Post-transcriptional mechanisms are responsible for the reduction in lipoprotein lipase activity in cardiomyocytes from diabetic rat hearts

Abstract

Lipoprotein lipase (LPL) activity is reduced in cardiomyocytes from rat hearts following the acute (4-5 day) induction of diabetes with 100 mg/kg streptozotocin. The molecular basis for this inhibitory effect of diabetes on LPL activity was investigated by measuring steady-state LPL mRNA content and the synthesis and turnover of LPL protein ([35S]methionine incorporation into immunoprecipitable LPL protein in pulse and pulse-chase experiments) in control and diabetic cardiomyocytes. LPL activity was reduced to approx. 50% of control in diabetic cardiomyocytes, but LPL mRNA levels and turnover (degradation) of newly synthesized LPL were unchanged. Synthesis of total protein and LPL were reduced to 72% and 71% of control respectively; therefore, relative rates of LPL synthesis were the same in control and diabetic cardiomyocytes. The diabetes-induced reduction in LPL synthesis was accompanied by a decrease in LPL mass to 78% of control, and a decrease in enzyme specific activity (0.48 to 0.33 m-unit/ng of LPL protein) since the decline in catalytic activity was greater than the decrease in LPL synthesis and mass. Thus, post-transcriptional mechanisms involving a reduction in LPL synthesis as part of a generalized decrease in total protein synthesis, together with a post-translational mechanism(s) that result in accumulation of inactive LPL protein, are responsible for the decreased LPL activity in cardiomyocytes from diabetic rat hearts.

Streptozotocin-Induced Diabetes Enhances Cardiac Heparin-Releasable Lipoprotein Lipase Activity in Spontaneously Hypertensive Rats

“…Previously, a decline in cellular LPL catalytic activity was suggested to result from post-transcriptional/translational mechanisms leading to an accumulation of inactive LPL protein in diabetic cardiomyocytes. 29 In perfused guinea pig hearts, LPL can move from parenchymal cells to the endothelial surface within 30 minutes. 39 In mouse hearts, this movement involves "jumping" of LPL between cell surface-associated heparan sulfate and other polyanions.…”

“…29 The assay for cell sonicate LPL activity was done essentially as described above except that 20 L of the cell sonicate was used and heparin (2 U/mL) was included in the assay.…”

Peroxisome proliferator-activated receptor-α ligands inhibit cardiac lipoprotein lipase activity

“…Assay of LPL activity. LPL activities were determined by measuring hydrolysis of a sonicated [ 3 H]triolein substrate emulsion (1,10). The assay contained 0.1 mM glycerol-[9,10-3 H]trioleate (6 mCi/mmol), 25 mM PIPES (pH 7.5), 0.05% (wt/vol) essentially FA-free BSA, 50 mM MgCl2, and 3% heat-inactivated chicken serum as the LPL activator.…”

“…LPL synthesis. The synthesis of LPL in cultured cardiomyocytes was determined by measuring the incorporation of [ 35 S]methionine into immunoprecipitable LPL protein, essentially as described previously by Carroll et al (10). After 3 h incubation in 10-cm dishes, cardiomyocytes were additionally incubated at 37°C for 6 h with [ 35 S]methionine (0.2 mCi/ml), or with 0.1 mCi/ml overnight, in DMEM buffer without methionine and cysteine but containing 1 mM glutamine and 0.2% (wt/vol) FA-free BSA.…”

Chylomicron and palmitate metabolism by perfused hearts from diabetic mice

“…3 H]-triolein substrate emulsion (7,29 The ex vivo working mouse heart preparation provides an experimental system with precise control of physiological factors (afterload, preload) and the supply of exogenous substrates in the perfusate (3,21). Isolated working mouse hearts were perfused with recirculating KHB buffer (total volume of 40 ml) containing 3% BSA, 0.4 mM CM-[…”