Role of HSP70 and the TLR4-MD2 Complex in the Vasculature: From Function to Dysfunction in Diabetes
Almeida De Oliveira, Amanda
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Diabetes impairs vascular functionality, which contributes to the pathophysiology of its associated complications. Recently, it was suggested that innate immune receptors assist in the development of diabetic vasculopathies, which sustains the pro-inflammatory state of the disease. Among these receptors, Toll-like receptor 4 (TLR4) is a strategic player as its activation prime the release of inflammatory mediators and reactive oxygen species (ROS), which are well-known pathological mechanisms linked to diabetic complications. The TLR4-Myeloid differentiation factor 2 (MD2) complex has gained much attention over the last decade, primarily because it interacts with self-produced molecules. In vascular tissues, the Heat-shock protein 70 (HSP70) is relevant as it may have antagonistic mechanisms (iHSP70: anti-inflammatory and eHSP70: proinflammatory) following crosstalk with TLR4 pathways. Interestingly, eHSP70 levels are increased in diabetic patients. Thus, it has been suggested that elevated eHSP70 levels lead to a reduced heat-shock response in intracellular compartments contributing to islet dysfunction and insulin resistance, critical elements of diabetes. Still, at this point, it is unclear if a similar process occurs within the vascular system. Vascular dysfunction is the single-most cause of cardiovascular complications in diabetic patients; therefore, unveiling the interplay between the TLR4-MD2 complex and HSP70 in the vasculature is of utmost importance. While it has been previously hypothesized that an interplay between TLR4 and HSP70 contributes to diabetic vasculopathies, it is yet to be confirmed whether the eHSP70/TLR4-MD2 axis mediates vascular dysfunction in diabetes. Furthermore, in muscle biology, evidence suggests that HSP70 could have a versatile range of functions, as genetic deletion of its inducible genes impairs Ca2+ handling, and consequently, cardiac and skeletal muscle contractility. However, it is unknown whether HSP70 is involved in vascular reactivity, an intrinsic physiological mechanism of blood vessels, which could also have implications for diabetes-induced vascular dysfunction. To narrow these gaps, this work independently investigated the role of HSP70 and the TLR4-MD2 complex in the vasculature: from function to dysfunction in diabetes. Our contributions are as follows. First, we uncovered that iHSP70 has an additional physiological role, the assistance of vascular reactivity. Then, we dissected that the mechanism by which this protein impacts contraction involves a complex interaction with Ca2+ dynamics. Next, we report that HSP70 plays a dual role in diabetes-induced vascular dysfunction: iHSP70 affects Ca2+ handling mechanisms, and eHSP70 modulates the TLR4-MD2 complex. Considering the alleged roles of the TLR4-MD2 complex in the vascular system, we subsequently examined this system’s contributions in an animal model of type 1 diabetes. We found that the activation of the TLR4-MD2 complex triggers hypercontractility and augments ROS production in the aorta of diabetic rats, which helps explain why the blockade of this complex lowers blood pressure in this animal model. Together, these newly discovered roles of HSP70, whether via interaction with Ca2+ handling mechanisms or the TLR4-MD2 complex, as well as the independent contributions of the TLR4-MD2 complex, push forward the field of vascular biology and opens research avenues for other diseased states associated with a dysfunctional vascular response.