Constitutive expression of c-myc does not relieve cAMP-mediated growth arrest in human lymphoid Reh cells

The Reh cell system is suitable for evaluating events important for control of proliferation independently of mechanisms involved in differentiation, as Reh cells are unable to differentiate. In the human pre-B cell line Reh, activation of adenylate cyclase by forskolin induces a five to tenfold rapid, transient down-regulation of steady-state c-myc RNA within 4 hours. Concurrently, the cells are strongly growth arrested in the G1 phase of the cell cycle. To clarify if the observed growth arrest could be relieved by constitutive expression of c-myc, an exogenous c-myc gene under constitutive promoter control was introduced into Reh cells by electroporation. The c-myc-expressing construct pDMmycHyg contained human c-myc exons 2 and 3 driven by the Mo-MLV LTR and conferred hygromycin resistance. Exogenous c-myc RNA transcripts and protein were constitutively expressed in the transfected clones at levels roughly twice as high as the level in nontransfected cells. Total c-myc protein levels were unchanged upon treatment of transfected clones with forskolin. Yet, the transfected cells were not released from growth arrest. Furthermore, the transfected Reh cells did not differentiate upon forskolin treatment. Constitutive overexpression of c-myc is therefore not sufficient for relieving forskolin-mediated effects on growth arrest in Reh cells.