Cyclin D1 is an important molecule for cell proliferation, of which role is to promote cell cycle progression from G1 to S phase by activating cdk4/6 that phosphorylate pRb. We found that there were two forms of cyclin D1 in proliferating mouse hepatic cells, one of which had molecular weight of 32.5 kDa (authentic form) and the other with 34 kDa (alternative form) in Western blot analysis. This study was purposed to characterize the alternative form of cyclin D1 with its relationship to cell proliferation. When cyclin D1 was analysed in diethylnitrosamine-induced mouse liver tumor tissues and non-tumor liver tissues, the two forms were seen in the tumor tissues, while no cyclin D1 was detected in the normal liver tissues. When the animals bearing hepatic tumors were subjected to partial hepatectomy, the double bands were seen not only in the tumor tissues but also in the non-tumor liver tissues. Furthermore, although only the authentic form was detected in mouse hepatic tumor cell lines under the non-proliferating condition, the two forms were seen under the proliferating condition, suggesting that the expression of the alternative form is related to cell proliferation. On the other hand, because the both forms of cyclin D1 were recognized by the anti-cyclin D1 antibodies that bind to the C-terminus of the cyclin D1 molecule, the two forms have the common C-terminus amino acid sequence. When the cell lysates from proliferating tumor cells were treated with alkaline phosphatase, a non-specific phosphatase, the alternative form did not show any changes, while the phosphorylated Akt used as control completely disappeared, indicating that the alternative form is not a phosphorylation product. 5'-RACE (rapid amplification of cDNA ends) of cyclin D1 cDNAs revealed that there was no variability in their coding sequence, although they had various length of the 5'-untranslated regions. These results indicate that the alternative form of cyclin D1 is not a phosphorylation product and has the same primary structure to the authentic form. Therefore, the alternative form is suggested to be a novel cyclin D1 molecule with unknown post-translational modification.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA