Intro to Pharm and Tox Topics   

Diacylglycerol and Protein Kinase C

Diacylglycerol (DAG) is a second messenger produced by phospholipase C (PLC) hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2):

Phosphatidic acid, a product of phospholipase D (PLD) can also be converted to DAG by phosphatidic acid phosphorylase.

There are two isoforms of PLD in the rat brain (?): sodium oleate-dependent PLD and ADP-dependent PLD (also known as ribosylation factor). There is growing evidence that PLD is activated by either PKC or small G-proteins:

G-protein PLC DAG PKC PLD

DAG binds to and activates most protein kinase C (PKC) isozymes. It is metabolized by DAG lipase to monoacylglycerol, or by DAG kinase into phosphatidic acid.

PKC are membrane-bound proteins that phosphorylate other proteins at Ser/Thr residues. PKC isozymes are reversibly activated by DAG and (irreversibly?) by exogenous administration of phorbol esters.

There are at least eleven PKC isotypes (?) that vary in activation profile and can be classified into four subgroups: classic, novel, atypical and mue. The classical PKC are activated by DAG, Ca2+ and phorbol esters. The novel PKC and mue PKC are activated by DAG and phorbol esters but not Ca2+. The atypical PKC are not activated by either DAG, phorbol esters or Ca2+.

The regulatory and catalytic domains of PKC are on the same polypeptide chain. The regulatory domain is responsible for tonic inhibition of the catalytic activity (a pseudosubstrate). Inhibition of the catalytic site by the pseudosubstrate is relieved by DAG. There is evidence that some phospholipids also activate PKC isozymes. Phosphorylation of PKCalpha and PKCbeta at threonine residues also seems to increase activity (but not by autophosphorylation).

PKC can be regulated by phorbol esters, cell-permeable analogs of DAG and Ca2+, inhibitor drugs, pseudosubstrate inhibitors, and phorbol-induced down-regulation (?).

Continue to "PLA2 and Arachidonic Acid" or take a quiz: [Q1] [Q2] [Q3] .

Need more practice? Answer the review questions below (after sponsor).

1- What is the activity of diacylglycerol?

2- What is the activity of DAG kinase?

3- What is the activity of PKC and were are they located?

4- List 4 subgroups of PKC

5- How are classical PKC activated?

6- How are novel and mue PKC activated?

7- How are atypical PKC activated?

8- What is the mechanism of PKC activation/inhibition?

9- What are other possible ways of activating PKC?

10- List 5 regulators of PKC.

11- What is the activity of phosphatidic acid phosphohydrolase?

12- List 2 PLD isoenzymes

13- How is PLD activated?

Continue scrolling to answers below (after sponsor).

Hey! DON'T PEEK!!! Finish the questions fist!

Answers:

1- What is the activity of diacylglycerol?
Activates most protein kinase C isozymes.

2- What is the activity of DAG kinase?
Metabolizes diacylglycerol into phosphatidic acid.

3- What is the activity of PKC and were are they located?
They are membrane bound and phosphorylate other proteins at Ser/Thr residues.

4- List 4 subgroups of PKC
classic
novel
atypical
mue

5- How are classical PKC activated?
By DAG, Ca2+ or phorbol esters

6- How are novel and mue PKC activated?
By DAG and phorbol esters.

7- How are atypical PKC activated?
By neither DAG, Ca2+ nor phorbol esters (???)

8- What is the mechanism of PKC activation/inhibition?
The regulatory and catalytic domains are on the same polypeptide and the regulatory domain exhibits tonic inhibition of the catalytic activity, as a pseudosubstrate. The inhibition is relieved by DAG.

9- What are other possible ways of activating PKC?
Some phospholipids activate PKC isozymes. Phosphorylation of PKCalpha and PKCbeta seems to increase activity (not autophosphorylation).

10- List 5 regulators of PKC.
phorbol esters
cell-permeable DAG and Ca2+ analogs
inhibitor drugs
pseudosubstrate inhibitors
phorbol-induced down-regulation (??)

11- What is the activity of phosphatidic acid phosphohydrolase?
Hydrolyzes phosphatidic acid to yield diacylglycerol.

12- List 2 PLD isoenzymes
sodium oleate-dependent PLD
ADP-dependent PLD (also known as ribosylation factor)

13- How is PLD activated?
There is growing evidence that PLD is activated by either PKC or small G-proteins:
G-protein PLC DAG PKC PLD

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