Constitutive secretion of serum albumin requires reversible protein tyrosine phosphorylation events in trans-Golgi

Abstract

Serum albumin secretion from rat hepatocytes proceeds via the constitutive pathway. Although much is known about the role of protein tyrosine phosphorylation in regulated secretion, nothing is known about its function in the constitutive process. Here we show that albumin secretion is inhibited by the tyrosine kinase inhibitor genistein but relatively insensitive to subtype-selective inhibitors or treatments. Secretion is also blocked in a physiologically identical manner by the tyrosine phosphatase inhibitors pervanadate and bisperoxo(1,10-phenanthroline)-oxovanadate. Inhibition of either the kinase(s) or phosphatase(s) leads to the accumulation of albumin between the trans-Golgi and the plasma membrane, whereas the immediate precursor proalbumin builds up in a proximal compartment. The trans-Golgi marker TGN38 is rapidly dispersed under conditions that inhibit tyrosine phosphatase action, whereas the distribution of the cis-Golgi marker GM130 is insensitive to genistein or pervanadate. By using a specifically reactive biotinylation probe, we detected protein tyrosine phosphatases in highly purified rat liver Golgi membranes. These membranes also contain both endogenous tyrosine kinases and their substrates, indicating that enzymes and substrates for reversible tyrosine phosphorylation are normal membrane-resident components of this trafficking compartment. In the absence of perturbation of actin filaments and microtubules, we conclude that reversible protein tyrosine phosphorylation in the trans-Golgi network is essential for albumin secretion and propose that the constitutive secretion of albumin is in fact a regulated process.

Secretion of Apolipoprotein E From Macrophages Occurs via a Protein Kinase A– and Calcium-Dependent Pathway Along the Microtubule Network

“…Thus apoE secretion can be considered a prototype for the understanding of the secretion of other macrophage proteins. Recent studies have demonstrated that constitutive secretion is regulated, 40,41 involving kinases, phosphatases, receptors, and, typically, the microtubule network. 42 Microtubules mediate long-range transport in vesicle traffic via kinesin and dynein motor proteins.…”

Regulation of Endogenous Apolipoprotein E Secretion by Macrophages

“…76,83,84 In view of previous studies indicating that apoE secretion could be stimulated by apoA-I and by HDL, 17,19,38,47 we hypothesized that even basal apoE secretion must be under some form of regulation. We showed that disruption of microtubules inhibited basal apoE secretion, whereas disruption of the actin skeleton had no effect.…”

Kalirin/Trio Rho Guanine Nucleotide Exchange Factors Regulate a Novel Step in Secretory Granule Maturation

“…However, it is noteworthy that use of the terms constitutive and constitutive-like for these pathways does not mean that these routes are not subject to control. Indeed, it is clear that the flow of secretory molecules through the constitutive pathway can be modulated by tyrosine phosphorylation of Golgi proteins (Webb et al, 2005) and retrieval of specific membrane proteins from maturing secretory granules is modulated by phosphorylationdependent recruitment of adaptor proteins (Eaton et al, 2000;Hinners et al, 2003;Kakhlon et al, 2006).…”