Significant growth retardance was exerted by p16INK4a compared with
the control. The protein was transduced the day before cell counting. Data shown are the mean ± standard deviation of triplicate wells or experiments. **p < 0.01. c. p16INK4a protein caused evident accumulation of A549 cells find more in G1 phase and a decrease of those in S phase at 48 h after subculture. Data shown are the mean ± standard deviation of three independent experiments. *p < 0.05. Discussion As a tumor suppressor, CDKN2A is an important gene because its inactivation abrogates two fundamental pathways, that of pRB and p53, both of which are involved in carcinogenesis and tumor progression. So far, the distinct tumor suppressive effects of p16INK4a and p14ARF have been established, but those of p12 have not. Furthermore, to the best of our knowledge, the effects of the three transcripts check details have not been compared.
The human A549 cell line is a good model to investigate the suppression effects of each of the three transcript variants. The advantages of this cell line are as follows: First, the CDKN2A locus is homozygously deleted in this cell line, such that there is no interference from the endogenous proteins. This is an important consideration since the effects of p16INK4a were shown in a previous study to be associated with endogenous p16INK4a status [23, 24]. Previous research in our laboratory also demonstrated that introduction
of p16INK4a neither suppresses growth nor decreases colony formation rates by Anip973 and AGZY83-a cells expressing endogenous wild-type p16INK4a . Second, the A549 cell line is wild-type in RB and p53. Therefore, p16INK4a and p14ARF plasmids can be expected to successfully act on the pRB and p53 pathways. As to the methods used in this study, the use of stable transfectants confers several advantages as it eliminates Rho a source of variability in transfection efficiency, which facilitated parallel comparison experiments. Furthermore, the characteristics of cells stably transfected with p16INK4a have been shown to differ from those transiently transfected with the vector; transient p16INK4a transfection induces apoptosis whereas stable transfection markedly suppresses cell growth and cloning efficiency . Research on p12 has been hindered as the gene is expressed in normal pancreas tissue, which is difficult to obtain in a well-preserved state. We successfully constructed a eukaryotic expression vector carrying p12 and were thus able to show that the gene acts as a proliferation inhibitor in A549 cells. Thus, our research provides evidence that p12 has tumor suppressive effects not only in pancreatic and cervical cancer cell lines, as previously reported, but also in a lung cancer cell line. The effects of p12 on other cell types will be investigated in future studies.