Abstract
Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is found predominantly in adipose tissue and muscle. We examined the mechanism of regulation of LPL in muscles composed of different fiber types (soleus, extensor digitorum longus, and heart) in fed, fasted, and hypothyroid rats. In all muscles, the detergent-extractable (EXT) fraction represented approximately 95% of total LPL activity and mass. LPL activity was similar in the heparin-releasable (HR) fractions of heart and soleus (predominantly type I fibers), while in the EXT fraction LPL activity in soleus was 418 +/- 48 nEq/min per g, and in heart was 272 +/- 30 nEq/min per g (P < 0.05). However, LPL activity in extensor digitorum longus (EDL, predominantly type II fibers) was considerably lower (7.9 +/- 0.8 nEq/min per g in EXT, P < 0.0001 versus heart and soleus). LPL immunoreactive mass followed a pattern similar to LPL activity. LPL mRNA levels were quantitated by both Northern blotting and reverse transcriptase-polymerase chain reaction (RT-PCR), and were approximately equal in heart and soleus, and 5-fold lower in EDL. In response to feeding, LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, although feeding had no effect on soleus or EDL. In hypothyroid animals, muscle LPL activity was increased by 3- to 4-fold in the HR (but not EXT) fractions of heart and soleus (P < 0.05), with no change in LPL mass or mRNA. Thus, muscles with oxidative, type I fibers expressed higher levels of LPL mRNA than muscles containing glycolytic, type II fibers.(ABSTRACT TRUNCATED AT 250 WORDS)
Highlights
Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is found predominantly in adipose tissue and muscle
LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, feeding had no effect on soleus or extensor digitorum longus (EDL)
To determine whether there was a similar difference in LPL protein between EDL, soleus and heart, LPL immunoreactive mass was measured by enzyme-linked immunosorbent assay (ELISA) (Fig. 1, C and D)
Summary
Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is found predominantly in adipose tissue and muscle. LPL activity in extensor digitorum longus (EDL, predominantly type I1 fibers) was considerably lower (7.9 + 0.8 nEq/min per g in EXT, P < 0.0001 versus heart and soleus). LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, feeding had no effect on soleus or EDL. Muscle LPL activity was increased by 3 - to 4-fold in the H R (but not EXT) fractions of heart and soleus (P < 0.05), with no change in LPL mass or mRNA. Soleus and heart from hypothyroid rats demonstrated increased H R LPL, with no change in LPL mass or mRNA, suggesting posttranslational regulation by thyroid hormone.-Ong, J.
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