SnoN and Ski protooncoproteins are rapidly degraded in response to transforming growth factor β signaling

Transforming growth factor β (TGF-β) regulates a variety of physiologic processes, including growth inhibition, differentiation, and induction of apoptosis. Some TGF-β-initiated signals are conveyed through Smad3; TGF-β binding to its receptors induces phosphorylation of Smad3, which then migrates to the nucleus where it functions as a transcription factor. We describe here the association of Smad3 with the nuclear protooncogene protein SnoN. Overexpression of SnoN represses transcriptional activation by Smad3. Activation of TGF-β signaling leads to rapid degradation of SnoN and, to a lesser extent, of the related Ski protein, and this degradation is likely mediated by cellular proteasomes. These results demonstrate the existence of a cascade of the TGF-β signaling pathway, which, upon TGF-β stimulation, leads to the destruction of protooncoproteins that antagonize the activation of the TGF-β signaling.

The catalytic subunit of yeast telomerase

Telomerase is an RNA-directed DNA polymerase, composed of RNA and protein subunits, that replicates the telomere ends of linear eukaryotic chromosomes. Using a genetic strategy described here, we identify the product of the EST2 gene, Est2p, as a subunit of telomerase in the yeast Saccharomyces cerevisiae. Est2p is required for enzyme catalysis, as mutations in EST2 were found to result in the absence of telomerase activity. Immunochemical experiments show that Est2p is an integral subunit of the telomerase enzyme. Critical catalytic residues present in RNA-directed DNA polymerases are conserved in Est2p; mutation of one such residue abolishes telomerase activity, suggesting a direct catalytic role for Est2p.

DNA damage enhances melanogenesis.

Although the ability of UV irradiation to induce pigmentation in vivo and in vitro is well documented, the intracellular signals that trigger this response are poorly understood. We have recently shown that increasing DNA repair after irradiation enhances UV-induced melanization. Moreover, addition of small DNA fragments, particularly thymine dinucleotides (pTpT), selected to mimic sequences excised during the repair of UV-induced DNA photoproducts, to unirradiated pigment cells in vitro or to guinea pig skin in vivo induces a pigment response indistinguishable from UV-induced tanning. Here we present further evidence that DNA damage and/or the repair of this damage increases melanization. (i) Treatment with the restriction enzyme Pvu II or the DNA-damaging chemical agents methyl methanesulfonate (MMS) or 4-nitroquinoline 1-oxide (4-NQO) produces a 4- to 10-fold increase in melanin content in Cloudman S91 murine melanoma cells and an up to 70% increase in normal human melanocytes, (ii) UV irradiation, MMS, and pTpT all upregulate the mRNA level for tyrosinase, the rate-limiting enzyme in melanin biosynthesis. (iii) Treatment with pTpT or MMS increases the response of S91 cells to melanocyte-stimulating hormone (MSH) and increases the binding of MSH to its cell surface receptor, as has been reported for UV irradiation. Together, these data suggest that UV-induced DNA damage and/or the repair of this damage is an important signal in the pigmentation response to UV irradiation. Because Pvu II acts exclusively on DNA and because MMS and 4-NQO, at the concentrations used, primarily interact with DNA, such a stimulus alone appears sufficient to induce melanogenesis. Of possible practical importance, the dinucleotide pTpT mimics most, if not all, of the effects of UV irradiation on pigmentation, tyrosinase mRNA regulation, and response to MSH without the requirement for antecedent DNA damage.