Immortalization of mouse myogenic cells can occur without loss of p16INK4a, p19ARF, or p53 and is accelerated by inactivation of Bax.
BACKGROUND
Upon serial passaging of mouse skeletal muscle cells, a small number of cells will spontaneously develop the ability to proliferate indefinitely while retaining the ability to differentiate into multinucleate myotubes. Possible gene changes that could underlie myogenic cell immortalization and their possible effects on myogenesis had not been examined.


RESULTS
We found that immortalization occurred earlier and more frequently when the myogenic cells lacked the pro-apoptotic protein Bax. Furthermore, myogenesis was altered by Bax inactivation as Bax-null cells produced muscle colonies with more nuclei than wild-type cells, though a lower percentage of the Bax-null nuclei were incorporated into multinucleate myotubes. In vivo, both the fast and slow myofibers in Bax-null muscles had smaller cross-sectional areas than those in wild-type muscles. After immortalization, both Bax-null and Bax-positive myogenic cells expressed desmin, retained the capacity to form multinucleate myotubes, expressed p19ARF protein, and retained p53 functions. Expression of p16INK4a, however, was found in only about half of the immortalized myogenic cell lines.


CONCLUSIONS
Mouse myogenic cells can undergo spontaneous immortalization via a mechanism that can include, but does not require, loss of p16INK4a, and also does not require inactivation of p19ARF or p53. Furthermore, loss of Bax, which appears to be a downstream effector of p53, accelerates immortalization of myogenic cells and alters myogenesis.
Nowak
Jonathan A
JA
Boston Biomedical Research Institute 64 Grove Street Watertown, Massachusetts 02472, USA. jonathan@nowaks.org
Malowitz
Jonathan
J
Girgenrath
Mahasweta
M
Kostek
Christine A
CA
Kravetz
Amanda J
AJ
Dominov
Janice A
JA
Miller
Jeffrey Boone
JB
D000818:Animals; D002455:Cell Division; D002460:Cell Line; D002478:Cells, Cultured; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D018345:Mice, Knockout; D018485:Muscle Fibers, Skeletal / Q000166:cytology* / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D019253:Proto-Oncogene Proteins c-bcl-2; D025803:Tumor Suppressor Protein p14ARF / Q000378:metabolism; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D051028:bcl-2-Associated X Protein
C494827:Bax protein, mouse; C417743:Cdkn2a protein, mouse; D019941:Cyclin-Dependent Kinase Inhibitor p16; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D025803:Tumor Suppressor Protein p14ARF; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein