Regulation of fos-lacZ fusion gene expression in primary mouse epidermal keratinocytes isolated from transgenic mice

Calcium regulation of keratinocyte differentiation

Calcium is the major regulator of keratinocyte differentiation in vivo and in vitro. A calcium gradient within the epidermis promotes the sequential differentiation of keratinocytes as they traverse the different layers of the epidermis to form the permeability barrier of the stratum corneum. Calcium promotes differentiation by both outside-in and inside-out signaling. A number of signaling pathways involved with differentiation are regulated by calcium, including the formation of desmosomes, adherens junctions and tight junctions, which maintain cell-cell adhesion and play an important intracellular signaling role through their activation of various kinases and phospholipases that produce second messengers that regulate intracellular free calcium and PKC activity, critical for the differentiation process. The calcium receptor plays a central role by initiating the intracellular signaling events that drive differentiation in response to extracellular calcium. This review will discuss these mechanisms.

Vanadium-induced apoptosis of HaCaT cells is mediated by c-fos and involves nuclear accumulation of clusterin

Vanadium exerts a variety of biological effects, including antiproliferative responses through activation of the respective signaling pathways and the generation of reactive oxygen species. As epidermal cells are exposed to environmental insults, human keratinocytes (HaCaT) were used to investigate the mechanism of the antiproliferative effects of vanadyl(IV) sulfate (VOSO(4)). Treatment of HaCaT cells with VOSO(4) inhibited proliferation and induced apoptosis in a dose-dependent manner. Inhibition of proliferation was associated with downregulation of cyclins D1 and E, E2F1, and the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). Induction of apoptosis correlated with upregulation of the c-fos oncoprotein, changes in the expression of clusterin (CLU), an altered ratio of antiapoptotic to proapoptotic Bcl-2 protein family members, and poly(ADP-ribose) polymerase-1 cleavage. Forced overexpression of c-fos induced apoptosis in HaCaT cells that correlated with secretory CLU downregulation and upregulation of nuclear CLU (nCLU), a pro-death protein. Overexpression of Bcl-2 protected HaCaT cells from vanadium-induced apoptosis, whereas secretory CLU overexpression offered no cytoprotection. In contrast, nCLU sensitized HaCaT cells to apoptosis. Our data suggest that vanadium-mediated apoptosis was promoted by c-fos, leading to alterations in CLU isoform processing and induction of the pro-death nCLU protein.

Regulation of protein kinase D during differentiation and proliferation of primary mouse keratinocytes

Diseased skin often exhibits a deregulated program of the keratinocyte maturation necessary for epidermal stratification and function. Protein kinase D (PKD), a serine/threonine kinase, is expressed in proliferating keratinocytes, and PKD activation occurs in response to mitogen stimulation in other cell types. We have proposed that PKD functions as a pro-proliferative and/or anti-differentiative signal in keratinocytes and hypothesized that differentiation inducers will downmodulate PKD to allow differentiation to proceed. Thus, changes in PKD levels, autophosphorylation, and activity were analyzed upon stimulation of differentiation and proliferation in primary mouse keratinocytes. Elevated extracellular calcium and acute 12-O-tetradecanoylphorbol-13-acetate (TPA) treatments induced differentiation and triggered a downmodulation of PKD levels, autophosphorylation at serine 916, and activity. Chronic TPA treatment stimulated proliferation and resulted in a recovery of PKD levels, autophosphorylation, and activity. Immunohistochemical analysis demonstrated PKD localization predominantly in the proliferative basal layer of mouse epidermis. Co-expression studies revealed a pro-proliferative, anti-differentiative effect of PKD on keratinocyte maturation as monitored by increased and decreased promoter activities of keratin 5, a proliferative marker, and involucrin, a differentiative marker, respectively. This work describes the inverse regulation of PKD during keratinocyte differentiation and proliferation and the pro-proliferative/anti-differentiative effects of PKD co-expression on keratinocyte maturation.

Biphasic effect of 1,25-dihydroxyvitamin D3 on primary mouse epidermal keratinocyte proliferation

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] has been proposed as a physiologic regulator of keratinocyte growth and differentiation. Utilizing a proliferative serum-free culture system, we have found that a physiologic (picomolar) concentrations this hormone stimulated proliferation of primary mouse epidermal keratinocytes; at higher (nanomolar to micromolar) doses, growth was inhibited by 1,25(OH)2D3. We investigated the nature of the signal transduction mechanism underlying the response to 1,25(OH)2D3 and observed little or no effect of either low or high concentrations of the hormone on cytosolic calcium levels or Fos expression. Furthermore, the protein kinase C inhibitor, Ro 31-7549, had very little effect on the growth inhibition induced by a high dose (1 microM) of 1,25(OH)2D3. This lack of rapid signal transduction events was consistent with the inability of a short (4-hour) exposure to 1,25(OH)2D3 to initiate a complete growth-inhibitory response as measured using [3H]thymidine incorporation. Our results indicate that physiologic concentrations of 1,25(OH)2D3 are required for optimal keratinocyte growth. Furthermore, we found no evidence of rapid effects of 1,25(OH)2D3 and suggest that in mouse epidermal keratinocytes, the response to this hormone is mediated by a slow transduction pathway, such as that activated by the intracellular 1,25(OH)2D3 receptor (VDR).