Insulin Stimulates the Phosphorylation and Activity of Farnesyltransferase via the Ras-Mitogen-Activated Protein Kinase Pathway*

Abstract

Farnesylation of p21Ras by farnesyltransferase (FTase) is obligatory for anchoring p21Ras to the plasma membrane, where it can be activated by growth factors. Insulin significantly stimulates the phosphorylation of the alpha-subunit of FTase (4-fold) and the enzymatic activity of FTase in 3T3-L1 fibroblasts and adipocytes. FTase activity was assessed by the amount of [3H] mevalonate (a precursor of farnesyl) incorporated into p21Ras in vivo and by quantitating the amount of farnesylated p21Ras before and after insulin administration. Insulin-stimulated phosphorylation of the alpha-subunit of FTase in 3T3-L1 fibroblasts and adipocytes was blocked by the mitogen-activated protein/extracellular-signal regulated kinase-kinase inhibitor, PD98059, but not by wortmannin or bisindolylmaleimide. Additionally, PD98059 blocked insulin-stimulated [3H]mevalonic incorporation and farnesylation of unprocessed p21Ras in both cell lines. Furthermore, expression of the dominant negative mutant of p21Ras precluded insulin-stimulated phosphorylation of the FTase alpha-subunit and activation of its enzymatic activity. In contrast, 3T3-L1 fibroblasts, expressing the constitutively active Raf-1, exhibited enhanced phosphorylation of the FTase alpha-subunit. It seems that insulin's effect on the phosphorylation and activation of FTase in both fibroblasts and adipocytes is mediated via the Ras pathway, resulting in a positive feedback augmentation of the cellular pool of farnesylated p21Ras.

USP17 Regulates Ras Activation and Cell Proliferation by Blocking RCE1 Activity

“…These modifications are facilitated by a series of enzymes including FTase (28), RCE1 (29 -31), and ICMT (32). Previously, modulation of FTase activity has been shown to occur through phosphorylation of the ␣-and ␤-subunits (44 -46), and this phosphorylation is induced in response to growth factor stimulation (46). However, no reports have indicated that either RCE1 or ICMT can be regulated, and to date it has been assumed that these enzymes are constitutively active.…”

Insulin-induced Adipocyte Differentiation

“…9). Shc, via its SH2 domain, and MAP kinase mediate insulin effect on FTase (20,21). FTase, in turn, farnesylates additional molecules of p21 ras and increases its availability for subsequent stimulation.…”

“…The photographs show cells on day 10 of each treatment. (20,21). FTase farnesylates additional p21 ras that mediates insulin effect on MAP kinase and CREB.…”

“…Only insulin promotes the phosphorylation of the ␣-subunit of the FTase and GGTase I and increases the activity of these enzymes. The effect of insulin appears to involve activation of the MAP kinase (20) and an additional signal involving the Shc SH2 domain (21) (Fig. 9).…”

“…We further demonstrated that prenylation of these proteins is significantly augmented by ambient hyperinsulinemia either present in insulin-resistant humans and animals or induced experimentally in controls (18,19). Insulin appears to promote the phosphorylation and activation of farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) by a mechanism independent of PI 3-kinase (20,21). Availability of prenylated p21 ras is required for the activation of MAP kinase (22,23).…”