Protein Tyrosine Phosphatases in the Vessel Wall

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Alarge and growing literature supports the involvement of protein tyrosine kinases in the regulation of vascular cell behavior. Many growth factor receptors are tyrosine kinases, and many cytosolic signaling events are regulated by protein tyrosine phosphorylation. Many of these mitogens, receptors, and intracellular kinases have been studied in vivo in various vascular injury and remodeling models. The biochemical counterpoint to these phosphorylation steps, ie, the reverse reaction of dephosphorylation, has received much less attention in vascular injury studies. This deficiency has begun to be addressed by the rigorous report of Wright et al in this issue.1 These authors have examined the expression in the artery wall of a family of enzymes, the protein tyrosine phosphatases (PTPases), which dephosphorylate phosphorylated tyrosine residues. It was originally believed that a small number of PTPases acted broadly to maintain a low basal level of tyrosine phosphorylation against which a large number of specific tyrosine kinases would act to regulate the phosphorylation of specific proteins. This initial concept has turned out to be incorrect. The first PTPase was cloned in 1988.2 3 Since then, at least 75 distinct PTPases have been identified,4 5 6 and it has been estimated that up to 2000 PTPases may be encoded in the mammalian genome.7 However, until the current report, little was known of the identity of the PTPases expressed by vascular tissue in vivo, their regulation of expression and activity, or their possible role in vascular remodeling. It is important to note that protein tyrosine kinases and PTPases catalyze opposite biochemical reactions, but they do not always play opposite physiological roles. SHP1 (also known as HCP, hematopoietic cell phosphatase) represents 1 well-characterized example of the multiple ways, both positive and negative, in which a PTPase may act as a regulator of cell function. SHP1 …