Macrophage receptor with collagenous structure (MARCO) is a dynamic adhesive molecule that enhances uptake of carbon nanotubes by CHO-K1 cells

Our plain-English summary of Macrophage receptor with collagenous structure (MARCO) is a dynamic adhesive molecule that enhances uptake of carbon nanotubes by CHO-K1 cells, by S. Hirano, Y. Fujitani, A. Furuyama, et al (Toxicology and Applied Pharmacology, 2012).

The study

• Authors: S. Hirano, Y. Fujitani, A. Furuyama, S. Kanno
• Journal: Toxicology and Applied Pharmacology
• Published: 2012;259(1):96-103
• Cited by: 29 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The toxicity of carbon nanotubes (CNTs), a highly promising nanomaterial, is similar to that of asbestos because both types of particles have a fibrous shape and are biopersistent. Here, we investigated the characteristics of macrophage receptor with collagenous structure (MARCO), a membrane receptor expressed on macrophages that recognizes environmental or unopsonized particles, and we assessed whether and how MARCO was involved in cellular uptake of multi-walled CNTs (MWCNTs). MARCO-transfected Chinese hamster ovary (CHO-K1) cells took up polystyrene beads irrespective of the particle size (20nm-1μm). In the culture of MARCO-transfected CHO-K1 cells dendritic structures were observed on the bottom of culture dishes, and the edges of these dendritic structures were continually renewed as the cell body migrated along the dendritic structures. MWCNTs were first tethered to the dendritic structures and then taken up by the cell body. MWCNTs appeared to be taken up via membrane ruffling like macropinocytosis, rather than phagocytosis. The cytotoxic EC(50) value of MWCNTs in MARCO-transfected CHO-K1 cells was calculated to be 6.1μg/mL and transmission electron microscopic observation indicated that the toxicity of MWCNTs may be due to the incomplete inclusion of MWCNTs by the membrane structure.

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In plain English

This was a laboratory study of cultured Chinese hamster ovary (CHO-K1) cells that the authors engineered to produce mouse MARCO, a receptor normally found on macrophages, which are scavenger cells. The cells were exposed to fluorescent polystyrene beads (20 nm, 200 nm and 1 μm) and to multi-walled carbon nanotubes (MWCNTs, average diameter 67 nm). In a toxicity test they were also exposed to UICC crocidolite asbestos. Cells switched on to make MARCO took up the beads rapidly, although some uptake also happened without MARCO. MARCO was found in branching membrane extensions that contained no F-actin. MARCO-expressing cells gathered cell-associated MWCNTs, whether inside the cells or bound to their surface, more rapidly than unstimulated cells. The nanotubes were caught on the extensions and then drawn into the cell body, in a way that looked more like macropinocytosis than typical phagocytosis. The lethality EC50 was 6.1 μg/mL for MWCNTs and 19.6 μg/mL for crocidolite. Electron microscopy showed membranes only partly enclosing the long nanotubes. The authors concluded that MARCO accelerated uptake of these particles and that loss of membrane integrity was probably responsible for MWCNT toxicity.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) cell culture experiment
• Who was studied: Chinese hamster ovary (CHO-K1 and T-REx-CHO) cells engineered to express murine MARCO (CHO-GFP-MARCO, TREX30-MARCO and TREX31-MARCO cells), compared with mock-transfected, untransfected or tetracycline-unstimulated cells
• Exposure: Multi-walled carbon nanotubes (average diameter 67 nm, lengths 1 to 20 μm, mostly at 10 μg/mL); fluorescent polystyrene beads of 20 nm, 200 nm and 1 μm; UICC crocidolite used for comparison in the cytotoxicity test
• Main results:
  • TREX30-MARCO and TREX31-MARCO cells stimulated with tetracycline to express MARCO took up polystyrene beads of 20 nm to 1 μm rapidly. Some beads were also taken up without tetracycline and by untransfected CHO-K1 cells.
  • In CHO-GFP-MARCO cells, MARCO was present in dendritic (branching) structures on the dish bottom, and F-actin was not present in these structures.
  • Over 1 to 4 hours, tetracycline-stimulated TREX30-MARCO cells gathered cell-associated MWCNTs more rapidly than unstimulated TREX30-MARCO cells. The measurement could not separate nanotubes inside the cells from those bound to the cell surface. Unstimulated cells also gathered MWCNTs, but much more slowly.
  • In CHO-GFP-MARCO cells, MWCNTs were first trapped on the dendritic structures and then taken up by the cell body, accumulating around the nucleus.
  • The authors judged that MWCNT uptake looked more like macropinocytosis (uptake through membrane ruffling) than typical zipper-mode phagocytosis.
  • In CHO-GFP-MARCO cells exposed for 24 hours, the lethality EC50 value was 6.1 μg/mL for MWCNTs and 19.6 μg/mL for crocidolite.
  • Transmission electron microscopy of CHO-GFP-MARCO cells exposed to MWCNTs for 6 hours showed that the membrane surrounding the nanotubes was not continuous, because the nanotubes were too long to be fully enveloped.
• The authors' conclusion: The authors concluded that MARCO adhered to polystyrene microspheres and MWCNTs and accelerated cellular uptake of these particles. They found that MWCNTs were internalized via membrane ruffling. They also concluded that loss of membrane integrity in MWCNT-associated organelles was probably responsible for MWCNT toxicity.
• Limitations the authors state:
  • The turbidity method used to measure cell-associated MWCNTs could not distinguish nanotubes inside the cells from those bound to the cell surface.

Topics

Medical terms in this study, explained in our Lexicon: Macrophage, Phagocytosis.

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