Response by Prisco et al to Letter Regarding Article, “Inflammatory Glycoprotein 130 Signaling Links Changes in Microtubules and Junctophilin-2 to Altered Mitochondrial Metabolism and Right Ventricular Contractility”

In Response:

We thank Li et al for raising important questions about our recent manuscript¹. These authors highlight limitations to our work including nonspecificity of the small molecule antagonist as compared to a genetic knockout, which we do not dispute. However, a small molecule inhibitor usually does not recapitulate a genetic knockout phenotype. Generally, an antagonist only partially blocks signaling, and does not completely suppress it. Consistent with this, our manuscript shows phosphorylated STAT3 levels in the right ventricle of SC-144 treated animals are still slightly higher than control¹.

In addition, we would like to highlight multiple papers from several independent laboratories demonstrating excess GP130-STAT3 signaling results in cardiac dysfunction. First, cardiac-specific overexpression of constitutively active GP130 is even more detrimental than GP130 knockout in a myocardial infarction model². Importantly, the reduction of STAT3 protein levels mitigates this phenotype². Second, knockout of suppressor of cytokine signaling-3 (SOCS3) leads to upregulation of the GP130-STAT3 axis and depresses cardiac function³. However, cardiac-specific deletion of GP130 abrogates STAT3 activation and significantly extends survival in SOCS3 knockout mice³. Third, expression of a dominant-negative GP130 transgene prevents the increase in STAT3 phosphorylation and cardiac hypertrophy along with normalizing sarcoplasmic reticulum Ca²⁺ ATPase 2 and brain natriuretic peptides mRNA levels in an abdominal aortic banding model⁴. Fourth, use of a GP130 neutralizing antibody preserves left ventricular ejection fraction in a rodent of myocardial infarction⁵. Our data appear to be congruent with multiple other groups demonstrating a pathological effect of excess GP130-STAT3 activation in cardiac biology. Thus, our synthesis of the available literature is that both the absence of and excess GP130 signaling have detrimental effects on cardiac function.

We focused on the JAK/STAT3 signaling pathway because STAT3 is believed to the predominant intracellular effector protein for GP130, and STAT3 is involved in microtubule remodeling. In the original description of SC-144, STAT3 and AKT activation with two GP130 cytokines was inhibited, but AKT-STAT3 signaling induced by GP130-independent molecules was not. Future studies evaluating the effects of SC-144 on other GP130 signaling pathways (ERK-MAPK, PI3K/AKT, and Src-YAP) in the right ventricle along with determining the therapeutic effects in a STAT3 antagonized model are warranted.

Finally, because we used differentiated H9c2 cells, which have extremely low proliferation rates, we propose depressed mitochondrial function is not solely due to changes in mitosis. However, we agree additional work needs to be performed to understand the microtubule-mitochondrial link in cardiac biology.