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Increased N-myristoyltransferase activity observed in rat and human colonic tumors
B A Magnuson1, R V Raju, T N Moyana
1Department of Pathology and Saskatoon Cancer Centre, College of Medicine, University of Saskatchewan, Saskatoon, Canada.
Background:
Colorectal cancer is one of the leading causes of cancer death in North America. Since treatment of colonic cancer remains difficult because of the lack of effective chemotherapeutic agents, it is important to continue to search for cellular functions that can be disrupted by chemotherapeutic drugs and inhibit the development or progression of this disease. Modification of proteins by myristoylation has been recognized as important in the function of various viral, oncogenic, and signal-transduction proteins and thus has been proposed as a target for chemotherapeutic drug design. However, the activity of the enzyme that catalyzes this modification, N-myristoyltransferase, has not been investigated in cancer relative to normal tissue.
Purpose:
The purpose of this study was twofold: 1) to investigate the activity of N-myristoyltransferase in azoxymethane-induced rat colonic cancer tissue compared with normal and normal-appearing rat colonic tissue and 2) to determine if similar differences would be observed in a small sample of human colonic tumors.
Methods:
N-Myristoyltransferase activity was determined in 45 colonic tissue specimens from Sprague-Dawley rats--10 given injections of the colon carcinogen, azoxymethane, and three untreated. Tissue specimens included 35 colonic tumors of varying pathologic stages, seven specimens of normal-appearing adjacent mucosa, and three specimens of normal colonic mucosa. Colectomy specimens from five patients were assayed for N-myristoyltransferase activity. Subcellular distribution of N-myristoyltransferase activity was determined. Synthetic peptides of known myristoylated proteins--pp60src and cyclic adenosine monophosphate-dependent protein kinase--were used in kinetic analyses of N-myristoyltransferase in colonic cancer and normal-appearing colonic tissue. All P values are two-tailed.
Results:
N-Myristoyltransferase activity was increased in rat colonic tumors compared with normal-appearing adjacent mucosa and normal mucosa (P = .0002). Elevation of N-myristoyltransferase activity was present in all tumors, including colonic polyps. Increased N-myristoyltransferase activity was also observed in human colonic tumors and was predominantly cytosolic. N-Myristoyltransferase of colonic cancer tissues had a similar Michaelis constant but an approximate twofold higher maximum velocity for both the pp60src- and cyclic adenosine monophosphate-dependent protein kinase-derived peptides compared with N-myristoyltransferase of normal-appearing tissue.
Conclusions:
This study demonstrates for the first time that N-myristoyltransferase activity is higher in colonic epithelial neoplasms than in normal-appearing colonic tissue and that an increase in N-myristoyltransferase activity appears at an early stage in colonic carcinogenesis.
Insights
N-myristoyltransferase (NMT) activity is elevated in both rat and human colorectal tumors compared to normal tissue. This finding suggests NMT may be an early indicator and potential therapeutic target for colorectal cancer.
Area of Science:
- Biochemistry
- Oncology
- Molecular Biology
Background:
- Colorectal cancer (CRC) is a leading cause of cancer mortality with limited effective chemotherapeutic options.
- Targeting cellular functions disrupted by drugs is crucial for CRC treatment.
- Protein myristoylation, catalyzed by N-myristoyltransferase (NMT), is vital for oncogenic and signal-transduction proteins, making NMT a potential therapeutic target.
Purpose of the Study:
- To investigate N-myristoyltransferase (NMT) activity in azoxymethane-induced rat colon cancer versus normal tissue.
- To assess if similar NMT activity differences exist in human colon tumors.
Main Methods:
- Assessed N-myristoyltransferase (NMT) activity in 45 rat colonic tissue specimens (tumors, normal-appearing mucosa, normal mucosa).
- Analyzed NMT activity in five human colon cancer specimens.
- Determined subcellular distribution and kinetic properties of NMT using specific peptide substrates.
Main Results:
- N-myristoyltransferase (NMT) activity was significantly higher in rat colon tumors than in normal or normal-appearing mucosa (P = .0002).
- Elevated NMT activity was observed in all rat tumors, including polyps, and in human colon tumors.
- Human colon tumor NMT was predominantly cytosolic, with higher maximum velocity compared to normal tissue.
Conclusions:
- N-myristoyltransferase (NMT) activity is demonstrably higher in colonic epithelial neoplasms than in normal colonic tissue.
- Increased NMT activity appears to be an early event in colorectal carcinogenesis, indicating its potential as a biomarker or therapeutic target.