(Received for publication, June 16, 1995; and in revised form, July 28, 1995)
From the
Individuals with insulin resistance show increased levels of PC-1 expression in skeletal muscle and fibroblasts, and in transfected cell lines that overexpress PC-1 there is a reduction in the insulin-stimulated insulin receptor tyrosine phosphorylation. As PC-1 is a type II transmembrane protein with extracellular phosphodiesterase and pyrophosphatase activity, increased expression of PC-1 at the cell surface will decrease extracellular adenosine triphosphate levels and increase extracellular adenosine levels. Consequently it is possible that PC-1-mediated insulin resistance could be caused either by a decrease in adenosine triphosphate or an indirect increase in adenosine levels. We have tested this hypothesis and find that the PC-1-mediated inhibition of insulin-stimulated insulin receptor autophosphorylation is not altered by agents that alter the level or action of adenosine. Further, a mutated PC-1 with a single amino acid change that abolishes the phosphodiesterase and pyrophosphatase activities is still able to inhibit insulin-stimulated insulin receptor phosphorylation. The results of these experiments indicate that the phosphodiesterase activity of PC-1 is not involved in the inhibition of insulin receptor autophosphorylation.
Insulin controls glucose homeostasis by regulating the production of glucose by the liver and the uptake of glucose into muscle and fat. If these actions of insulin are inadequate, glucose levels rise and diabetes mellitus develops. The most common form of diabetes mellitus is non-insulin-dependent diabetes mellitus (type II diabetes). While the cause(s) of non-insulin-dependent diabetes mellitus is unknown, this form of diabetes is accompanied by insulin resistance, an inability of insulin to appropriately trigger cellular responses. Except in rare individuals, this insulin resistance is not the result of alterations of the number or structure of the insulin receptors; rather the defect lies distal to the insulin receptor in the insulin signaling pathway (for reviews see (1, 2, 3, 4) ).
We have demonstrated that in muscle and fibroblasts of many patients with insulin resistance there is increased expression of PC-1(5) . When the mammary epithelial cell line, MCF-7, is transfected with a PC-1 cDNA expression vector, expression of PC-1 is increased and the cells become insulin-resistant(5) . PC-1 is a type II (extracellular C terminus) transmembrane glycoprotein(6) . Although initially identified in plasma cells, it is now known to be expressed in a wide variety of cell types(7) . PC-1 is also known as nucleotide pyrophosphatase/alkaline phosphodiesterase I, which has pyrophosphatase as well as phosphodiesterase activity(8) . These activities of PC-1 suggested two hypotheses as to how PC-1 could decrease insulin receptor phosphorylation. One possibility is that the pyrophosphatase activity leads to a loss of ATP either at the cell membrane or in an intracellular trafficking pool, and this reduction in ATP could impact negatively on insulin signaling. Another possibility is that extracellular ATP hydrolysis could lead indirectly to an increase in extracellular adenosine, which could act through one of the adenosine receptors to reduce the response of PC-1 expressing cells to insulin. Both of these hypotheses rely on the pyrophosphatase activity of PC-1.
We have tested these hypotheses in two ways. First we have used pharmacological agents to either alter the level of extracellular adenosine or the signaling through the adenosine receptor. None of these manipulations altered the insulin resistance in either control MCF7 cells or in MCF7 cell lines that express PC-1. In addition we have introduced an amino acid change into PC-1 that abolishes the pyrophosphatase and phosphodiesterase activity. When an expression plasmid encoding this variant is introduced into MCF7 cells, the protein is expressed without these two enzymatic activities. This variant is, however, as active as the wild type PC-1 protein in its ability to reduce the insulin-stimulated autophosphorylation of the insulin receptor kinase. Taken together these data suggest that the phosphodiesterase activity of PC-1 is not involved in the inhibition of insulin receptor phosphorylation.
Figure 1: Insulin dose response in the presence of adenosine, adenosine deaminase, or 8-PT. Control MCF7 cells expressing neo only (neo) or MCF7 cells expressing PC-1 (PC22) were treated for 1 h with either adenosine (10 µM), 8-PT (2 µM), or adenosine deaminase (7.5 units/ml) for 1 h and stimulated with insulin at the indicated concentrations for 15 min, and the phosphorylation of the insulin receptor was assayed. Results are presented as a percentage of the insulin receptor phosphorylation (IR-P) detected in the control cells at 100 nM insulin.
