Mutant p53 enhances MET trafficking and signalling to drive cell scattering and invasion

Tumour-derived mutant p53 proteins promote invasion, in part, by enhancing Rab coupling protein (RCP)-dependent receptor recycling. Here we identified MET as an RCP-binding protein and showed that mutant p53 promoted MET recycling. Mutant p53-expressing cells were more sensitive to hepatocyte growth factor, the ligand for MET, leading to enhanced MET signalling, invasion and cell scattering that was dependent on both MET and RCP. In cells expressing the p53 family member TAp63, inhibition of TAp63 also lead to cell scattering and MET-dependent invasion. However, in cells that express very low levels of TAp63, the ability of mutant p53 to promote MET-dependent cell scattering was independent of TAp63. Taken together, our data show that mutant p53 can enhance MET signalling to promote cell scattering and invasion through both TAp63-dependent and -independent mechanisms. MET has a predominant role in metastatic progression and the identification of mechanisms through which mutations in p53 can drive MET signalling may help to identify and direct therapy.

Effect of acylglyceride content on the structure and function of reconstituted high density lipoprotein particles

The effects of different acylglycerides on the conformation and charge of apolipoprotein A-I (apoA-I) have been investigated in reconstituted high density lipoproteins (LpA-I). Various amounts of diacylglycerol (DG) and triacylglycerol (TG) were incorporated into sonicated spherical LpA-I particles containing 2 molecules of apoA-I and 80 molecules of phospholipid. Inclusion of 30 molecules of TG into the LpA-I particle increases the net negative charge of apoA-I (-8.5 to -9.3 mV), but has little effect on the amount and thermodynamic stability of the alpha helices in apoA-I. Incorporation of 30 molecules of DG into the lipoprotein complex promotes a small increase in the alpha-helix content and stability, but greatly increases the net negative charge of apoA-I (-8.5 to -11.2 mV). Inclusion of DG increases the immunoreactivity of two epitopes in the N terminus of apoA-I, but decreases the exposure of a domain closer to the C terminus (residues 148;-186) of the apoprotein. In contrast, TG increases the exposure of epitopes over the entire apoA-I molecule; TG increases the immunoreactivity of epitopes for 13 different monoclonal antibodies to apoA-I. Incubations with purified lecithin:cholesterol acyltransferase show that cholesterol esterification is stimulated by DG, but inhibited by TG. The data show that TG and DG have different effects on apoA-I structure and function and this suggests that the TG-to-DG ratio in HDL may directly affect the metabolism of this lipoprotein class. - Braschi, S., C. R. Coffill, T. A-M. Neville, D. M. Hutt, and D. L. Sparks. Effect of acylglyceride content on the structure and function of reconstituted high density lipoprotein particles. J. Lipid Res. 2001. 42: 79;-87.

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.

Diacylglycerol is the preferred substrate in high density lipoproteins for human hepatic lipase

The hydrolysis of HDL phospholipids (PL) and glycerides by hepatic lipase (HL) has been investigated in native and reconstituted HDL particles (Lp2A-I). Fasting, normolipidemic HDL exhibit total lipid hydrolytic rates of between 10 and 36 nM FA/h per microM PL. Of the total fatty acids liberated with HDL3 only 1% are from triolein (TG), while 49% are from diolein (DG) and 50% are from PL. A spherical reconstituted particle containing 2 molecules of apoA-I, 120 molecules of PL, and 20 molecules of TG exhibits a total lipid hydrolytic rate of 18 nM FA/h per microM PL and 93% of the fatty acids liberated are from PL. Inclusion of 40 molecules of TG into the Lp2A-I particle doubles the rate of fatty acid hydrolysis by HL through a stimulation of TG hydrolysis. Further addition of 10 molecules of DG to the Lp2A-I complex has no effect on the overall rates of hydrolysis, but changes the substrate specificity, wherein 61% of the fatty acids are from DG and both TG and PL hydrolytic rates are significantly reduced. Increasing the amount of DG in the Lp2A-I particle further stimulates total lipid hydrolysis by raising DG hydrolytic rates at the expense of PL and TG hydrolysis. A particle containing 10 molecules of TG and 40 molecules DG yields the fastest lipid hydrolytic rate of 143 nM FA/h per microM PL, which constitutes 96% DG hydrolysis, 3% TG hydrolysis, and 1% PC hydrolysis. These data indicate that hepatic lipase acts primarily as a surface lipid lipase with HDL particles. DG is the preferred substrate of HL in HDL and the HDL-DG content regulates the hydrolysis of both PL and TG by HL.