious interpretation. While the study clearly demonstrates downstream activation of AMPK-Parkin signaling in cardiac tissue and primary cardiomyocyte preparations, the available data do not conclusively establish the cellular origin of this signaling response. Specifically, in vivo analyses were performed on whole-heart tissue, and in vitro signaling readouts were derived from mixed primary cell preparations, neither of which directly resolves cell-type specific receptor engagement. As a result, the extent to which the observed molecular effects can be attributed to direct cardiomyocyte GLP-1R signaling, as opposed to contributions from other cardiac cell populations, remains uncertain.

Why the current evidence is insufficient

A key assumption underlying the proposed mechanism is that ventricular cardiomyocytes express functional GLP-1R. This assumption remains a matter of ongoing debate in the cardiovascular field. Studies employing genetically engineered Glp1r reporter models and rigorously validated monoclonal antibodies have consistently localized GLP-1R expression predominantly to atrial cardiomyocytes and vascular endothelial or smooth muscle cells, with little or no detectable expression in ventricular cardiomyocytes[2,3]. Although low-level GLP-1R transcripts have been reported in ventricular tissue homogenates, such bulk measurements do not resolve the specific cellular source and cannot distinguish cardiomyocyte expression from that of interstitial or vascular cell populations. In addition, antibody-based detection of GLP-1R is known to be susceptible to false-positive signals when non-validated reagents are used, further complicating receptor localization[4-6].

In the study by Zhu et al[1], downstream signaling events, including activation of AMPK and Parkin, are interpreted as evidence of cardiomyocyte GLP-1R engagement. However, downstream pathway activation, while mechanistically informative, does not by itself establish the cellular locus of receptor signaling. In the absence of direct receptor localization—such as co-immunostaining with cardiomyocyte-specific markers, lineage-resolved receptor reporting, or genetic receptor perturbation—these signaling readouts cannot be unambiguously attributed to cardiomyocytes. As such, the inference from pathway activation to cardiomyocyte-specific receptor engagement remains uncertain.

This uncertainty is further compounded by methodological considerations related to neonatal rat cardiomyocyte (NRCM) cultures. Even when standard enrichment protocols are applied, NRCM preparations are well recognized to retain a non-trivial proportion of non-myocyte populations, including endothelial cells[7]. Endothelial cells are well established to express functional GLP-1R and to mount robust AMPK signaling responses upon glucagon-like peptide-1 (GLP-1) stimulation[8]. Consequently, biochemical analyses performed on whole-well lysates cannot distinguish cardiomyocyte-derived signals from those originating in contaminating non-myocyte populations. Without quantitative assessment of culture purity or cell-type resolved analytical approaches, the cellular source of the observed AMPK activation cannot be definitively assigned.

A testable alternative interpretation

Taken together, these considerations raise the possibility that liraglutide-induced AMPK activation may, at least in part, arise from non-myocyte cell populations such as endothelial cells and influence cardiomyocytes indirectly through paracrine signaling. We emphasize that this represents a hypothesis grounded in established aspects of vascular biology and known patterns of GLP-1R expression, rather than a definitive or comprehensive replacement mechanism. This alternative interpretation is introduced solely to illustrate the limitations of the current evidence in resolving cell-type specific receptor engagement, not to advance a competing mechanistic model.

Importantly, such a hypothesis does not challenge the validity or robustness of the cardioprotective phenotype reported by Zhu et al[1]. Instead, it underscores that the observed molecular signaling responses, while biologically meaningful, cannot be uniquely attributed to cardiomyocytes based on the available experimental design. Framing this possibility, therefore, serves to clarify the boundary between conclusions that are directly supported by the data and those that remain inferential, and reinforces the need for cell-type resolved validation when mechanistic claims are made.

Approaches to resolve mechanistic attribution

To distinguish direct cardiomyocyte signaling from indirect endothelial-mediated effects, future studies could incorporate several targeted approaches aimed at resolving cell-type specific receptor engagement. First, a quantitative assessment of NRCM culture purity, including systematic evaluation of endothelial and other non-myocyte markers, would provide essential context for interpreting biochemical and signaling readouts obtained from mixed primary cell preparations[9,10]. Such information would clarify the extent to which observed molecular responses can be attributed to cardiomyocytes rather than contaminating cell populations.

Second, direct localization of GLP-1R using rigorously validated monoclonal antibodies, genetically encoded reporter models, or fluorescently labeled GLP-1 analogues would help establish whether functional receptor expression is present in ventricular cardiomyocytes under the experimental conditions studied[2,4,11]. Cell-type specific localization would substantially strengthen inferences regarding receptor mediated signaling.

Third, conditional genetic deletion of Glp1r in cardiomyocytes as compared with endothelial cells would provide a causal framework for assigning cellular targets of liraglutide action in vivo. Such approaches would allow phenotypic and signaling outcomes to be interpreted in a cell-type resolved manner rather than inferred from downstream pathway activation alone.

Finally, separated or Transwell-based co-culture systems could be used to determine whether cardiomyocyte protection requires direct exposure to liraglutide or can be mediated by soluble factors derived from adjacent non-myocyte populations. Together, these complementary strategies would enable mechanistic conclusions to be grounded in cell-type resolved evidence and would help align pathway-level observations with precise cellular attribution.

Conclusion

In summary, Zhu et al[1] convincingly demonstrate that liraglutide improves cardiac phenotype in experimental DCM, providing robust evidence for cardioprotective effects at the tissue and cellular response levels. Nevertheless, the current evidence does not yet definitively support the conclusion that these benefits arise from direct cardiomyocyte GLP-1R signaling. In the absence of cell-type resolved receptor localization and signaling analyses, the cellular origin of the observed AMPK-Parkin pathway activation remains incompletely defined. Given the unresolved status of ventricular cardiomyocyte GLP-1R expression and the known presence of non-myocyte populations in primary cardiomyocyte preparations, we suggest that mechanistic interpretations be framed with appropriate caution. Clarifying the cellular targets through which GLP-1-based therapies exert their cardiac effects will be essential not only for accurate mechanistic understanding but also for guiding precise translational strategies in diabetic heart disease.