Figure 3:
Expression and effects of PC-1 and PC-1
(T256S). A, cell lines (MCF-7) were derived from
co-transfections with plasmids pRKneo and either pRK-PC-1 or pRK-PC-1
(T256S). Colonies were isolated in the presence of G418, and cell lines
derived from individual colonies were analyzed for expression of PC-1
(or variants) by a combination of Western blotting and pyrophosphatase
assay. Two control cell lines expressing neo (neo5 and neo8),
three cell lines expressing wild type PC-1 (PC6, PC8, and PC22), and
three cell lines expressing the pyrophosphatase-deficient PC-1 (PD-F1,
PD-A2, and PD-A7) were analyzed. A, Western analysis of PC-1
proteins using an anti-peptide antiserum as described under
``Materials and Methods.'' B,
[-
P]ATP pyrophosphatase assays on the same
cell lines described in A. C, insulin-stimulated
insulin receptor phosphorylation (IR-P) in the cell lines
described in A and B. Results are expressed as a
percentage of the IR phosphorylation detected in the neo5 cell line at
100 nM insulin.
An increase in the expression of PC-1 in MCF7 cells causes these cells to have a diminished response to insulin as measured by the ability of insulin to stimulate phosphorylation of the insulin receptor (5) . To facilitate an analysis of the mechanism by which the PC-1-mediated inhibition occurs we have developed a 96-well plate-based assay that specifically measures insulin-mediated insulin receptor phosphorylation (Fig. 1). As described under ``Materials and Methods'' the assay is dependent on an insulin-mediated signal and is determined by the use of two well characterized monoclonal antibodies. The insulin-dependent response seen in this assay parallels the response determined by Western blotting analysis (data not shown).
It has been reported that adenosine can alter insulin sensitivity (see below), and it is known that ATP is required for phosphorylation of the insulin receptor. Thus we hypothesized that the pyrophosphatase activity associated with PC-1 could cause insulin resistance by increasing adenosine or by decreasing ATP. This hypothesis was tested initially using adenosine receptor agonists and antagonists. Insulin at 100 nM induced a comparable phosphorylation of the insulin receptor whether or not adenosine was present. This lack of effect of adenosine was observed when the adenosine was present for 19 h at either 100 or 10 mM ( Fig. 2and data not shown) or for 1 h at 100 or 10 mM ( Fig. 2and data not shown). Extracellular adenosine is converted to inosine by exogenous adenosine deaminase. If PC-1 were leading to an increased level of extracellular adenosine and this was causing the insulin resistance, then PC-1-transfected cells would have an increase in their response to insulin in the presence of adenosine deaminase. As can be seen in Fig. 2the presence of adenosine deaminase in the culture medium for the 19 h prior to the insulin response assay did not significantly alter the sensitivity of either wild type or the PC-1-transfected cells. We also tested whether the adenosine receptor antagonist 8-PT was able to alter the response of the cells to insulin. As shown in Fig. 2, the PC-1-induced insulin resistance was not altered by the presence of 8-PT when present for 19 h at 3 µM (Fig. 2) or 1 µM (data not shown).
Figure 2: The actions of adenosine receptor agonists and antagonists on PC-1-mediated inhibition of insulin-stimulated insulin receptor phosphorylation. MCF7 cell lines expressing either neo only (graycolumns) or neo and PC-1 (solidcolumns) were treated with the indicated drugs. In their presence the ability of insulin to stimulate insulin receptor phosphorylation was measured. As described under ``Materials and Methods'' the drugs were present for either 19 or 1 h prior to insulin addition. The uppernumber refers to the concentration of drug present for the -19 to -1 h prior to the insulin stimulation, and the number at the bottom refers to the concentration present in the 1-h period prior to the insulin stimulation. The results are presented as a percentage of the insulin-stimulated insulin receptor phosphorylation (IR-P) detected in the control (neo) cells in the absence of drug and at an insulin dose of 100 nM.
