Initial studies suggested that insulin increases diacylglycerol and activates protein kinase C (PKC) in BC3H-1 myocytes. In these earlier studies, insulin was found to translocate PKC-β, but the presence of PKC-ϵ was not appreciated. More recently, the presence of PKC-ϵ was documented, but PKC-β was not detected, and it was questioned whether insulin activates PKC in BC3H-1 myocytes [Stumpo, D.J., Haupt, D.M. and Blackshear, P.J. (1994)J. Biol. Chem. 269:21184–21190]. We questioned whether insulin translocates PKC-ϵ in BC3H-1 myocytes, and re-evaluated the question of whether myocytes truly contain a PKC-β isoform whose existence can be verified by its response to phorbol ester treatment. We found that PKC-ϵ was acutely translocated by insulin and phorbol esters from the cytosol to the membrane fraction in BC3H-1 myocytes; in addition, PKC-ϵ, like PKC-α, was depleted by chronic phorbol ester treatment. We also found that BC3H-I myocytes containing a 76,000 Mr PKC-β isoform that is acutely translocated and subsequently depleted by phorbol esters. Moreover, chronic phorbol ester treatment induced an 84,000 Mr PKC-β2 isoform that appeared to be persistently translocated and activated, as suggested by studies of myristoylated arginine-rich C kinase substrate (MARCKS) phosphorylation. We conclude that: (1) insulin acutely translocates PKC-ϵ, as well as PKC-β, in BC3H-1 myocytes; and (2) PKC-β is not truly downregulated by phorbol esters in BC3H-1 myocytes.
Available at: http://works.bepress.com/thomas-arnold/3/