Prenylated protein methyltransferase of rat cerebellum is developmentally co-expressed with its substrates

Stringent structural requirements for anti-Ras activity of S-prenyl analogues

The carboxy terminal S-farnesylcysteine of Ras oncoproteins is required for their membrane anchorage and transforming activities. We showed previously that S-farnesylthiosalicylic acid (FTS) affects the membrane anchorage of activated H-Ras in EJ cells and inhibits their growth. We report here on structural elements in S-prenyl derivatives that specifically inhibit the growth of EJ cells, but not of untransformed Rat-1 cells. Inhibition of the Ras-dependent extracellular signal-regulated protein kinase (ERK), of DNA synthesis and of EJ cell growth were apparent after treatment with FTS or its 5-fluoro, 5-chloro and 4-fluoro derivatives or with the C20 S-geranylgeranyl derivative of thiosalicylic acid. The 4-Cl-FTS analogue was a weak inhibitor of EJ cell growth. The 3-Cl-FTS analogue and the FTS carboxyl methyl ester were inactive, as were the C10 S-geranyl derivative of thiosalicylic acid, farnesoic acid, N-acetyl-S-farnesyl-L-cysteine and S-farne-sylthiopropionic acid. The structural requirements for anti-Ras activity of S-prenyl analogues thus appear to be rather stringent. With regard to chain length, the C15 farnesyl group linked to a rigid backbone seems to be necessary and sufficient. A free carboxyl group in an appropriately rigid orientation, as in thiosalicylic acid, is also required. Halogenic substitutents on the benzene ring of the thiosalicylic acid are tolerated only at position 5 or 4. This information may facilitate the design of potent Ras antagonists and deepen our understanding of the mode of association of Ras with the plasma membrane.

Pancreatic exocrine secretion is blocked by inhibitors of methylation

A number of early experiments suggested a relationship between methyl group metabolism and the exocrine secretion of the pancreas. These included nutritional studies showing that ethionine, the ethyl analog of methionine which inhibits cellular methylation reactions, is a specific pancreatic toxin. Other studies indicated that protein carboxymethylation might be involved. We now show that in vivo ethionine inhibits amylase secretion from freshly isolated rat pancreatic acini, while in vitro ethionine inhibits amylase secretion from the AR42J pancreatic cell line. S-Adenosylhomocysteine (SAH) is a product inhibitor of all methyltransferase reactions involving S-adenosylmethionine (SAM), and treatments that elevate cellular levels of SAH such as inhibition of S-adenosylhomocysteine hydrolase and the in vitro addition of adenosine and homocysteine result in the inhibition of amylase secretion in both isolated pancreatic acini and AR42J cells. Measurement of SAM and SAH levels in AR42J cells shows that inhibition of secretion is more closely related to elevation of SAH levels than to a decrease in the SAM/SAH ratio. Small G-proteins are carboxymethylated on the C-terminal prenylated cysteine and inhibitors of membrane-associated prenylcysteine methyltransferase, N-acetylfarnesylcysteine, N-acetylgeranylgeranylcysteine, and farnesylthioacetic acid (FTA), block secretion in AR42J cells. N-Acetylgeranylcysteine is not an inhibitor of the methyltransferase and does not inhibit amylase secretion. FTA inhibits membrane-associated prenylcysteine methyltransferase from AR42J cells with a Ki in the 45-69 microm range. These results suggest that a methylation event is needed for pancreatic exocrine secretion which may be the reversible methylation of a G-protein involved in signal transduction or membrane trafficking.

Characterization of the prenylated protein methyltransferase in human endometrial carcinoma

The processing of ras and of other GTP-binding proteins includes a final reversible step in which the carboxy terminal prenylated cysteine is methylated by the enzyme prenylated protein methyltransferase (PPMTase). The significance of this modification and of the role of PPMTase in human tumors has yet to be fully elucidated. Here we characterize the PPMTase of human endometrial carcinomas (tumors in which the frequency of ras gene mutations is relatively high) and compare it to the PPMTase of the normal endometrium. Our results show that in both types of tissues the enzyme is bound to the membranes. It can utilize synthetic substrates such as N-acetyl-S-farnesyl-L-cysteine (Km = 18-20 microM) and is blocked by the PPMTase inhibitor S-farnesylthioacetic acid (Ki = 2 microM). In vitro methylation assays and [alpha-32P]GTP blot-overlay assays showed that the major endogenous PPMTase substrates are small GTP-binding proteins. Methylation of these proteins in vitro is blocked by farnesylthioacetic acid. The kinetic properties of PPMTase from the carcinomas and the normal tissues are very similar. However, levels of PPMTase activity (but not of its endogenous substrates) are higher in the carcinomatous endometrium than in the normal one. The elevated enzyme activity is restricted to the crude mitochondrial fraction (8.0 +/- 0.4 vs. 5.4 +/- 0.1 pmol N-acetyl farnesylcysteine methyl ester formed/min/mg protein by the carcinoma and by the normal endometrial preparations, respectively). As this fraction is enriched in plasma membranes, it appears that the elevated enzyme activity could be related to ras protein methylation; if so, selective methylation blockers might inhibit the growth of endometrial carcinomas.