These experiments were all carried out at 0, 1, and 100 nM insulin (data not shown for 0 and 1 nM insulin). To determine whether an adenosine-related effect could be detected at other insulin concentrations, the experiments were repeated using adenosine, adenosine deaminase, or 8-PT at doses that are known to be effective and a range of insulin concentrations. As can be seen in Fig. 1there was no significant alteration in the insulin-stimulated insulin receptor phosphorylation at insulin doses ranging from 0.5 to 100 nM. This lack of effect was consistently noted (n = 3) in both the wild type MCF7 cells and in the PC-1-transfected cells.
These pharmacological interventions suggested that the insulin resistance was not mediated through an increase in the production of adenosine. To test this possibility in a more direct fashion and also to test the possibility that the PC-1 effect was due to a reduction in ATP levels, we introduced a single amino acid change into the PC-1 protein that allowed protein expression but abolished the pyrophosphatase and phosphodiesterase activity. The relevant amino acids of PC-1 were identified based on homology with bovine intestinal 5`-nucleotide phosphodiesterase. It has been demonstrated that a threonine at the active site is required for catalysis mediated by this bovine phosphodiesterase(15) . The threonine is thought to contribute to catalysis by forming an intermediate bond with the phosphate. A conserved series of amino acids including a threonine (at amino acid 256) is present in PC-1. Accordingly this threonine was changed to a serine. The cDNAs encoding either the wild type PC-1 (Thr-256) or the variant (Ser-256) were cloned into an expression vector under the control of the cytomegalovirus promoter, and the plasmids co-transfected with a plasmid that confers G418 resistance into MCF7 cells. Clones were selected in the presence of G418, expanded, and tested for PC-1 expression, first by Western analysis and then for phosphodiesterase activity. Three clones that expressed the wild type PC-1 protein and three clones that expressed the Ser-256 variant were identified by Western blot analysis using anti-peptide antiserum (see ``Materials and Methods''). As seen in Fig. 3there was a small amount of PC-1 in the control cell lines (neo5 and neo8) as detected by Western blot analysis. The three cell lines expressing wild type PC-1 (PC6, PC8, and PC22) and the three cell lines expressing the mutant PC-1 (PD-F1, PD-A2, PD-A7) all had comparable immunoreactive PC-1 of the appropriate size. All of these cell lines were then assayed for pyrophosphatase activity using radioactive ATP as a substrate (see ``Materials and Methods''). The two control cell lines expressing the neomycin-selective marker had activities of 7.5 and 7.8 pmol of ATP hydrolyzed per min per mg of protein; the cell lines expressing wild type PC-1 had activities of 21.8, 29.4, and 31.2 pmol of ATP hydrolyzed per min per mg of protein. While the cell lines expressing the mutated PC-1 demonstrated a significant expression of PC-1 protein by Western blot analysis, the pyrophosphatase activity of these mutant PC-1 cell lines was indistinguishable from the neo-only cell lines, 6.4, 7.8, and 9.2 pmol of ATP hydrolyzed per min per mg of protein.
These three groups of cell lines (expressing either neo, wild type, PC-1, or the phosphodiesterase-deficient version of PC-1) were then assayed for the ability of insulin to stimulate phosphorylation of the insulin receptor. As demonstrated ( Fig. 1and 2), the expression of wild type PC-1 led to a clear reduction in the ability of insulin to stimulate the phosphorylation of the insulin receptor (Fig. 3). When the cell lines expressing the pyrophosphatase-deficient version of PC-1 were assayed for insulin-stimulated IR phosphorylation, we found that these cell lines were as resistant to insulin-stimulated insulin receptor phosphorylation as were the cell lines that expressed the wild type PC-1.
To evaluate whether the results obtained with the cell lines expressing the phosphodiesterase-deficient PC-1 were dependent on the insulin concentration the experiment was repeated using a range of insulin concentrations. The PC-1-mediated inhibition of insulin-stimulated insulin receptor phosphorylation could be detected at insulin doses ranging from 0.5 to 100 nM insulin. A comparable dose-dependent inhibition was also observed for all of the cell lines expressing the pyrophosphatase-deficient PC-1 (Fig. 4).
