Molecular BIology of Cancer Topics             

APC

Familial Adenomatous Polyposis (FAP) accounts for about 1% of colonorectal cancer cases, and is characterized by frequent bening adenomatous polyps (hundresds per patioent), some of which progress to carcinomas. Compared with the normal colon mucosa, . the mucosal surface of the colon in familial polyposis is essentially a carpet of small adenomatous polyps. There is a 100% risk over time for development of adenocarcinoma, so a total colectomy is done, generally before age 20.

Nearly all human colon cancers, whether FAP or sporadic, have an APC mutation. APC mutation occurs early in human colon carcinogenesis. APC functions as a "gatekeeper" in colon epithelial cells, maintaining a constant cell number in the renewing cell population. Rb (in retinoblast cells) is another example of a gatekeeper protein. Inactivation of the gatekeeper is required for net cell division. A mutation in the gatekeeper leads to a permanent imbalance of cell division over cell death. Mutations in other genes in the presence of the Gatekeeper do not lead to a permanent imbalance of cell division over cell death (?).

There are two populations of colonic crypt epithelial cells: stem cells and functional cells. Stem cells proliferate, remain undifferentiated, and have a low APC protein concentration. As the stem cells differentiate and migrate ‘up’ the crypt they become functional cells. Functional cells do not proliferate, are terminally differentiated and have a high APC protein concentration. Eventually, functional cells will undergo apoptosis and be sloughed off into the lumen.

In normal colon cells, GSK-3b and APC target b-catenin for degradation. b-catenin sequesters APC, but once released by GSK-3b phosphorylation, APC stimulates migration and differentiation. In both normal embryonic cells and colon cancer or melanoma cells b-catenin is not degraded and accumulates, binding to Tcf-Lef and triggering gene expression, leading to cell proliferation at the same time that migration is inhibited. In the absence of APC, Tcf-Lef and beta-catenin are constitutively active.

APC mutants cannot bind beta-catenin and fail to block TCF/b-catenin transcriptional activity. Colon cancer cell lines with wild-type APC have a constitutively active mutant beta-catenin.

The multiple intestinal neoplasia (Min) mouse has a germline mutation in APC. Tumor number varies among strains of Min mice
A second locus was identified which affects Min tumor yield: MOM (modifier of Min), which codes for phospholipase A2. Phospholipase A2 (PLA2) cleaves arachidonic acid from membrane phospholipids. Arachidonic acid is a precursor to prostaglandins. Phospholipase A2 (PLA2) releases arachadonic acid from a membrane phosphlipid. Cyclooxygenase (COX) converts arachadonic acid to prostaglandin PGG2, a precursor of other prostaglandins and thromboxanes. Aspirin and other Nonsteroidal Anti-Inflammatory Drugs (NSAIDs) inhibit COX activity and thus has analgesic and antiinflammatory effects. Since mutant PLA2 increases the number of tumors, could aspirin be used as cancer chemoprevention or therapy? Prostaglandings affect cell growth by activating the PPAR nuclear receptors, which are transcription factors. The COX inhibitor piroxicam was succesfully used as chemoprevention in mouse Min/APC tumors.

COX-2 is expressed in intestinal cancers but not in normal intestinal tissue. While COX-1 is expressed in tumor and in normal tissue, COX-2 is only expressed in tumors (polyps). COX-2 expression is also induced in many human cancers compared to normal tissue: colon, pancreas, breast, lung and prostate. COX-2 knockout results in fewer tumors in MinAPC-/- mice. A COX-2 specific inhibitor like MF tricyclic has antitumor activity in ApcMin mice. Men who used aspirin 2 or more times per week were 32% less likely to develop colon cancer and 50% less likely to develop advanced (metastatic) colon cancer. Non-aspirin NSAIDs seem to be more effective than aspirin. Long-term sulindac use significantly decreased adenoma incidence.

People at risk of colon cancer can be identified by detecting APC defects, either inherited APC mutation (in FAP) (check any tissue, ?) or sporadic (check colon tissue, ?). Options for those with high colon cancer risk (i.e. with mutant APC) include removal of colon before malignant carcinoma develops, frequent colonoscopies to detect cancer early (and then remove colon), and chemoprevention with NSAIDs or COX-2 selective inhibitor and frequent colonoscopies.

Continue to "Therapy" or take a quiz: [Q1].

Need more practice? Answer the review questions below.

