Role of insulin resistance in decreasing lipoprotein lipase activity in tumor-bearing rats

The role of insulin resistance in the tumor-induced decrease in tissue lipoprotein lipase (LPL) activity was studied in vivo and vitro in methylcholanthrene-induced sarcoma-bearing rats. Intraperitoneal (ip) injection of 2 U of regular insulin resulted in high-adipose LPL activity in control rats (CTR) of 122.0±42.4 U/mg tissue, but it had little effect on tumor-bearing rats (TBR), which showed a value of only 9.6±5.5 U/mg tissue (P=0.002). When adjusted for serum insulin concentrations, adipose LPL activity remained significantly different between the TBR and CTR at 0.19±0.17 and 0.78±0.29 U/mg tissue, respectively (P=0.02). Following the in vitro incubation with either 1.44 g/l glucose of 1×10^–8 U insulin of adipose tissue fragments obtained from the TBR and the CTR, measurable LPL activity was maintained in the tissue from the CTR for 2h but not in that from the TBR. These results suggest that the decreased LPL activities seen in the tumor-bearing state may be mediated by insulin resistance.