Reduction of Oncoprotein Transformation In Vitro by Albumin

Abstract

For src oncoproteins to effect transformation, they must be co-translationally modified with a myristoyl group in a reaction catalyzed by the enzyme Nmyristoyltransferase (NMT). NMT catalyzes a biochemical protein modification process called iV-myristoylation. Many of the myristoylated cellular proteins are known to be involved in signal transduction and oncogenesis (7). The importance of myristoylation of proteins in tumorigenesis was first suggested by studies demonstrating that myristoylation of the viral oncogene pp6O' is required for membrane association and cell transformation (23). Activation of ppoXr^ has been reported in human colon carcinoma (4), colonic tumor-derived cell lines (4,5), and colonic polyps with a high potential for developing into cancer (<5). It has been reported that changes in the amino-terminal sequence of pp60 lead to decreased membrane association and decreased in vivo tumorigenicity (2). We have demonstrated for the first time that the activity of NMT in animal and human colon cancer tissues is markedly higher than that in corresponding normal tissue (7). This finding suggests that this enzyme may be involved in colorectal cancer development. These lines of evidence support the hypothesis that myristoylation of proteins may be involved in the pathogenesis of cancer. Consequently, NMT is proposed as a novel molecular target for anticancer drug design (8). As indicated above, inhibitors of NMT could possibly form the basis of a new approach to cancer treatment. In an investigation to find inhibitors of NMT, we discovered, for the first time, that human serum albumin is a potent inhibitor of A'-myristoylation of the oncoprotein-derived peptide substrate in vitro (Fig. 1) by human NMT cloned from the HepG2 hepatocarcinoma cell line expressed in Escherichia coli (9). Background information (70) indicates that mutant albumin-deficient rats have hyperlipidemia and are susceptible to the induction by various carcinogens of tumors of the bladder, kidney, stomach, intestine, and subcutaneous tissues (77). Albumin-deficient rats (i.e., analbuminemic rats) are mutants resulting from a defect in splicing of the albumin messenger RNA precursor resulting from the deletion of seven bases in the intron that is called HI, which is responsible for the absence of albumin production in these animals (10). Hyperlipidemia associated with the lack of albumin may change the membrane lipid composition, altering the permeability to certain carcinogens (11). We have also reported earlier that NMT activity is low in liver cytosol (72). It is tempting to suggest that low levels of NMT activity could be related to increased levels of albumin in the liver (75). Albumin content has been reported to be low in the elderly (14); furthermore, increased expression of pp60 in gastric mucosa of aged rats has also been reported (75). Taken together, these results suggest that albumin may play a key role in reducing oncoprotein transformation and the development of cancer. RAJALA V. S. RAJU