TGF-beta 1 regulates the expression of multiple max-interacting transcription factors in Balb/MK cells: Implications for understanding the mechanism of action of TGF-beta 1

Appropriate transforming growth factor-beta1 (TGF-beta1) signaling is requi red to preserve homeostasis of diverse tissues during development, At the c ellular level, one function of TGF-beta1 that is critical for preserving ho meostasis is the ability to arrest cell growth. TGF-beta1 arrests growth by blocking the function of the c-myc proto-oncogene. c-myc function is deter mined by the level of c-myc expression relative to other Max-interacting tr anscription factors, and TGF-beta1 has been shown to inhibit c-myc expressi on by inhibiting c-myc transcription. However, whether TGF-beta1 might also increase the expression of a Max-interacting factor that blocks myc functi on by competing with myc for Max binding is not known. Therefore, we determ ined the effect of TGF-beta1 on the expression of Max-interacting transcrip tion factors in Balb/MK cells, We found unexpectedly that Balb/MK cells exp ress both N-myc and c-myc, The pattern of N-myc expression during the cell cycle differs from that of c-myc, indicating that mRNA accumulation is cont rolled by mechanisms specific to each gene, TGF-beta1 rapidly inhibits N-my c mRNA expression; thus N-myc is a novel target of TGF-beta1 in Balb/NK cel ls. More importantly, we found that TGF-beta1 induces the expression of the putative tumor suppressor genes Mad4 and Mxi1 in both the Balb/MK and Mv1L u cell lines. Mad4 and Mxi1 are novel targets of TGF-beta1, known to inhibi t cell growth by antagonizing the interaction of Myc with Max. Thus, our re sults suggest that the induction of Mad4 and Mxi1 may function in tandem wi th the inhibition of N-myc and c-myc to mediate the growth inhibitory funct ion of TGF-beta1.