Combined light and electron microscopy using diaminobenzidine photooxidation to monitor trafficking of lipids derived from lipoprotein particles

Curr Pharm Biotechnol. 2012 Feb;13(2):331-40. doi: 10.2174/138920112799095338.

Abstract

Diaminobenzidine (DAB) photooxidation is a method for conversion of fluorescent signals into electron-dense precipitates that are visible in the electron microscope. Recently, we have applied this method to analyze organelles involved in holo-high density lipoprotein (HDL) particle uptake at the ultrastructural level. In the present work we extended the spectrum of molecules visualized via photooxidation to monitor the uptake of HDL-derived lipids in HepG2 cells. By the combined light-electron microscopic method and with the aid of the DAB photooxidation technique, it became possible for the first time to visualize different intracellular pathways of lipoprotein particle-derived lipids and analyze the compartments involved at the ultrastructural level. HDL-Alexa 568 was used to visualize holo-HDL particle uptake. Reconstituted HDL particles containing the fluorescent cholesterol analogues Bodipy-cholesterol, Bodipy-cholesteryl oleate, or cholesteryl Bodipy-ester were used to visualize uptake of the HDL-associated sterol. In Bodipy-cholesteryl oleate and cholesteryl Bodipy-ester, the cholesterol moiety or the fatty acid moiety is fluorescently labeled, respectively; in contrast, Bodipy-cholesterol is an analogue of free cholesterol. The cellular compartments involved in their intracellular routes after uptake were analyzed in the fluorescence and electron microscope after DAB photooxidation. Bodipy-cholesterol was found to be localized in tubular endosomes and multivesicular bodies (MVBs), in the trans-Golgi network, and in stacked Golgi cisternae. In contrast, HepG2 cells incubated with HDL containing Bodipy-cholesteryl oleate or cholesteryl Bodipyester gave an uptake pattern comparable to that of holo-HDL particles, with MVBs being involved. Bodipy-cholesteryl oleate was also found in lysosomes. These results indicate that HDL-derived cholesterol and cholesteryl ester are transported by different intracellular pathways in HepG2 cells. Thus, the DAB photooxidation method enables the analysis of intracellular transport of lipoprotein particle-derived lipids at the light and at the ultrastructural level.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 3,3'-Diaminobenzidine / chemistry*
  • Biological Transport / physiology
  • Boron Compounds / chemistry
  • Cholesterol / metabolism
  • Cholesterol Esters / metabolism
  • Endosomes / metabolism
  • Fluorescence
  • Hep G2 Cells
  • Humans
  • Light
  • Lipid Metabolism / physiology*
  • Lipoproteins, HDL / metabolism*
  • Lipoproteins, HDL / ultrastructure
  • Lysosomes / metabolism
  • Microscopy, Electron / methods*
  • Microscopy, Fluorescence / methods
  • Multivesicular Bodies / metabolism
  • Oxidation-Reduction
  • Photochemical Processes
  • Tumor Cells, Cultured
  • trans-Golgi Network / metabolism

Substances

  • 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
  • Boron Compounds
  • Cholesterol Esters
  • Lipoproteins, HDL
  • 3,3'-Diaminobenzidine
  • Cholesterol