Research area 1: Molecular mechanism of myocardial fibrotic remodeling
|Figure 1: GSK-3β crosstalk with pro-fibrotic TGF-β1-SMAD-3 pathway|
Fibrosis affects nearly every tissue in the body. It is a leading cause of morbidity and mortality, as highly aggressive fibrotic processes eventually lead to organ malfunction and death. In reference to myocardial diseases, virtually every form of heart disease is associated with expansion and activation of the cardiac fibroblast compartment, the primary source of ECM production and fibrosis. Despite its enormous impact on human health, there are currently no approved treatments that specifically target fibrosis. Although the direct evidence on the role of cardiac fibroblasts in normal cardiac function is lacking, it is believed that cardiac fibroblasts play a central role in the maintenance of ECM in the normal heart.
Recent pioneering work from our lab propose the concept that cardiac fibroblasts are not mere bystanders, acting only in fibrosis, but are crucial mediators of myocardial hypertrophy and adaptive responses in the heart injury. We demonstrated that GSK-3β is phosphorylated (inhibited) in fibrotic tissues from ischemic human and mouse hearts. Using two different fibroblast-specific GSK-3β knockout mouse models, we demonstrated that deletion of GSK-3β, specifically in cardiac fibroblasts, leads to fibrogenesis and profound scarring in the post-ischemic heart. Deletion of GSK-3β also induces a pro-fibrotic, myofibroblast phenotype in isolated cardiac fibroblasts, in mouse embryonic fibroblasts (MEFs) and in post-MI hearts deleted for GSK-3β. This report was the first to study MI-induced fibrotic remodeling employing cardiac fibroblast-specific gene targeting. Furthermore, this study is the first to demonstrate the effect of cardiac fibroblast-specific gene targeting on global cardiac function and adverse remodeling post-MI.
We administered a small molecule inhibitor of SMAD-3 (SIS3) in vivo to test if hyper-activation of SMAD-3 is really responsible for excessive fibrotic responses observed in cardiac fibroblast-specific GSK-3β KO mice. SIS3 treatment significantly blunted scar expansion in KOs hearts compared to KOs treated with vehicle. Moreover, SIS3 administration nearly abolished the detrimental phenotype of cardiac fibroblast specific GSK-3β deletion as evidenced by restored ventricular function and chamber dimensions. Together, these findings provide strong evidence that hyper-activation of SMAD-3 is largely responsible for the detrimental phenotype following selective inhibition or deletion of GSK-3β in cardiac fibroblasts. Furthermore, we demonstrated that GSK-3β directly interacts with SMAD-3 in human heart, and cardiac fibroblasts. Inhibition/deletion of GSK-3β leads to increased phosphorylation of SMAD-3 at the carboxyl terminus (S423/425) and decreased phosphorylation at the n-terminus (S204). Thus our study suggests that GSK-3β exerts its effect on TGF-β1-SMAD-3 signaling specifically by regulating both the C-terminal domain as well as the linker region of SMAD-3. Taken together the available evidence suggests that inhibition/deletion of GSK-3β induces the fibrotic phenotype by activating the SMAD-3 pathway. Currently we are exploring the role of GSK-3β, Wnt/β-catenin and TGF-β-1-SMAD-3 signaling network in myocardial fibrosis in diseased heart. We believe, a better understanding of the GSK-3β, β-catenin and SMAD-3 signaling network may provide a promising new therapeutic target for the management of myocardial fibrosis in the diseased hearts.
Research area 2: GSK-3 family and myocardial biology
|Fig. 2 GSK-3 family and myocardial biology|
Myocardial infarction induced cardiac remodeling and heart failure is the leading cause of deaths worldwide. Recent studies indicate that Glycogen synthase kinase-3 (GSK-3) could be a promising target to treat the cardiac remodeling. GSK-3 is the ubiquitously expressed Ser/Thr kinase which was initially identified as a negative regulator of glycogen synthase, the rate limiting enzyme in glycogen synthesis. Intensive study over the past three decades has helped define the role of the GSK-3 family in a variety of physiological and pathophysiological processes including heart failure. GSK-3 regulates a wide variety of cellular functions including metabolism, transcription, translation, cell proliferation, differentiation and survival. The family consists of two isoforms, α and β, which are 98% identical within their kinase domains but differ substantially in their N- and C-terminal sequences. Unlike most protein kinases, GSK-3 is typically active in unstimulated cells and is inhibited in response to a variety of inputs, including growth factors. Because GSK-3-mediated phosphorylation of substrates usually leads to inhibition of those substrates, the end result of growth factor-mediated inhibition of GSK-3 is typically functional activation of its downstream substrates.
