Pathophysiology

Pathophysiology

I-13. Secondary hyperlipoproteinemia syndromes

続発性高リポタンパク血症に関連する症候群

Secondary Hyperlipoproteinemia — Overview

Increased plasma lipoproteins as a consequence of another disease. Most common causes:

  • Diabetes mellitus
  • Excessive alcohol consumption
  • Liver disease
  • Hypothyroidism
  • Lupus

~1/3 of patients have a multiplex (multiple) secondary cause. Obesity is also a cause but considered too obvious to list.

Diabetic Dyslipidemia

  • Altered production/elimination of lipoproteins: ↑TAG, ↓HDL, postprandial lipemia.
  • Type 1 DM: ↑TAG, ↓HDL — resolves with insulin replacement.
  • Type 2 DM: ↑TAG, ↓HDL, plus small-dense LDL (sdLDL) — highly atherogenic; glycemic control alone does not fully normalize lipids; common in prediabetes too; needs diet + weight control (the core problem is insulin resistance, not just glucose).

Pathomechanism (ApoB degradation/stabilization)

  1. ApoB synthesis is uncontrolled (in everyone); normally degraded afterward.
  2. ApoB bound to fatty acids escapes degradation; insulin normally promotes ApoB degradation — absent in diabetes.
  3. Insulin resistance → ↑hormone-sensitive lipase → ↑lipolysis → ↑FFA to liver → ApoB stabilized → ↑hepatic ApoB → ↑VLDL synthesis.
  4. ↓Lipoprotein lipase activity → more TAG/VLDL in circulation.
  5. VLDL transfers TAG to LDL/HDL; LDL/HDL transfer cholesteryl-ester to VLDL (CETP).
  6. LDL → sdLDL (atherogenic → explains ↑CV risk); HDL → ApoA-I excreted by kidney → ↑atherosclerosis risk.

Alcohol Dyslipidemia

  • The “sharpest double-edged sword” — U-shaped curve:
    • Low intake: ↓CHD/AMI/PVD/stroke (↓LDL, ↑HDL).
    • High intake: ↑CV risk + fatty liver (steatosis) from ↑TAG synthesis → alcoholic fatty liver disease.
  • Recovers shortly after abstinence (longer if chronic alcoholic).

Dyslipidemia in Kidney Disease

  • Albumin loss → ↓plasma oncotic pressure → liver compensates with ↑protein/lipoprotein production (↑ApoB → ↑VLDL).
  • ↓LPL & hepatic lipase activity → ↓lipoprotein degradation; ↑LDL-receptor degradation.
  • Net: ↑total cholesterol and ↑LDL cholesterol.

Dyslipidemia in Hypothyroidism

  • TSH affects lipid metabolism (↑LDL production, ↓LDL catabolism).
  • Abnormalities: ↑total cholesterol, ↑LDL, ↑TAG.
  • Treatment: L-thyroxine normalizes the lipid profile.

一問一答

▶What is secondary hyperlipoproteinemia, and what are its most common causes?

Increased plasma lipoproteins as a consequence of another disease; most common causes are diabetes mellitus, excessive alcohol consumption, liver disease, hypothyroidism, and lupus.

▶What are the characteristic lipid abnormalities of diabetic dyslipidemia?

Increased triglycerides, decreased HDL, and postprandial lipemia.

▶How does dyslipidemia differ between Type 1 and Type 2 diabetes?

T1DM: ↑TAG, ↓HDL that resolves with insulin replacement. T2DM: ↑TAG, ↓HDL plus highly atherogenic small-dense LDL; glycemic control alone does not fully normalize lipids.

▶How does insulin normally regulate ApoB, and what happens in diabetes?

Insulin normally promotes ApoB degradation; in diabetes this is absent, so ApoB is stabilized, increasing hepatic ApoB and VLDL synthesis.

▶How does insulin resistance increase free fatty acid delivery to the liver?

Insulin resistance increases hormone-sensitive lipase → increased lipolysis → more FFA to the liver → ApoB stabilization → increased VLDL synthesis.

▶How does CETP activity generate small-dense LDL in diabetes?

VLDL transfers TAG to LDL/HDL while LDL/HDL transfer cholesteryl ester to VLDL (via CETP); the TAG-enriched LDL is then remodeled into atherogenic small-dense LDL.

▶Why does HDL fall in diabetic dyslipidemia, increasing atherosclerosis risk?

HDL is remodeled so that ApoA-I is excreted by the kidney, lowering HDL and raising atherosclerosis risk.

▶Describe the U-shaped curve of alcohol and cardiovascular risk.

Low intake lowers CHD/AMI/PVD/stroke risk (↓LDL, ↑HDL), whereas high intake raises CV risk and causes fatty liver (steatosis) from increased TAG synthesis.

▶Is alcohol-related dyslipidemia reversible?

Yes — it recovers shortly after abstinence, though recovery takes longer in chronic alcoholics.

▶What is the mechanism of dyslipidemia in kidney disease (nephrotic)?

Albumin loss lowers plasma oncotic pressure, so the liver compensates with increased protein/lipoprotein production (↑ApoB → ↑VLDL); LPL and hepatic lipase activity fall and LDL-receptor degradation increases.

▶What is the net lipid effect of kidney disease?

Increased total cholesterol and increased LDL cholesterol.

▶What lipid abnormalities occur in hypothyroidism and how are they treated?

Increased total cholesterol, LDL, and triglycerides (TSH increases LDL production and decreases LDL catabolism); L-thyroxine normalizes the lipid profile.

▶What fraction of patients with secondary hyperlipoproteinemia have a multiplex (multiple) cause?

About one third.

▶Why is it important to distinguish secondary from primary dyslipidemia before treating?

Because the underlying causative disease must be identified and treated; secondary dyslipidemia is a consequence of another disorder.

▶Why does glycemic control alone fail to normalize lipids in Type 2 diabetes?

Because the core problem is insulin resistance rather than glucose alone, so diet and weight control are also required.

▶Why does reduced lipoprotein lipase activity raise circulating triglycerides in diabetes?

Decreased LPL activity reduces TAG clearance, leaving more triglyceride-rich VLDL in the circulation.

▶Why is ApoB stabilization central to diabetic VLDL overproduction?

ApoB synthesis is constant but normally followed by degradation; when bound to fatty acids (and without insulin-driven degradation) ApoB escapes breakdown, so more is available to assemble VLDL.

▶Besides the listed causes, what obvious condition also causes secondary dyslipidemia?

Obesity — considered too obvious to list explicitly among the main causes.

▶Why is small-dense LDL in T2DM clinically important?

It is highly atherogenic and helps explain the increased cardiovascular risk that persists despite glucose control.

▶When is alcohol intake cardioprotective versus harmful, in one phrase?

Low intake is protective (↓LDL, ↑HDL); high intake is harmful (↑CV risk and hepatic steatosis).