Pathogenesis of colorectal carcinoma and therapeutic implications: the roles of the ubiquitin-proteasome system and Cox-2

Abstract

Pathways of the molecular pathogenesis of colorectal carcinoma have been extensively studied and molecular lesions during the development of the disease have been revealed. High up in the list of colorectal cancer lesions are APC (adenomatous polyposis coli), K-ras, Smad4 (or DPC4-deleted in pancreatic cancer 4) and p53 genes. All these molecules are part of important pathways for the regulation of cell proliferation and apoptosis and as a result perturbation of these processes lead to carcinogenesis. The ubiquitin-proteasome system (UPS) is comprised of a multi-unit cellular protease system that regulates several dozens of cell proteins after their ligation with the protein ubiquitin. Given that among these proteins are regulators of the cell cycle, apoptosis, angiogenesis, adhesion and cell signalling, this system plays a significant role in cell fate and carcinogenesis. UPS inhibition has been found to be a pre-requisite for apoptosis and is already clinically exploited with the proteasome inhibitor bortezomib in multiple myeloma. Cyclooxygenase-2 (Cox-2) is the inducible form of the enzyme that metabolizes the lipid arachidonic acid to prostaglandin H2, the first step of prostaglandins production. This enzyme is up-regulated in colorectal cancer and in several other cancers. Inhibition of Cox-2 by aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) has been found to inhibit proliferation of colorectal cancer cells and in epidemiologic studies has been shown to reduce colon polyp formation in genetically predisposed populations and in the general population. NSAIDs have also Cox-independent anti-proliferative effects. Targeted therapies, the result of increasingly understanding carcinogenesis in the molecular level, have entered the field of anti-neoplastic treatment and are used by themselves and in combination with chemotherapy drugs. Combinations of targeted drugs have started also to be investigated. This article reviews the molecular pathogenesis of colorectal cancer, the roles of UPS and Cox-2 in it and puts forward a rational for their combined inhibition in colorectal cancer treatment.

1

Sequence of molecular events leading to colorectal cancer. In A, the sequence taking place in hereditary syndrome familial adenomatous polyposis and the majority of sporadic cases is depicted. In B, events happening in hereditary non-polyposis colorectal cancer syndrome and most of the remaining sporadic cases are pictured. Epigenetic events such as CpG islands methylation hap-pen in both pathogenetic pathways but appear to be more frequent in B.

2

The canonical Wnt/β-catenin signalling pathway. A. When Wnt1 is active, kinase GSK3β is inactivated and β-catenin is free to enter the nucleus and begin transcription in co-operation with TCF4. B. When Frizzled is not ligated by Wnt1, GSK3β remains active and in co-operation with APC, axin, conductin and Casein kinase II phosphorylates β-catenin, which is then targeted for proteasome degradation.TCF4 remains associated with the inhibitor Groucho and transcription is not initiated.

3

β-catenin regulation and functions. Kinase GSK3β mediated phosphorylation leads to proteasome-dependent β-catenin degradation, while β-calpain-mediated degradation is proteasome-independent. GSK3β is inhibited by Wnt signalling as well as by kinases ILK and PKC. β-catenin functions as a transcription co-factor for TCF4 and FoxO, a component of adherens junction interacting with E-cadherin and a direct inhibitor of NF-βB.

4

K-ras initiated signalling. Four major K-ras initiated signalling pathways and down-stream events are pictured.

5

TGFβ signalling and interactions with K-ras.

6

p53 regulation and function. E3 ligases mdm2, ARF-BP/Mule, COP1 and Pirh2 keep p53 under control by ubiquitinating it for proteasome degradation. p14 inhibits both mdm2 and ARF-BP/Mule. ATM and Chk1/2 kinases activate p53. Depending on co-acti-vators recruited p53 activation leads to either apoptosis or cell cycle arrest.

7

Cox-2 regulation.Transcription factors and pathways regulating Cox-2 transcription.TIA-1 translational silencing of Cox-2 mRNA is also depicted.

8

Prostaglandins production and action.

9

Arachidonic acid and main enzymes of its production and metabolism.Arachidonic acid increase stimulates production of pro-apoptotic lipid ceramide while arachidonic depletion by FACL4, Cox-2 and LOX enzymes negates this effect.