Differentiation-inducing factor-1, a morphogen of Dictyostelium, induces G(1) arrest and differentiation of vascular smooth muscle cells

-Differentiation-inducing factor-1 (DIF-1) is a morphogen that induces diff erentiation of Dictyostelium. Recently, DIF-1 has been shown to inhibit pro liferation and induce differentiation in tumor cells, although the underlyi ng mechanisms remain unknown. In this study, we examined the effects of DIF -1 on the proliferation and differentiation of vascular smooth muscle cells , to explore novel therapeutic strategies for atherosclerosis. DIF-1 nearly completely inhibited DNA synthesis and cell division in mitogen-stimulated cells. DIF-1 inhibited the phosphorylation of the retinoblastoma protein a nd the activities of cyclin-dependent kinase (Cdk) 4, Cdk6, and Cdk2, which phosphorylate the retinoblastoma protein. DIF-1 strongly suppressed the ex pression of cyclins D1, D2, and D3, as well as those of cyclins E and A, wh ich normally began after that of the D-type cyclins. The mRNAs for the smoo th muscle myosin heavy chains SM1 and SM2 were expressed in quiescent cells in primary culture, and these expression levels decreased after mitogenic stimulation. In the presence of DIF-1, the rate of the reduction was signif icantly decelerated. Moreover, the addition of DIF-1 to dedifferentiated ce lls induced the expressions of SM1 and SM2, accompanied by a reduction in t he level of SMemb, a nonmuscle-type myosin heavy chain. Therefore, DIF-1 se emed to interrupt a very early stage of G(1), probably by suppressing the e xpressions of the D-type cyclins. Furthermore, this compound may prevent ph enotypic modulation and induce differentiation of vascular smooth muscle ce lls.