Serum stimulation of Phosphate uptake into 3T3 cells

Abstract

The stimulation by calf serum of phosphate uptake into 3T3 cells results from a change in maximum velocity of the transport process with no change in the Michaelis constant. Only arsenate among a series of inorganic structural analogs of phosphate inhibited phosphate uptake indicating a high specificity for the process. The arsenate inhibition was competitive in nature. Papaverine, theophylline, and protaglandin E1, drugs known to maintain high intracellular levels of cAMP, had little effect on serum stimulated phosphate uptake. The phosphate uptake stimulating factor(s) in serum could be distinguised from the 3T3 cell survival and migration factors by stability characteristics, but this factor(s) could not be completely separated from a uridine uptake stimulation activity or growth promoting activity using a variety of serum fractionation procedures. Only partial stimulation of the uptake process was achieved with any one serum fraction indicating a multiplicity of serum components is probably involved in this process. Because of the rapidity of serum activation of phosphate uptake and its apparent independence of intracellular cyclic nucleotide levels, it is suggested that serum factors may stimulate phosphate uptake by inducing structural changes in the phosphate carrier system.

Nutrient uptake and control of animal cell proliferation

“…case of Pi, early uptake stimulation in 3T3 cells appears to be a result of a change in the observed Vm,, (42,43). Following addition of 25% dialyzed serum to quiescent 3T3 cells, an increase in the rate of Pi uptake can be detected within 5 min (42).…”

“…This concept must be modified since hypotheses based on the pleiotypic response usually invoke one or more cyclic nucleotides as a universal regulator of transport, metabolism, and cell growth (40,41). However, it has been demonstrated by several methods that early serum-stimulated changes in Pi (42,43) and glucose (46,47) uptake are not mediated by changes in CAMP.…”

Elevated Phosphate Activates N-ras and Promotes Cell Transformation and Skin Tumorigenesis

“…Inorganic phosphate (Pi) is a common dietary component that may directly alter cell and tissue behavior in just such a manner. Almost four decades ago studies noted that contact inhibited 3T3 cells responded to serum stimulation with a rapid increase in Pi transport (31) and described Pi as a limiting nutrient in proliferation (4-6) capable of actively altering cell growth properties (32) and transformation (7). Recent in vitro results suggest that Pi is capable of stimulating specific signal transduction pathways including ERK1/2 and Akt (8, 12, 13) and suggest a paradigm shift from viewing inorganic phosphate as a passive requirement in these processes to an active regulator, thereby defining a novel mitogenic signal.…”

The role of metals in carcinogenesis: biochemistry and metabolism.

“…Arsenate was found to enter yeast cells by the phosphate transport system (3). Arsenate was a competitive inhibitor of phosphate uptake into 3T3 mouse fibroblast cells, however, chromate and vanadate showed very little inhibition of the phosphate transport system (4). Vanadate crossed the red blood cell membrane by the anion-exchange system responsible for phosphate transport (5).…”