Increased amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals

Abstract

Activation of the Ras signalling pathway by GTP loading of Ras proteins is the critical step that connects the events at the plasma membrane with cellular and nuclear responses [1]. The replacement of p21Ras-bound GDP with GTP is facilitated by guanine nucleotide exchange factors that convert inactive p21Ras-GDP proteins into the active p21Ras-GTP conformation, enabling p21Ras-GTP to transduce external signals to internal events [2,3]. Conversely, the return of the active form of p21Ras into the inactive form is preceded by the hydrolysis of p21Ras-GTP to GDP [1±4], a process that is not only intrinsic to Ras proteins but also increased by the GTPase activating protein (GAP).Only post-translationally modified Ras proteins (those that are farnesylated and carboxymethylated) are translocated to the plasma membrane whereby they can be activated by the guanine nucleotide exchange factors [5]. Farnesylation of p21Ras is catalyzed by the enzyme farnesyltransferase (FTase) [6,7] and is an obligatory initial step that targets Ras proteins to the plasma membrane for future activa- Diabetologia (1999) Summary We have recently demonstrated that insulin activates farnesyltransferase (FTase) and thereby increases the amounts of cellular farnesylated p21Ras in 3T3-L1 fibroblasts, adipocytes and vascular smooth muscle cells. We postulated that hyperinsulinaemia might considerably increase the the cellular pool of farnesylated p21Ras available for activation by other growth factors. To examine the role of in vivo hyperinsulinaemia in regulating farnesylated p21Ras, we measured the amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals. Liver, aorta, and skeletal muscle of ob/ob mice, and mice made obese and hyperinsulinaemic by injection of gold-thioglucose contained greater amounts of farnesylated p21Ras than tissues of their lean normoinsulinaemic counterparts. Similarly, farnesylated p21Ras was increased (67 vs 35 % in control animals, p < 0.01) in the livers of hyperinsulinaemic Zucker rats (fa/fa). Reduction of hyperinsulinaemia by exercise training (2 h/day for 7±8 weeks) resulted in decreases in the amounts of farnesylated p21Ras in these animals. Increased farnesylated p21Ras in hyperinsulinaemic animals reflected increasing increments in the activity of FTase in ob/ob mice (2-fold increase) and fa/fa Zucker rats (3.5-fold increase), while the total amounts of Ras proteins remained unchanged. In contrast to insulin-resistant hyperinsulinaemic animals, denervated insulin-resistant rat soleus muscle (in the presence of normoinsulinaemia) showed normal amounts of farnesylated p21Ras. In summary, these data confirm increased amounts of farnesylated p21Ras in tissues of hyperinsulinaemic animals. [Diabetologia (1999) 42: 310±316]

Mitogenic action of insulin: friend, foe or ‘frenemy’?

“…Thus, liver, aorta and skeletal muscle of insulin-resistant ob/ob mice and fa/fa rats contained increased amounts of farnesylated Ras [18]. Reduction of hyperinsulinaemia by exercise training resulted in decreased amounts of farnesylated Ras in Zucker fa/fa rats [18]. Induction of fetal hyperinsulinaemia by direct infusion of insulin into the fetus and by fetal or maternal infusions of glucose resulted in significant increases in the activity of farnesyltransferase and the amounts of farnesylated Ras in fetal liver, skeletal muscle, fat and white blood cells [19].…”

“…Thus, liver, aorta and skeletal muscle of insulin-resistant ob/ob mice and fa/fa rats contained increased amounts of farnesylated Ras [18]. Reduction of hyperinsulinaemia by exercise training resulted in decreased amounts of farnesylated Ras in Zucker fa/fa rats [18].…”

“…Overall, the ability of insulin to potentiate action of IGF-1, EGF, PDGF and VEGF has been observed in a variety of tissues, including vascular smooth muscle cells, endothelial cells, adipocytes, fibroblasts, liver cells and breast cancer cells [9,10,13,[15][16][17][18][19][20][21]. This effect of insulin has been consistently observed whenever metabolic insulin resistance along the PI3K pathway is present.…”

Insulin Signals to Prenyltransferases via the Shc Branch of Intracellular Signaling

“…It appears that activation of the Ras/ MAPK 1 and phosphatidylinositol (PI) 3-kinase pathways is necessary for the nuclear effects of insulin (1-6). Both DNA synthesis and transcription regulation have been shown to involve the MAPK and PI 3-kinase pathways (1-6).)We have recently identified another aspect of the mitogenic influence of insulin: its ability to stimulate the prenylation of the Ras family of GTPases (7,8). Insulin promotes the phosphorylation and activation of farnesyltransferase (FTase) and geranylgeranyltransferases (GGTases) I and II (9 -11).…”

Is Diabetes a Risk Factor for Colorectal Cancer?

“…Second, in addition to its direct mitogenic effect, insulin may prime cells to the effects of specific growth factors by influencing farnesylation of ras [17], a key step in the localization of ras in the plasma membrane that is required for its transforming capabilities. Insulin increases the pool of farnesylated ras protein [18,19], thereby priming the cellular response of growth factors dependent on the ras pathway. Mutations that lead to overactivation of ras proteins play a role in progression of adenomas to cancer and have been found in over 50 % of colorectal cancers [20].…”