Skin pigmentary abnormalities are seen as aesthetically unfavorable and have led to the development of cosmetic and therapeutic treatment modalities of varying efficacy. Hence, several putative depigmenting agents aimed at modulating skin pigmentation are currently being researched or sold in commercially available products. In this review we will discuss the regulation of processes that control skin complexion coloration. This includes direct inhibition of tyrosinase and related melanogenic enzymes, regulation of melanocyte homeostasis, alteration of constitutive and facultative pigmentation and down-regulation of melanosome transfer to the keratinocytes. These various processes, in the complex mechanism of skin pigmentation, can be regulated individually or concomitantly to alter complexion coloration and thus ameliorate skin complexion diseases.

Modification of skin complexion coloration has traditionally been accomplished by interruption or attenuation of melanogenesis and/or melanosome transfer. Post-transfer modification of pigmented melanosomes provides an attractive and distinct avenue of modulating skin pigmentation. The processing of melanosomes during keratinocyte (KC) terminal differentiation and the degradative variability observed between light and dark skin (LS and DS) remains enigmatic. To evaluate this, we developed a model system to investigate the loss of fluorescently labeled and isolated melanosomes by cultured human KCs. The extent of melanosome loss has been qualitatively assessed using transmission electron microscopy and indirect immunofluorescence with confocal microscopy, and quantitatively assessed using flow cytometry analysis. Results show that melanosomes are incorporated into the cytoplasm of both light and dark keratinocytes (LKCs and DKCs) and trafficked to a perinuclear region. Within 48  hours, confocal microscopy images suggest that LKCs display accelerated melanosome loss. This time-dependent decrease in carboxyfluorescein diacetate (CFDA) fluorescence was then quantitatively analyzed using flow cytometry. Consistent with the results of the confocal analysis, over a 48-hour time frame, LKCs appear to lose melanosomes more efficiently than DKCs. These experiments show that melanosomes are more rapidly lost in KCs derived from LS as opposed to DS.

Degradation of melanosomes in light skin (LS, i.e. phototype I/II) appears to occur more rapidly than dark skin (DS, i.e. phototype IV/V). Hydrolytic enzymes known to reside and be expressed in a differential pattern within the interfollicular epidermis are implicated in playing a role in epidermal differentiation and potentially melanosome degradation. The aim of this present study was to evaluate the differential expression of hydrolytic enzymes that may correlate with physiological and phenotypic differences seen between DS and LS. Expression of six hydrolytic enzymes was confirmed by microarray analysis of the suprabasal epidermis from LS and DS. Specific lysosomal hydrolases identified by microarray analysis were analyzed by indirect immunofluorescence (IIF) and immunoblot analysis. Immunogold electron microscopy (IEM) was completed to visualize cellular expression of the hydrolytic enzyme cathepsin L2 (Cath L2) and biochemical assay was performed to ascertain Cath L2 activity. Immunoblotting of light and dark epidermal lysates demonstrated that of the six enzymes initially analyzed, both prostatic acid phosphatase (ACPP) and Cath L2 were reproducibly upregulated in DS and LS, respectively. IIF and IEM analyses of Cath L2 in tissue confirmed this differential expression. Biochemical analysis of Cath L2 in light and dark epidermal lysates displays increased activity of Cath L2 in LS. The results of this study confirm differential expression of ACPP and Cath L2 in DS and LS at gene and protein level. Additionally, Cath L2 displays increased activity in LS-derived epidermal lysates. This study indentified two acid hydrolases that may play a role in melanosome degradation and pigment processing.