Cell Structure Chapter 15 Study Questions

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What is the difference between paracrine and endocrine signaling?

In paracrine signaling, molecules released by one or a few cells affect nearby cells. In endocrine signaling, hormones are carried throughout the body to act on any target cell that has a receptor for that hormone.

How does signaling by hydrophobic molecules like steroid hormones differ from signaling by peptide hormones?

Hydrophobic molecules like steroid hormones can diffuse through the plasma membrane and bind to cytosolic or nuclear receptors. Hydrophilic molecules (like peptide hormones) cannot cross the plasma membrane, so they act by binding to receptors on the cell surface.

How does aspirin reduce inflammation and blood clotting?

Aspirin inhibits the enzyme cyclooxygenase, which atalyzes the first step in the syntehsis of prostaglandin and thromboxanes from arachidonic acid.

Hormones that activate a receptor coupled to G s stimulate the proliferation of thyroid cells. How would inhibitors of cAMP phosphodiesterase affect the proliferation of these cells?

Inhibition of cAMP phosphodiesterase would result in elevated levels of cAMP, which would stimulate cell proliferation.

The epinephrine receptor is coupled to G s, whereas the acetylcholine receptor (on heart muscle cells) is coupled to G i. Suppose you were to construct a recombinant molecule containing the extracellular sequences of the epinephrine receptor joined to the cytosolic sequences of the acetylcholine receptor. What effect would epinephrine have on cAMP level in cells expressing such a recombinant receptor?

The recombinant molecule would function as an epinephrine receptor coupled to Gi. Ephinepherine would therefore inhibit adenylyl cycllase, lowering intracellular cAMP levels.

How would acetylcholine affect these cells?

Acetylcholine would have no effect, since it would not bind to the recombinant receptor.

Platelet-derived growth factor (PDGF) is a dimer of two polypeptide chains. How would PDGF monomers affect signaling by the PDGF receptors?

PDGF monomers would not induce receptor dimerization. Since this is the first critical step in signaling from receptor protein-tyrosine kinases, they would be unable to stimulate the PDGF receptor.

You have generated a truncated version of the EGF receptor that lacks the tyrosine kinase domain. Expression of this truncated receptor inhibits the response of cells to EGF. Why?

The truncated receptor would act as a dominant negative mutant because it would dimerize with the normal receptor. The dimers would be inactive because they would be unable to cross-phosphorylate.

How would overexpression of protein phosphatese 1 affect the inducation of cAMP-inducatible genes in response to hormone stimulation of target cells?

Protein phosphatase 1 dephosphorylates serine residues that are phosphorylated by protien kinase A. Cyclic AMP inducible genes are activated by CREB, which is phosphorylated by protein kinase A, so overexpression of protein phosphatase 1 would inhibit their induction.

Would protein phosphatase 1 affect the function of cAMP-gated ion channels?

Protein phosphated 1 would not affect the activity of cAMP-gated ligand channels, since these channels are opened directly by cAMP binding rather than by protein phosphorylation.

How does the PI 3-kinase/Akt pathway regulate cellular protein synthesis in response to growth factor stimulation?

Akt phosphorylates the protein kinase GSK-3 which regulates the translation factor eIF2B. In addition, Akt regulates translation by phosphorylating S6 kinase and the EIF4E binding protine 4E-BP1.

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