S-Adenosylmethionine decarboxylase gene expression is regulated by the cAMP signal transduction pathway in H-ras transformed fibrosarcoma cells capable of malignant progression

HSG cells differentiated by culture on extracellular matrix involves induction of S-adenosylmethione decarboxylase and ornithine decarboxylase

The human salivary gland (HSG) epithelial cell line can differentiate when cultured on extracellular matrix preparations. We previously identified >30 genes upregulated by adhesion of HSG cells to extracellular matrix. In the current studies, we examined the role of one of these genes, the polyamine pathway biosynthetic enzyme S-adenosylmethionine decarboxylase (SAM-DC) and the related enzyme, ornithine decarboxylase (ODC), on HSG cell differentiation during culture on extracellular matrix. HSG cells cultured on fibronectin-, collagen I gel-, and Matrigel-coated substrates for 12-24 h upregulated SAM-DC and ODC mRNA expression and enzyme activity compared to cells cultured on non-precoated substrates. After 3-5 days, HSG cells grown on Matrigel- or collagen I gel-coated substrates acquired a differentiated phenotype: the cells showed changes in culture morphology and increased expression of salivary gland differentiation markers (vimentin, SN-cystatin, and alpha-amylase). Further, culturing the cells on substrates precoated with an anti-beta1-integrin-antibody promoted differentiation-like changes. HSG cells cultured on collagen I- or Matrigel-coated substrates rapidly entered the cell cycle but showed decreased cell proliferation at longer times. In contrast, cell proliferation was enhanced on fibronectin-coated substrates compared to cells on non-precoated substrates. Treatment with the polyamine synthesis inhibitors, difluoromethylornithine (DFMO), and methylglyoxal bis-(guanylhydrazone) (MGBG), inhibited cell proliferation and delayed (3)H-thymidine incorporation in HSG cells cultured on all of the substrates. Further, inclusion of DFMO and MGBG inhibited or delayed acquisition of the differentiated phenotype in HSG cells cultured on Matrigel- or collagen I gel-coated substrates. This suggests that the adhesion-dependent expression of SAM-DC and ODC contributes to extracellular matrix-dependent HSG cell differentiation.

Characterization of the cAMP binding site of purified S-adenosyl-homocysteine hydrolase from bovine kidney

The enzyme S-adenosyl-homocysteine hydrolase (AdoHcyase) which catalyzes the reversible hydrolysis of AdoHcy to adenosine and homocysteine is an adenosine binding protein. In the present study we examined the characteristics of [(3)H]cAMP binding to purified AdoHcyase from bovine kidney in comparison with the high affinity adenosine binding site of AdoHcyase. AdoHcyase exhibits one [(3)H]cAMP binding site with an affinity of K(d)=23.1+/-1.1nM and a B(max) of 116.6+/-3.8pmol/mg protein. Binding of [(3)H]cAMP obeyed a monophasic reaction with a k(+1) value of 0.035min/M. The dissociation of AdoHcyase-[(3)H]cAMP complex exhibited a time- and temperature-dependent character. After a 240min incubation at 0 degrees only 5-10%, however, at 20 degrees 90% were displaceable. Adenosine and cAMP displace each other with similar affinities of EC(50) 57nM vs. EC(50) 65nM. 2'-Deoxyadenosine, N(6)-methyladenosine, and NECA displace 25nM [(3)H]cAMP and 10nM [(3)H]adenosine with EC(50) values of 94, 90 and 80nM, respectively. All other nucleosides studied, adenine, inosine, adenosine-2',3'-dialdehyde, 2-chloroadenosine, aristeromycin, and adenine nucleotides were only week competitors for [(3)H]cAMP and [(3)H]adenosine. These compounds displace [(3)H]cAMP and [(3)H]adenosine with equal potencies. Our data indicate that the binding site for nanomolar concentrations of cAMP and adenosine at the AdoHcyase appears to be identical. The physiological implications of a cAMP binding site at the AdoHcyase remain to be established.

Growth factor-mediated altered expression and regulation of S-adenosylmethionine decarboxylase in a H-ras transformed cell line capable of malignant progression

Mammalian S-adenosylmethionine decarboxylase (SAMDC) is a regulatory activity, which is involved in the biosynthesis of polyamines. The polyamines, namely putrescine, spermidine, and spermine, are essential for mammalian cell proliferation. SAMDC expression was examined in a H-ras transformed cell capable of metastasis formation. Serum stimulation of these cells resulted in increased SAMDC mRNA and enzyme activity expression. The effect of several physiologically relevant growth factors on SAMDC expression was also determined. SAMDC mRNA expression was increased in response to epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) stimulation but was unaffected by transforming growth factor beta(1) (TGF-beta(1)) and platelet derived growth factor (PDGF). Increased SAMDC enzyme activity occurred in response to exposure to EGF, bFGF, TGF-beta(1), and PDGF. The EGF and bFGF mediated alterations in SAMDC mRNA expression were apparently not due to alterations in the transcriptional apparatus but occurred partly through post-transcriptional mechanisms involving increased SAMDC message stability. EGF and bFGF were able both to cooperate with cycloheximide, an inhibitor of protein synthesis, to augment the expression of SAMDC mRNA. Furthermore, studies with NIH-3T3 fibroblasts transfected with either the normal basic fibroblast growth factor coding sequence that lacks a known secretory signal sequence or a chimeric bFGF sequence that targets the growth factor to the secretory pathway revealed that increased SAMDC expression occurred only in those cells which contained the chimeric bFGF sequence that targets the growth factor to the secretory pathway suggesting that the increase in expression of SAMDC occurs through an autocrine mechanism. Increased ornithine decarboxylase (ODC) expression was found to occur in both types of bFGF transfected cells suggesting that altered ODC expression in response to bFGF stimulation may occur through both autocrine and intracrine mechanisms. In addition, a correlation was found to exist between SAMDC expression and regulation in response to growth factor stimulation and malignant potential. This correlation supports the view that growth factor induced alterations in SAMDC expression, although not sufficient on their own to induce metastasis, are important in the promotion and establishment of events important to the phenotype expressed by H-ras transformed cells capable of malignant progression.