Figure 4: Dose-dependent IR phosphorylation in PC-1 and PC-1(T256S) cell lines. The phosphorylation of the IR at increasing insulin concentrations was determined in a control cell line (neo5), a cell line expressing wild type PC-1 (PC8), and three cell lines expressing the pyrophosphatase-deficient PC-1 (PD-F1, PD-A2, and PD-A7). The results are expressed as a percentage of the IR phosphorylation (IR-P) seen in the neo5 cell line at 100 nM insulin.
We had hypothesized that PC-1 could reduce the
response to insulin by either reducing ATP or by increasing adenosine.
Intracellular ATP is required for the autophosphorylation of the
insulin receptor, and it might be expected that PC-1 could locally
reduce ATP levels sufficiently to reduce IR phosphorylation. This
possibility is unlikely since PC-1 is a type II transmembrane protein
with the phosphodiesterase and pyrophosphatase activity outside the
cell(6) . It has been shown, however, that increased expression
of PC-1 can lead to an increase in intracellular pyrophosphate levels
possibly within vesicles associated with the movement of PC-1 to the
plasma membrane (16) . A related hypothesis was that a
PC-1-mediated increase in extracellular adenosine could mediate the
effects on insulin-stimulated IR phosphorylation. Adenosine interacts
with and signals through the P-purinergic receptors, which
are further categorized as either A
, A
,
A
, or A
adenosine receptors on the basis of
selective binding of pharmacological agents. A further distinction
between the various adenosine receptors is made on the basis of the
intracellular signaling pathways used. The A
receptor
appears to couple exclusively to and activates adenylate cyclase. In
contrast A
and A
activation decreases adenylate
cyclase activity, and in addition the A
receptor affects
potassium and calcium channels and can couple to a variety of
intracellular messenger systems including G
proteins(17, 18, 19) . That adenosine can
influence insulin-mediated signals is well established although whether
adenosine acts to increase or decrease insulin sensitivity appears to
be very much dependent on the cells of the organ system used and which
insulin-mediated event is measured. Adenosine increases insulin
sensitivity in isolated fat cells as measured by insulin-mediated
glucose phosphorylation and insulin-mediated
anti-lipolysis(20, 21, 22) . In contrast, the
effects of adenosine on insulin-stimulated glucose uptake in muscle are
less clear. Challiss and colleagues (23) have shown that
adenosine receptor antagonists (adenosine deaminase, 8-PT) will
increase the sensitivity of isolated muscle strips to insulin. In
contrast, Vergauwen et al.(24) reported that
adenosine receptor antagonists (caffeine,
8-cyclopentyl-1,3-dipropylxanthine) will decrease the effectiveness of
insulin in vivo. In this latter study the adenosine had no
effect on insulin action when the muscle was at rest but did decrease
insulin-stimulated glucose uptake when the muscles were electrically
stimulated.
At least for MCF7 cells, there does not appear to be any effect of adenosine on the response to insulin, as measured by the autophosphorylation of the insulin receptor. In addition, data from both our pharmacological intervention studies and the analysis of the cell lines expressing the pyrophosphatase-negative PC-1 are concordant in demonstrating that the PC-1-mediated inhibition of insulin-stimulated insulin receptor autophosphorylation is not mediated either by the hydrolysis of ATP or the generation of adenosine.
The
mechanism by which PC-1 inhibits insulin receptor autophosphorylation
is not known. It has been suggested that PC-1 has both intrinsic kinase
and phosphatase activities (25, 26) and that PC-1
interacts directly with the fibroblast growth factor
receptor(25) . As it is likely that any kinase or phosphatase
activity associated with PC-1 would be extracellular, it is unclear how
these kinase/phosphatase activities would impact on insulin signaling.
Furthermore, it has been demonstrated that PC-1 may not be a true
kinase; the radiolabeling by [-
P]ATP may be
due to the presence of a covalent ATP-PC-1 intermediate formed during
the cleavage of the pyrophosphate bond in the ATP(27) . The
PC-1 protein also includes amino acid sequence motifs that are
characteristic of tyrosine kinase phosphorylation sites, somatomedin B
domain signature motifs, and divalent cation binding
domains(28) . (
)We are currently determining whether
these regions may be involved in the inhibition of insulin-stimulated
phosphorylation of the insulin receptor.