THE ACTIVITY OF DIFFERENTIATION FACTORS INDUCES APOPTOSIS IN POLYOMAVIRUS LARGE T-EXPRESSING MYOBLASTS

It is commonly accepted that pathways that regulate proliferation/diff erentiation processes, if altered in their normal interplay, can lead to the induction of programmed cell death. In a previous work we repor ted that Polyoma virus Large Tumor antigen (PyLT) interferes with in v itro terminal differentiation of skeletal myoblasts by binding and ina ctivating the retinoblastoma antioncogene product. This inhibition occ urs after the activation of some early steps of the myogenic program. In the present work we report that myoblasts expressing wild-type PyLT , when subjected to differentiation stimuli, undergo cell death and th at this cell death can be defined as apoptosis. Apoptosis in PyLT-expr essing myoblasts starts after growth factors removal, is promoted by c ell confluence, and is temporally correlated with the expression of ea rly markers of myogenic differentiation. The block of the initial even ts of myogenesis by transforming growth factor beta or basic fibroblas t growth factor prevents PyLT-induced apoptosis, while the acceleratio n of this process by the overexpression of the muscle-regulatory facto r MyoD further increases cell death in this system. MyoD can induce Py LT-expressing myoblasts to accumulate RB, p21, and muscle-specific gen es but is unable to induce G0(0) arrest. Several markers of different phases of the cell cycle, such as cyclin A, cdk-2, and cdc-2, fail to be down-regulated, indicating the occurrence of cell cycle progression . It has been frequently suggested that apoptosis can result from an u nbalanced cell cycle progression in the presence of a contrasting sign al, such as growth factor deprivation. Our data involve differentiatio n pathways, as a further contrasting signal, in the generation of this conflict during myoblast cell apoptosis.