Differential Expression Pattern Of S –Adenosylmethionine Synthetase Isoenzymes During Rat Liver Development

Abstract

The pattern of expression of liver-specific and extrahepatic S-adenosylmethionine (SAM) synthetase in developing rat liver was established by determining steady-state levels of the respective messenger RNAs (mRNAs) and protein content. Levels of liver-specific SAM synthetase mRNA increased progressively from day 20 of gestation, increased 10-fold immediately after birth, and reached a peak at 10 days of age, decreasing slightly by adulthood. Conversely, mRNA levels of extrahepatic isoenzyme decreased toward birth, increased threefold in the newborn, and decreased further in the postnatal life, reaching a minimum in the adult. Similar expression profiles were observed in isolated hepatocytes, indicating that both mRNAs are differentially regulated in the same cell type. Western blot analysis showed that levels of immunoreactive liver-specific isoenzyme followed a trend similar to the mRNA, indicating that developmental regulation of this enzyme is mediated at the mRNA level. Developmental patterns of expression of albumin and alpha-fetoprotein (AFP) mRNAs were closely related to those for liver-specific and extrahepatic isoenzymes, respectively. Therefore, it is suggested that liver-specific SAM synthetase may be a marker for hepatocyte differentiation. Incubation of primary cultures of hepatocytes from 21-day-old fetuses with permeant cyclic adenosine monophosphate (cAMP) analogues elicited an up-regulation of the mRNA for the liver-specific isoenzyme with a concomitant down-regulation of the extrahepatic message, suggesting a physiological role for the increased postnatal glucagonemia in the control of this isoenzyme switching. In contrast with the isoenzyme expression profiles, the levels of SAM, the product of SAM synthetase reaction, were determined to be greater during gestation than in immediate postnatal periods. These results indicate that synthesis and utilization of SAM may be regulated differentially in fetal and adult hepatocytes.

Role of Methionine Adenosyltransferase α2 and β Phosphorylation and Stabilization in Human Hepatic Stellate Cell Trans‐Differentiation

“…MATβ regulates the activity of the MATα2 subunit by lowering the inhibition constant (K i ) for SAMe and the Michaelis constant (K m ) for methionine (Halim et al, 1999). MAT1A is expressed in adult liver (Gil et al, 1996) whereas MAT2A and MAT2B are widely expressed in extrahepatic tissues (Gil et al, 1996; Horikawa et al, 1992). MAT2A and MAT2B are also expressed in rapidly dividing and de-differentiated liver (Gil et al, 1996).…”

“…MAT1A is expressed in adult liver (Gil et al, 1996) whereas MAT2A and MAT2B are widely expressed in extrahepatic tissues (Gil et al, 1996; Horikawa et al, 1992). MAT2A and MAT2B are also expressed in rapidly dividing and de-differentiated liver (Gil et al, 1996). Quiescent HSCs of the liver express MAT2A but not MAT1A (Shimizu-Saito et al, 1992).…”

Immunoassays of Methionine Adenosyltransferase Activity, S-adenosylmethione and Their Applications

“…Both MAT-I and MAT-III are present in adult liver cells. MAT-II is a heterotetramer formed by MAT2a encoding the catalytic subunit of α2 and MAT2b gene encoding β subunit, present in cells other than liver, embryonic liver and hepatoma cells [3]. MAT isoforms differ in their kinetic and regulatory properties.…”

“…Current methods to measure MAT activity are: (1)high performance liquid (HPLC) and mass spectrometry (LC/MS/MS) method to determine of SAM [3,4] synthesized; (2)malachite green and ammonium molybdateto determine the content of inorganic phosphorus Pi [10]. Method (1) is a direct method yet it has a few limitations: (a) Inaccurate as synthesized SAM cannot be measure immediately after it was synthesized due to the lengthy and harsh sample preparation, manipulation and measurement processes, the highly instable nature of SAM makes (1) more undesirable.…”

Proteomic analysis of human hepatoma cells expressing methionine adenosyltransferase I/III

“…In mammalian cells three distinct MAT isoforms are known [10–13], which are distributed in a tissue specific manner [14]. MATI/III, products of gene MAT1A , predominate in adult liver [15] while MATII, encoded by gene MAT2A is common to the rest of the tissues, as well as fetal liver, but is progressively replaced by MATI/III during development [16,17]. Although all three MAT forms share 85% of the aminoacidic sequence [18,19], they differ in their kinetic and regulatory properties [14].…”