We are interested to define the critical role of GSK-3 isoforms in the pathophysiology of cardiac remodeling and heart failure. We have employed different genetic loss of function model to define the role of GSK-3 isoforms in the developmental process and adult heart functions. GSK-3α and β have both unique and overlapping function in the pathophysiology of cardiac diseases. We have reported that germline deletion of GSK-3β is embryonic lethal due to hyper-proliferation of cardiomyoblasts that lead to near obliteration of ventricular cavities. Conversely, germline deletion of GSK-3α does not affect the embryonic development and pathophysiology of adult mice up to four months of age but leads to functional deterioration with aging. Our findings revealed that GSK-3α is a critical regulator of beta-adrenergic receptor responsiveness and cAMP production.
Further studies with cardiomyocyte specific conditional deletion models of GSK-3α and GSK-3β suggest that both isoforms promote ventricular remodeling and dysfunction post-myocardial infarction (MI) and deletion is protective. Interestingly, GSK-3α found to be a critical regulator of scar expansion post-MI. Study of underlying mechanisms of cardiac remodeling in these models reveals that GSK-3 regulates cardiomyocyte deaths and proliferation in the stressed heart.
Research area 3: Cardio-oncology
|Fig. 3 Signaling Pathways implicated in Cardio-Oncology|
The primary aim of this project is to minimize cardiotoxicity during cancer treatment and cardiovascular risks in cancer survival patients. The discipline known as “cardio-oncology” or “onco-cardiology” seems to be growing at a rapid pace, driven at least in part by the fact that cancer patients typically have cardiovascular disease and vice versa, demanding a team approach of cardiologists and oncologists. Another driver of this apparent boom in interest is the increasing awareness of the toxicities, in particular cardiac toxicities, associated with cancer therapeutics. Initially centered on anthracycline- and trastuzumab-induced cardiotoxicity, similar issues are now being faced with some of the so-called “targeted therapeutics,” which largely target protein kinases that are activated or overexpressed, and thus drive growth of various cancers. Cancer survival has improved over the years due to newer and better forms of treatment, so called targeted therapy. Targeted anticancer drugs were initially thought to affect tumors but not normal tissue in which kinases were not constitutively active. Thus, the hope of targeted therapy was one of high efficacy, with minimal adverse effects compared with traditional chemotherapy. However, unexpected reports of cardiotoxicity from approved targeted drugs suggest that these agents are not magic bullets. In fact, as long as a kinase maintained expression in the heart, targeting that kinase in cancer could, theoretically, cause cardiotoxicity. Indeed, there are numerous overlapping signaling pathways that drive tumorigenesis but that are also required for cardiomyocyte survival or function. Cardiotoxicity with kinase inhibitors (KIs) could be said to occur “predictably” if a given molecular target participates in basic cardiac function.
We are entering a new era, one in which personalized medicine in cancer patients is becoming a reality. The success of drug development in cancer, while limiting cardiotoxicity, will require a multidisciplinary approach, with collaborations between oncologists, cardiologists, pharmacologists, and toxicologists, as well as a strong commitment from industry and the regulatory agencies to ongoing translational research. This evolving discipline demands that cardiotoxicity be considered, both in preclinical and clinical scenarios. This issue should be foremost in the mind of investigators during protocol development, enrollment, and follow-up. This should also include post approval surveillance of patients treated with these drugs for it is then that problems will likely appear when patients with significant comorbidities begin to use these agents. Because small molecule KIs are believed to be the foundation of future cancer treatments, researchers with a particular expertise in this field will be required. Fortunately, there seems to be a growing interest in cardio-oncology at the basic, clinical, and funding agency level (National Cancer Institute and National Heart, Lung, and Blood Institute) and that leads to optimism.
At this end, we are examining the underlying mechanisms that drive the cardiotoxicity of the KIs. This will allow an understanding of the real and potential power of modern cancer therapeutics. We are also examining the preclinical models for predicting cardiotoxicity, including human induced pluripotent stem cells (hiPSC) and zebrafish, with the hope of eventually being able to identify problematic agents before their use in patients. We plan to validate our screening results in vivo with small animal models. We believe that a combination of these multiple screening models will provide valuable preclinical predictive toxicology data to cancer therapeutics.