Keratin Subunit Expression in Human Cultured Melanocytes and Mouse Neural Crest Cells Without Formation of Filamentous Structures

The synthesis of keratin is considered to occur in epithelial and epidermal cells. Previous studies have not reported on keratin synthesis within melanocytes that derive from neural crest cells. Epithelial and neural crest cells originally develop from ectodermal tissue. We previously reported that the expression of keratin is a universal phenomenon seen in cultured melanoma cell lines, as demonstrated by two-dimensional polyacrylamide gel electrophoresis, western blot, and electron microscopy analyses. To further investigate the specificity of keratin function in melanocytic cells, we first examined the presence of keratin proteins in cultured human melanocytes, and unexpectedly found keratin subunits in melanocytes by the above-mentioned procedures. The keratin (K) subunits were composed of K1, K5, K8, K10, K14, K16, and K18, together with vimentin. Neural crest cells, which contain immature embryonic melanocytes developing from ectoderm, already expressed keratins; however, under electron microscopy, the expressed keratin did not form filamentous structures. Although the ATP synthase α-chain, which is expressed universally in cultured epidermal tumor cell lines, was also expressed in cultured melanocytes and neural crest cells, a novel malignant melanoma-related protein (MMRP) was absent in melanocytes and neural crest cells. We concluded that keratin subunits are present in both cells, but do not construct keratin filaments