Phosphatidylinositol 3-kinase/Akt-dependent and -independent protection against apoptosis in normal human melanocytes.

Normal human melanocytes require the synergistic action of several growth-promoting agents for their growth in serum-free medium. The ability of four representative growth promoting agents including insulin, 12-O-tetradecanoylphorbol-13-acetate (TPA), basic fibroblast growth factor (bFGF), and 3-isobutyl-1-methylxanthine (IBMX), (iTbI) to protect melanocytes against apoptosis was examined. Also, the involvement of phosphatidylinositol (PI) 3-kinase and Akt, one of the downstream targets of PI 3-kinase, in the survival signaling pathway was examined. The percentage of apoptotic cells was negligible when the cells were grown in the presence of iTbI. Deprivation of iTbI from the culture medium for 72 h caused approximately 30% of melanocytes to undergo apoptosis and this was suppressed to variable extents by the addition of one of the iTbI to the medium. Insulin and TPA protected against apoptosis almost completely, whereas bFGF and IBMX rescued melanocytes from apoptosis to a lesser extent. Wortmannin, an inhibitor of PI 3-kinase, potently inhibited the protective effect of insulin on melanocytes, whereas it did not block the ability of TPA, bFGF, or IBMX to rescue the cells from apoptosis. Furthermore, apoptosis of melanocytes induced by deprivation of iTbI was prevented almost completely by infection with an adenovirus vector encoding a constitutively active mutant of either PI 3-kinase or Akt. These results indicate that melanocytes can operate both PI 3-kinase/Akt-dependent and -independent mechanisms for protection against apoptosis and that activation of the PI 3-kinase/Akt pathway is sufficient for protection against apoptosis induced by deprivation of growth-promoting agents.

Defining the conditions for the generation of melanocytes from human embryonic stem cells.

Because of their undifferentiated nature, human embryonic stem cells (hESCs) are an ideal model system for studying both normal human development and the processes that underlie disease. In the current study, we describe an efficient method for differentiating hESCs into a melanocyte population within 4-6 weeks using three growth factors: Wnt3a, endothelin-3, and stem cell factor. The hESC-derived melanocytes expressed melanocyte markers (such as microphthalmia-associated transcription factor and tyrosinase), developed melanosomes, and produced melanin. They retained the melanocyte phenotype during long-term cell culture (>90 days) and, when incorporated into human reconstructed skin, homed to the appropriate location along the basement membrane in the same manner as epidermis-derived melanocytes. They maintained a stable phenotype even after grafting of the reconstructs to immunodeficient mice. Over time in culture, the hESC-derived melanocytes lost expression of telomerase and underwent senescence. In summary, we have shown for the first time the differentiation of hESCs into melanocytes. This method provides a novel in vitro system for studying the development biology of human melanocytes.

Reversal of melanocytic malignancy by keratinocytes is an E-cadherin-mediated process overriding beta-catenin signaling.

Loss of E-cadherin in melanoma cells frees them from keratinocytes-mediated proliferation and phenotypic control, which can be restored by forced E-cadherin expression. In this study, E-cadherin and its derivatives were introduced into metastatic melanoma line 1205Lu. E-cadherin and E-cadherin-alpha-catenin fusion protein were functional in mediating cell adhesion, downregulating MCAM(4) in coculture, and inhibiting proliferation regardless of beta-catenin expression levels and activation status. In contrast, cytoplasmic domain-deleted (E-cadDeltaCYT) derivative was not able to reverse malignancy. The results indicate that E-cadherin-mediated cell adhesion is required for keratinocyte-mediated control of melanocytic cells, which can override proliferative activity of beta-catenin.