CLONING OF MID-G(1) SERUM RESPONSE GENES AND IDENTIFICATION OF A SUBSET REGULATED BY CONDITIONAL MYC EXPRESSION

The emergence of cells from a quiescent G(0) arrested state into the c ell cycle is a multistep process that begins with the immediate early response to mitogens and extends into a specialized G(1) phase. Many i mmediate early serum response genes including c-fos, c-myc, and c-jun are transcriptional regulators. To understand their roles in regulatin g cell cycle entry and progression, the identities of their regulatory targets must be determined. In this work we have cloned cDNA. copies of messenger RNAs that are either up- or down-regulated at a mid-G(1) point in the serum response (midserum-response [mid-SR]). The mid-SR p anel is expected to include both direct and indirect targets of immedi ate early regulators. This expectation was confirmed by the identifica tion of several transcriptional targets of conditional c-myc activity. In terms of cellular function, the mid-SR class is also expected to i nclude execution genes needed for progression through G(1) and into S- phase. DNA sequence data showed that the mid-SR panel included several genes already known to be involved in cell cycle progression or growt h transformation, suggesting that previously unknown cDNAs in the same group are good candidates for other G(1) execution functions. In func tional assays of G(0) --> S-phase progression, c-myc expression can by pass the requirement for serum mitogens and drive a large fraction of G(0) arrested cells through G(1) into S-phase. However, beyond this ge neral similarity, little is known about the relation of a serum-driven progression to a myc-driven progression. Using the mid-SR collection as molecular reporters, we found that the myc driven G(1) differs qual itatively from the serum driven case. Instead of simply activating a s ubset of serum response genes, as might be expected, myc regulated som e genes inversely relative to serum stimulation. This suggests that a myc driven progression from G(0) may have novel properties with implic ations for its action in oncogenesis.