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1- What are the main general characteristics of FAP?
Accounts for about 1% of colonorectal cases, occurs with very frequent bening adenomatous polyps (hundreds per patient) some of which preogress into carcinomas, and the main gene involved is APC.

2- Describe the main pathology and prognosis of FAP..
Compared to a normal colon mucosa, tha of an FPA patient is mostly covered by small adenomatous polyps. There is a 100% risk for development of adenocarcinoma, so a total colectomy is done usually before age 20.

3- Which gene is mostly associated with FAP and sporadic adenomatous colon cancer?
APC mutation occurs early in human colon cancer.

4- What is the main function of APC?
APC functions as a "gatekeeper" protein in colon epithelial cells, maintaining a constant cell number in the renewing cell population.

5- What is a gatekeeper protein?
Inactivation of a gatekeeper protein is required for net cell division. Mutation in the gatekeeper leads to permanent imbalance of cell division over cell death. In the presence of a wild-type gatekeeper, mutations in other genes do not lead to permanent imbalance.

6- List 2 gatekeeper proteins.
APC
Rb

7- List two populations of colonic crypt epithelial cells.
stem cells
functional cells

8- Describe colonic crypt stem cells.
They proliferate, remain undifferentiated and migrate up the crypt to become differentiated functional cells.

9- Describe colonic crypt functional cells.
They do not proliferate, are terminally differentiated and have a high APC protein concentration. They eventually undergo apoptosis and are sloughed off into the lumen.

10- Describe the mechanism of action of APC protein in normal colon cells.
GSK-3b and APC target b-catenin for degradation. b-catenin squesters APC, but once released by GSK-3b phosphorylation, APC stimulated migration and differentiation while b-catenin is degraded.

11- Describe the mechanism of action of APC protein in normal embryonic cells.
GSK-3b is inactivated by extracellular signals thus APC stays sequestered by b-catenin, inhibiting differentiation and migration. Free b-catenin is not degraded and accumulates, and can bind Tcf-Lef and thus serve as transcription factor, triggering gene expression.

12- Describe the mechanism of action of APC in cancer cells.
Mutant APC cannot bind b-catenin and fail to recruit GSK-3b to tag b-catenin for degradation, thus APC is inactive (no differentiation nor migration) and b-catenin can activate transcription by binding Tcf-Lef.

13- What is the cancer-causing mutation in colon cancer cells with wild-type APC?
Colon cancer cells with wild-type APC have a constitively active b-catenin mutant. Mutant b-catenin that is constitutively active would have the same effect as mutant APC.

14- What is modifier of Min (MOM)?
A second locus identified in mice with an APC germline mutation that affects the number of tumors, it codes for phospholipase A2 (PLA2).

15- What is the function of phospholipase A2?
Cleaves arachidonic acid from membrane phospholipids, a precursor of prostaglandins.

16- What is the function of cyclooxygenase (COX)?
Converts arachidonic acid to prostaglandin PGG2, a precursor of other prostaglandins and tromboxanes.

17- Explain the rationale for using aspirin and other non-steroidal anti-inflamatory drugs as preventive chemotherapy.
Prostaglandins affect cell growth by activating PPAR nuclear receptors. NSAIDs inhibit COX activity, thus the production of prostaglandings. Since mutant PLA2 increases the number of tumors, NSAID therapy should decrease the number of tumors in patients with APC mutation. The COX inhibitor piroxicam was successfully used as chemoprevention in mice with APC tumors. Men who used aspirin 2 or more times per week were 32% less likely to develop colon cancer and 50% less likely to develop metastatic colon cancer.

18- What are the differences in expression of COX isoenzymes in tumors versus normal tissue?
While COX-1 is expressed in both tumors and normal tissue, COX-2 is expressed in intestinal tumors but not in normal tissue. COX-2 expression is also induced in other human cancers like pancreas, breast, lung and prostate.

19- What is the importance of COX-2 in colon cancer chemoprevention?
COX-2 knockout in APC -/- mice results in fewer tumors. A COX-2 specific inhibitor like MF tricyclic has antitumor actvity in APC -/- mice.

20- How is the risk of colon cancer determined for a patient?
People at risk of colon cancer can be identified by detecting APC deffects, either inherited (testing any tissue) or sporadic (testing colon tissue).

21- What are the options for patients at high risk of colon cancer?
Removal of the colon before malignant carcinoma develops, frequent colonoscopies to detect cancer early, and chemoprevention with NSAIDs or COX-2 selective inhibitors.