Effects of growth hormone on lipoprotein lipase and hepatic lipase

Lipoprotein lipase (LPL) is a key enzyme in the regulation of the flux of fatty acids. LPL hydrolyses triglycerides in chylomicrons and very-low-density lipoproteins (VLDL), forming intermediate- (IDL) and low-density lipoproteins (LDL). Hepatic lipase (HL) is a related enzyme with a more restricted tissue distribution than LPL; HL is mainly engaged in the turnover of IDL and of high-density lipoproteins (HDL). Both enzymes can be released from their endothelial sites by heparin and their activities measured separately in post-heparin plasma (PHP). The PHP-LPL activity decreases in hypophysectomized rats and this effect is reversed by growth hormone (GH) therapy. However, GH seems to have no effect, or an inhibitory effect, on PHP-LPL activity in humans. Muscle and adipose tissues are the main sources of PHP-LPL activity. One week of GH therapy of hypophysectomized rats increases skeletal muscle and heart LPL activity. In this model, GH has little or no effect on LPL activity in adipose tissue. However, GH has been shown to decrease LPL activity in isolated rat adipose tissue. Insulin-like growth factor-I therapy decreases and insulin therapy increases LPL activity in adipose tissue of hypophysectomized rats, whereas these therapies have no effect on LPL activity in muscle tissue. The LPL activity in human adipose tissue is reduced both in vivo and in vitro after administration of GH while the LPL mRNA level is unchanged. The effect of GH on HL activity has been studied in PHP and liver. Several studies in the rat indicate that GH increases PHP-HL and liver HL activity, at least partly at the level of mRNA expression. In humans, GH has been shown to have variable effects on PHP-HL activity; this variability is probably to some extent dependent on different experimental set-ups. Although GH therapy increases hepatic secretion of VLDL, serum triglyceride levels decrease as a result of GH therapy in the hypophysectomized rat. An increase in HL and LPL activity by GH therapy is in line with these findings. In summary, GH is involved in the regulation of both LPL and HL activity but the effects and mechanisms of action of GH in the regulation of LPL and HL activity in different tissues are not yet fully elucidated.