Keratinocyte transglutaminase: regulation and release

TIG3 tumor suppressor-dependent organelle redistribution and apoptosis in skin cancer cells

TIG3 is a tumor suppressor protein that limits keratinocyte survival during normal differentiation. It is also important in cancer, as TIG3 level is reduced in tumors and in skin cancer cell lines, suggesting that loss of expression may be required for cancer cell survival. An important goal is identifying how TIG3 limits cell survival. In the present study we show that TIG3 expression in epidermal squamous cell carcinoma SCC-13 cells reduces cell proliferation and promotes morphological and biochemical apoptosis. To identify the mechanism that drives these changes, we demonstrate that TIG3 localizes near the centrosome and that pericentrosomal accumulation of TIG3 alters microtubule and microfilament organization and organelle distribution. Organelle accumulation at the centrosome is a hallmark of apoptosis and we demonstrate that TIG3 promotes pericentrosomal organelle accumulation. These changes are associated with reduced cyclin D1, cyclin E and cyclin A, and increased p21 level. In addition, Bax level is increased and Bcl-XL level is reduced, and cleavage of procaspase 3, procaspase 9 and PARP is enhanced. We propose that pericentrosomal localization of TIG3 is a key event that results in microtubule and microfilament redistribution and pericentrosomal organelle clustering and that leads to cancer cell apoptosis.

Type I transglutaminase accumulation in the endoplasmic reticulum may be an underlying cause of autosomal recessive congenital ichthyosis

Type I transglutaminase (TG1) is an enzyme that is responsible for assembly of the keratinocyte cornified envelope. Although TG1 mutation is an underlying cause of autosomal recessive congenital ichthyosis, a debilitating skin disease, the pathogenic mechanism is not completely understood. In the present study we show that TG1 is an endoplasmic reticulum (ER) membrane-associated protein that is trafficked through the ER for ultimate delivery to the plasma membrane. Mutation severely attenuates this processing and a catalytically inactive point mutant, TG1-FLAG(C377A), accumulates in the endoplasmic reticulum and in aggresome-like structures where it is ubiquitinylated. This accumulation results from protein misfolding, as treatment with a chemical chaperone permits it to exit the endoplasmic reticulum and travel to the plasma membrane. ER accumulation is also observed for ichthyosis-associated TG1 mutants. Our findings suggest that misfolding of TG1 mutants leads to ubiquitinylation and accumulation in the ER and aggresomes, and that abnormal intracellular processing of TG1 mutants may be an underlying cause of ichthyosis.

Commitment to differentiation and expression of early differentiation markers in murine keratinocytes in vitro are regulated independently of extracellular calcium concentrations

In the epidermis, one of the earliest characterized events in keratinocyte differentiation is the coordinate induction of a pair of keratins specifically expressed in suprabasal cells, keratin 1 (K1) and keratin 10 (K10). Both in vivo and in vitro, extracellular calcium is necessary for several biochemical and structural changes during keratinocyte differentiation. However, it has been unclear if calcium serves as a differentiation signal in keratinocytes. In these studies, expression of suprabasal keratin mRNA and protein is used to test whether the initial differentiation of primary mouse keratinocytes in vitro is dependent on changes in the concentration of extracellular calcium. K1 mRNA was expressed at low levels in cultures of keratinocytes growing on plastic in 0.05 mM calcium but in attached cells was not further induced by increases in the concentration of extracellular calcium. Suspension of the keratinocytes into semi-solid medium induced a rapid and substantial increase in both expression of K1 mRNA and in the percentage of cells expressing suprabasal keratin proteins. The induction was unaffected by the concentration of calcium in the semi-solid medium and could not be enhanced by exposing attached cells to higher calcium before suspension. The induction of K1 mRNA could be inhibited by exposure of the keratinocytes to either EGF or fibronectin. These results suggest that commitment of mouse keratinocytes to terminal differentiation is independent of extracellular calcium and may be regulated primarily by extracellular factors other than calcium.

Primary structure of keratinocyte transglutaminase

The nucleotide and deduced amino acid sequences of the coding regions of human and rat keratinocyte transglutaminases (protein-glutamine: amine gamma-glutamyltransferase; EC 2.3.2.13) have been determined. These yield proteins of approximately 90 kDa that are 92% identical, indicative of the conservation of important structural features. Alignments of amino acid sequences show substantial similarity among the keratinocyte transglutaminase, human clotting factor XIII catalytic subunit, guinea pig liver tissue transglutaminase, and the human erythrocyte band-4.2 protein. The keratinocyte enzyme is most similar to factor XIII, whereas the band-4.2 protein is most similar to the tissue transglutaminase. A salient feature of the keratinocyte transglutaminase is its 105-residue extension beyond the N terminus of the tissue transglutaminase. This extension and the unrelated activation peptide of factor XIII (a 37-residue extension) appear to be added for specialized functions after divergence of the tissue transglutaminase from their common lineage.

Proteolytic release of keratinocyte transglutaminase

Human keratinocytes express a particulate transglutaminase that can be released from the membrane by limited proteolysis with trypsin or plasmin to yield a form that is congruent to 80 kDa. The enzyme from cultured cells was also releasable by endogenous proteolysis to yield a catalytically active fragment of congruent to 80 kDa. Endogenous release was strongly dependent upon temperature and Ca2+ concentration and was inhibited by iodoacetate, but not by leupeptin, antipain or phenylmethanesulphonyl fluoride. These phenomena raise the possibility of partial translocation of transglutaminase activity to the cytoplasm by proteolysis to which the enzyme is subject during terminal differentiation. In addition, hydrodynamic measurements showed that the endogenously released enzyme was monomeric in solution (79 kDa), whereas that solubilized by hydroxylamine without proteolysis appeared dimeric (190 kDa). The latter dimeric state may reflect either an altered conformation of the enzyme or post-translational modification beyond fatty acid esterification.