PROTEIN SECRETION BY HEPATOCYTES

Danny TaheriLinda Amo-MensahVishal Kinkhabwala

Andrea Whitley

THE QUESTION:

• Beginning with protein synthesis in membrane-bound ribosomes, hepatocytes secrete proteins into the circulation via which of the following mechanisms?

• A: Active transport through the cell membrane• B: Diffusion through the cell membrane• C: Transport by microtubules and exocytosis• D: Transport in vesicles and exocytosis• E: Transport through pores in the cell membrane

PROTEIN SYNTHESIS AND SECRETION

• Central Dogma of Biology: DNA mRNA PROTEIN

• DNA is transcribed into mRNA in the nucleus

• mRNA is then TRANSLATED into a protein by the ribosomes in the cytoplasm

TRANSLATION• The mRNA is translated

into protein in the ribosome through the action of adding Amino Acids to make a polypeptide

• These ribosomes can either be Free Polysomes in the cytoplasm, or Membrane-Bound Ribosomes, attached to the Rough Endoplasmic Reticulum

Post-Translational Modification

• The polypeptide is then released into the cytoplasm, ready for post-translational modifications

• The proteins then enter the Golgi Apparatus, where they are packaged into vesicles, which then move to the plasma membrane, where they are released into circulation

Free vs. Membrane-Bound Ribosomes

• FREE:• Proteins formed from

free ribosomes, are released into the cytosol and used within the cells

• They are moved about anywhere in the cytosol but are excluded from the cell nucleus.

Free vs. Membrane-Bound Ribosomes

• MEMBRANOUS• They are attached to

the cell and synthesize proteins in the membranous cell organelles

• The organelles they bound to are rough endoplasmic reticulum.

• The newly produced polypeptide chains are inserted direct into the rough endoplasmic reticulum by the ribosome undertaking vectorial synthesis and are then transported to their various destinations through the secretory pathways.

• Bound ribosomes usually produce proteins that are used within the plasma membrane or are expelled from the cell via exocytosis

HEPATOCYTES

• Primary cell type in the liver

• One major function is synthesis and secretion of Plasma Proteins

SECRETED PROTEINS

• 1. Albumin: Maintains osmotic pressure

• 2. Alpha-Fetoprotein: High levels detected in congenital anomalies (i.e. spina bifida)

• 3. C-Reactive Protein: Major role in inflammatory response

• 4. Transferrin: Transfers iron from duodenum to body tissues

SECRETED PROTEINS

• 5. Plasminogen:• Thrombolitic – breaks

down clots• Active form of Plasmin

• 6. Clotting Factors• Most important:• CF 1: Fibrinogen• CF 2: Prothrombin

BACK TO THE QUESTION

• Beginning with protein synthesis in membrane-bound ribosomes, hepatocytes secrete proteins into the circulation via which of the following mechanisms?

• A: Active transport through the cell membrane• B: Diffusion through the cell membrane• C: Transport by microtubules and exocytosis• D: Transport in vesicles and exocytosis• E: Transport through pores in the cell membrane

A: Active Transport through the Cell Membrane

• WHY It Could Be:• Proteins would have to cross through a cell membrane to exit

the cell (Wang and Boyer, 2004)• ATP (is a component of active transport) is often a component

of protein secretion (Chen and Tai, 1985)

• WHY It Isn’t: • Movement through the Golgi Apparatus would involve

vesicles, and active transport does not use vesicles • No gradients or pumps No Active Transport used (Wang and

Boyer, 2004)

B: Diffusion through the Cell Membrane

• WHY It Could Be:• Proteins would need to cross cell membrane to exit cell

• WHY It Isn’t:• Passive Diffusion is not possible due to the polar nature of

proteins and their inability to freely cross the phospholipid bilayer (Wang and Boyer, 2004)

C: Transport by Microtubules and Exocytosis

• WHY It could be:• Exocytosis (through vesicles) is a MAJOR component of

protein secretion

• WHY It isn’t:• NO use of microtubules seen in this process (Wang and Boyer,

2004)

E: Transport through Pores in the Cell Membrane

• WHY It could be:• Release of certain substances from a cell does move through

pores

• WHY It isn’t:• Protein synthesis and secretion does not usually go through

pores but rather bud off through membrane vesicles (Wang and Boyer, 2004)

D: Transport in Vesicles and Exocytosis

• WHY It Is:• Proteins are packaged into vesicles and then released from

the membrane via exocytosis (Wang and Boyer, 2004)• Consistent with examination of the overall process!

References• Boron, W. F., & Boulpaep, E. L. (2009). Uptake, Processing, &

Secretion of Compounds by Hepatocytes. In Medical physiology: A cellular and molecular approach. Philadelphia, PA: Saunders/Elsevier.

• Chen, L., & Tai, P. (1985). ATP is essential for protein translocation into Escherichia coli membrane vesicles. Proceedings of the National Academy of Science, 82, 4384-4388.

• University of Washington (2012). Cell Secretion. Retrieved from http://courses.washington.edu/conj/cell/secretion.htm

• Wang, L., & Boyer, J. (2004). The Maintenance and Generation of Membrane Polarity in Hepatocytes. Science Frontier, 39(4), 892-899.