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Protein kinase C betaII (PKC betaII) has been implicated in proliferation of the intestinal epithelium. To investigate PKC betaII function in vivo, we generated transgenic mice that overexpress PKC betaII in the intestinal epithelium. Transgenic PKC betaII mice exhibit hyperproliferation of the colonic epithelium and an increased susceptibility to azoxymethane-induced aberrant crypt foci, preneoplastic lesions in the colon. Furthermore, transgenic PKC betaII mice exhibit elevated colonic beta-catenin levels and decreased glycogen synthase kinase 3beta activity, indicating that PKC betaII stimulates the Wnt/adenomatous polyposis coli (APC)/beta-catenin proliferative signaling pathway in vivo. These data demonstrate a direct role for PKC betaII in colonic epithelial cell proliferation and colon carcinogenesis, possibly through activation of the APC/beta-catenin signaling pathway.

The role of cell-specific metabolism in benzene toxicity was examined in both murine and human bone marrow. Hemopoietic progenitor cells and stromal cells are important control points for regulation of hemopoiesis. We show that the selective toxicity of hydroquinone at the level of the macrophage in murine bone marrow stroma may be explained by a high peroxidase/nicotanimide adenine dinucleotide phosphate, reduced [NAD(P)H]:quinone oxidoreductase (NQO1) ratio. Peroxidases metabolize hydroquinone to the reactive 1,4-benzoquinone, whereas NQO1 reduces the quinones formed, resulting in detoxification. Peroxidase and NQO1 activity in human stromal cultures vary as a function of time in culture, with peroxidase activity decreasing and NQO1 activity increasing with time. Peroxidase activity and, more specifically, myeloperoxidase, which had previously been considered to be expressed at the promyelocyte level, was detected in murine lineage-negative and human CD34+ progenitor cells. This provides a metabolic mechanism whereby phenolic metabolites of benzene can be bioactivated in progenitor cells, which are considered initial target cells for the development of leukemias. Consequences of a high peroxidase/NQO1 ratio in HL-60 cells were shown to include hydroquinone-induced apoptosis. Hydroquinone can also inhibit proteases known to play a role in induction of apoptosis, suggesting that it may be able to inhibit apoptosis induced by other stimuli. Modulation of apoptosis may lead to aberrant hemopoiesis and neoplastic progression. This enzyme-directed approach has identified target cells of the phenolic metabolites of benzene in bone marrow and provided a metabolic basis for benzene-induced toxicity at the level of the progenitor cell in both murine and human bone marrow.

The reverse transcription-polymerase chain reaction was used to examine alternative splicing at each of the three fibronectin exons known to undergo alternative splicing, i.e. extra domain A (ED-A), extra domain B (ED-B), and type III connecting sequence (IIICS). Ratios of fibronectin mRNAs with or without a given exon were determined in several rat tissues and human cell lines during aging in vivo and cellular senescence in vitro. We demonstrate that statistically significant shifts in the alternative splicing of fibronectin occur during aging in vivo and in vitro. Since all three alternatively spliced exons are spliced out at a higher frequency in aging tissues and cells, the fibronectin protein produced by old cells should be slightly smaller than that obtained from young cells. The reverse transcription-polymerase chain reaction demonstrates tissue-specific patterns of alternative splicing in several tissues. Whereas fibronectin mRNAs from adult rat tissues were found to range from 0 to 25% ED-A+ and from 0 to 10% ED-B+, fibronectin mRNAs from cultured cell lines were found to be approximately 50-60% ED-A+ and 15-25% ED-B+. We observed similarity in splicing of fibronectin RNA by the different cultured cell lines obtained from many tissues and attribute this observation to the effect of growth factors. We demonstrate that serum deprivation; placement of cells into primary culture; and growth factors such as transforming growth factor beta 1, retinoic acid, and 1,25-dihydroxyvitamin D3 can all change the alternative splicing of fibronectin pre-mRNA in the ED-A, ED-B, and type III connecting sequence exons. Possible mechanisms for the regulation of the alternative splicing of fibronectin RNA by growth factors are discussed.