These data support the following model (Fig. 6.4). Activation of Fyn in caveolae by insulin or stress leads to phosphorylation of caveolin-1 on Tyr14, which leads to the recruitment of Csk, and phosphorylation and inhibition of resident Src-family kinases. This feedback mechanism is supported by a number of observations [24,31]. Oxidative stress simultaneously activates Src-family kinases and their negative regulator Csk. Induction of oxidative stress leads to concomitant increases in both active site and Csk inhibitory site phosphorylation of Fyn in many cell types. Basal Csk activity varied from cell type to cell type, but was very low in fibroblasts from wild-type mice and absent from fibroblasts from Csk knockout mice. However, basal Src-family kinase activity was low in all cases. Therefore, Src-family kinases are maintained in an inactive conformation in fibroblasts through a
Fig. 6.4 Phosphocaveolin recruits Csk to the caveolae, attenuating Fyn. Csk (c-terminal Src kinase) is a negative regulator of Src-family kinases. Csk must be targeted to its substrates in membranes via an SH2 domain-mediated interaction with a tyrosine-phosphorylated protein. This activates Csk,
Fig. 6.4 Phosphocaveolin recruits Csk to the caveolae, attenuating Fyn. Csk (c-terminal Src kinase) is a negative regulator of Src-family kinases. Csk must be targeted to its substrates in membranes via an SH2 domain-mediated interaction with a tyrosine-phosphorylated protein. This activates Csk, leading to phosphorylation of Src-family kinases at their inhibitory site (pY527) and attenuation of their activity. Csk binds specifically to phosphocaveolin only after stimulation with insulin or oxidative stress, and is activated under these conditions.
mechanism that is independent of Csk phosphorylation. In contrast to basal activity, oxidative stress-induced Src-family kinase activity was much greater in Csk-/-cells than in Csk+/+ cells, indicating that loss of Csk activity leads to dysregulation of the Src-family kinases, but only after activation. Therefore, Csk plays a modulatory, not a strictly regulatory role for Src-family kinases in these cells. This is in marked contrast to the mechanism of regulation of Src-family kinases described in lymphocytes. In these cells, Src-family kinases are constitutively phosphorylated by Csk, and activated by dephosphorylation of this site .
Phosphocaveolin in the Loop
Csk must be recruited to its substrates via SH2-mediated binding to a tyrosine-phosphorylated targeting protein. A number of observations implicate phosphocaveolin in the feedback inhibition of Src-family kinases [24,31]. Src-family kinases are highly enriched in caveolae, due to both acylation and direct binding to cav-eolin. Caveolin-1 is phosphorylated in response to stimuli that activate Fyn, including oxidative stress and insulin, and stress-induced phosphorylation of caveolin requires Fyn. Stress-induced caveolin phosphorylation is self-limiting and attenuated by Csk: caveolin is phosphorylated to a much greater extent in Csk-/- cells than in the Csk+/+ cells. Csk binds specifically to phosphocaveolin, which is one of only two tyrosine-phosphorylated proteins associated with Csk in fibroblasts and adipocytes. Association with Csk increases significantly in response to either insulin or stress. These data indicate that binding to phosphocaveolin is a major mechanism for the regulation of Csk in response to stress and insulin. Therefore, the role of caveolin phosphorylation in signal transduction becomes a question of the role that regulation of caveolar Src-family kinase activity plays in signal transduction.
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Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...