Tyrosinase is considered to be the rate-limiting enzyme for the biosynthesis of melanin in epidermal melanocytes, and thus tyrosinase activity is thought to be a major regulatory step in melanogenesis. To determine whether the rate of pigment production was controlled at the level of tyrosinase gene expression, we developed a culture system capable of generating large populations of pure human melanocytes and then measured both melanin content as determined spectrophotometrically by absorption at 475 nm and mRNA levels as detected by hybridization with cloned cDNA Pmel 34, encoding human tyrosinase. We examined the relationship between pigment content and tyrosinase mRNA levels among human melanoma and melanocyte lines with very different levels of basal pigmentation; between two clones of a single human melanoma line, one pigmented and one amelanotic; and sequentially in melanocytes before and after simulation with isobutylmethylxanthine to increase melanin content per cell. Using Northern blot analysis and in-situ hybridization we found no correlation between tyrosinase message levels and melanin content, suggesting that posttranscriptional regulation of tyrosinase and/or other events determine the rate of pigment synthesis in human melanocytes.

This study examines how the microenvironment created by fluid in chronic wounds influences the growth of dermal fibroblasts. Newborn fibroblasts, which are known to grow rapidly, were used as a model system to explore how chronic wound fluid affects the growth of regenerative fibroblasts. Wound fluid was collected from patients with chronic venous leg ulcers (duration longer than two months). The biological properties of this fluid were then further characterised to elucidate its molecular effects on cell growth. Results indicate that chronic wound fluid dramatically inhibited the growth of newborn dermal fibroblasts. This growth inhibitory effect was variable among donors, reversible and heat-sensitive. The inhibitory effect was due not to cytotoxicity or impaired plating efficiency of these cells, but to specific interference with the cell cycle. Chronic wound fluid arrested newborn fibroblast growth by preventing entry into the S-phase, or DNA synthesis-phase, of the cell cycle. In contrast to its effects on newborn fibroblasts, chronic wound fluid either stimulated or had a minimal effect on fibroblasts which had been cultured from the edge of chronic venous leg ulcers and healthy tissue on the upper thigh in the same patient. This may partially account for the impaired healing seen in chronic venous leg ulcers.

Histone acetylation induces chromatin opening by perturbing higher-order chromatin compaction and folding, suggesting that histone acetylation and deacetylation dynamics are central to chromosome condensation or decondensation. The condensation of chromosomes during mitosis is an essential prerequisite for successful chromosome segregation. In this study, we depleted three representative histone acetyltransferases (HATs; p300, CBP, and P/CAF) using shRNAs to explore their role in regulating mitotic progression and chromosome segregation. We showed that HAT depletion severely interfered with the normal timing of mitotic progression, and it reduced condensin subunit levels. The predominant response to HAT depletion, in both human primary and cancer cells, was a mitotic catastrophe following aberrant mitotic arrest. Alternatively, adaptation to HAT depletion, particularly in cancer cells, led to multinucleation and aneuploidy. Interestingly, mitotic catastrophe induced by HAT depletion appeared to be coupled to the signaling process of H2AX phosphorylation and foci formation, independently of DNA doublestrand breaks and DNA damage. Taken together, our results provide novel molecular evidence that HAT proteins maintain mitotic chromatin assembly and integrity as a cellular determinant of mitotic cell death.