Activation of a cAMP pathway and induction of melanogenesis correlate with association of p16(INK4) and p27(KIP1) to CDKs, loss of E2F-binding activity, and premature senescence of human melanocytes.
There is strong evidence that the senescent phenotype, whether induced by telomere shortening, oxidative damage, or oncogenic stimuli, is an important tumor suppressive mechanism. The melanocyte is a cell of neural crest origin that produces the pigment melanin and can develop into malignant melanomas. To understand how malignant cells escape senescence, it is first crucial to define what genes control senescence in the normal cell. Prolonged exposure to high levels of cAMP results in accumulation of melanin and terminal differentiation of human melanocytes. Here we present evidence that activation of a cAMP pathway correlates with multiple cellular changes in these cells: (1) increased expression of the transcription factor microphthalmia; (2) increased melanogenesis; (3) increased association of the cyclin-dependent kinase inhibitors (CDK-Is) p27(KIP1) and p16(INK4) with CDK2 and CDK4, respectively; (4) failure to phosphorylate the retinoblastoma protein (pRB); (5) decreased expression of E2F1, E2F2, and E2F4 proteins; (6) loss of E2F DNA-binding activity; and (7) phenotypic changes characteristic of senescent cells. Senescent melanocytes have potent E2F inhibitory activity, because extracts from these cells completely abolished E2F DNA-binding activity that was present in extracts from the early proliferative phase. We propose that increased activity of the CDK-Is p27 and p16 and loss of E2F activity in human melanocytes characterize a senescence program activated by the cAMP pathway. Disruption of cAMP-mediated and melanogenesis-induced senescence may cause immortalization of human melanocytes, an early step in the development of melanomas.
Haddad
M M
MM
Departments of Cell Biology and Dermatology, Baylor College of Medicine and VAMC, Houston, Texas 77030, USA.
Xu
W
W
Schwahn
D J
DJ
Liao
F
F
Medrano
E E
EE
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D016922:Cellular Senescence / Q000502:physiology*; D000242:Cyclic AMP / Q000502:physiology; D051358:Cyclin-Dependent Kinase 4; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000235:genetics / Q000378:metabolism*; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D050703:E2F2 Transcription Factor; D050704:E2F4 Transcription Factor; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008543:Melanins / Q000096:biosynthesis; D008544:Melanocytes / Q000166:cytology* / Q000201:enzymology; D008545:Melanoma / Q000201:enzymology / Q000235:genetics / Q000473:pathology; D008850:Microphthalmos / Q000235:genetics; D008869:Microtubule-Associated Proteins / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein / Q000378:metabolism; D056465:Retinoblastoma-Binding Protein 1; D016196:S Phase / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology; D012878:Skin Neoplasms / Q000201:enzymology / Q000235:genetics / Q000473:pathology; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D025521:Tumor Suppressor Proteins
D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; D050703:E2F2 Transcription Factor; C493761:E2F2 protein, human; D050704:E2F4 Transcription Factor; C493762:E2F4 protein, human; D008543:Melanins; D008869:Microtubule-Associated Proteins; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D000242:Cyclic AMP; C495900:CDK4 protein, human; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases