ABSTRACT. At present, the effect of cyclin D2 implicated in cell cycle regulation, differentiation, and oncogenic transformation is not fully confirmed. To better elucidate the role of cyclin D2 in controling the cell proliferation, cyclin D2 expression level was determined at the early initiation and promotion stages during the in vitro two-stage transformation process of Balb/3T3 A31 cells. N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced G2/M-arrested cells expressed low level of cyclin D2 mRNA, while the contact-inhibited nonproliferating cells expressed high level of cyclin D2 mRNA. In the transformed proliferating cells at the promotion stage, cyclin D2 mRNA was not expressed. These data suggest that cyclin D2 expression may be associated with the type of growth arrest and nonproliferating state, but not with the cell proliferation and transformation.KEY WORDS: cyclin D2, G2/M arrest, transformation.J. Vet. Med. Sci. 64(3): 201-205, 2002 The D-type cyclins synthesized during the early G1 phase consist of three family members, cyclins D1, D2, and D3. Cyclin D1, encoded by bcl-1/PRAD1 oncogene, is an important cell cycle positive regulator facilitating the G1 progression toward the S phase similar to cyclin E and PCNA [23]. Cyclins D2 and D3 were cloned as a consequence of their homologies to cyclin D1 [10,15,23], and cyclin D2 was also independently identified to be encoded by a gene mutated with proviral insertion [8]. Aberrant expression of cyclin D2 has been linked to the human male germ cell tumorigenesis [9]. Moreover, overexpression of the cyclin D2 has been determined in chronic B-cell malignancies [5] and is known to be closely correlated with the gastric cancer progression and prognosis [19,20]. Cyclin D2 was expressed highly in transformed cells as well [6]. In addition, this cyclin has been linked to the differentiation of neural cell populations [16,17] and the activation of macrophage [21]. Furthermore, coexpression of cyclin D2 and Ha-Ras under low-serum conditions can induce a senescence-like phenotype [12]. However, several papers also reported that cyclin D2 was overexpressed in various nonproliferating states [11,14,18]. These results suggest that cyclin D2 may have different roles depending on its levels of expression and cell type. Therefore, to further understand the role of cyclin D2 on the cell proliferation, we determined the cyclin D2 mRNA expression level in G2/M-arrested cells initiated by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), contact-inhibited cells, and proliferating cells during the in vitro two-stage transformation process of Balb/ 3T3 A31 cells.
MATERIALS AND METHODSChemicals: MNNG, cadmium chloride (CdCl 2 ), Tri reagent, and other commonly used chemicals were purchased from Sigma (St. Louis, MO, U.S.A.). Dulbecco's modified Eagle's medium (DMEM), DMEM/F12 medium and bovine calf serum (BCS) were obtained from GIBCO-BRL (Gaithesburg, MD, U.S.A.). Digoxigenin (DIG)-RNA labeling and detection kit, and β-actin plasmid cDNA were purchased from Boehringer Mannheim (Ma...