Loss of protein kinase C δ isozyme immunoreactivity in human adenocarcinomas

Abstract

Protein kinase C (PKC) has been implicated in the control of colonic epithelial proliferative activity and in the process of malignant transformation. In the present study, we assessed by histone IIIS phosphorylationin vitro, total PKC activity, and the subcellular distribution of this activity in human adenocarcinomas and surrounding uninvolved mucosa from six patients. In these same tissues, we also examined the isozyme profile of PKC by immunoblotting. Total PKC activity and the subcellular distribution of PKC activity was not significantly different in mucosa compared to corresponding values in the tumors. Extracts of both human mucosa and tumors reacted with antibody to PKC isozymes α, β, δ and ζ but did not react with antibody to the γ and ɛ isozymes. The antibodies employed were directed against rabbit brain PKC (α, β, γ) or peptide sequences deduced from rat cDNA (γ, δ, ɛ, and ζ). Accordingly, the apparent absence of the epsilon isozyme in human mucosa and adenocarcinoma may be due to failure to conserve the relevant sequence rather than to loss of the isozymeper se. No statistically significant differences were noted in subcellular distribution of any of the isozymes in the tumors compared to mucosa. However, the subcellular distribution of the δ isozyme was highly variable in the tumors. Total PKC β immunoreactivity and that of the soluble, but not particulate, fraction were both significantly lower in homogenates of adenocarcinomas compared to corresponding values in surrounding mucosa, when expressed as a function of protein. However, these differences in PKC β were abolished when results were expressed as a function of tissue DNA content. By contrast, immunoreactivity of the δ isozyme of PKC in adenocarcinomas was markedly and consistently lower (78–98%) compared to surrounding mucosa, whether results were expressed as a function of DNA or protein. Loss of immunoreactivity of the δ isozyme of PKC may play a role in the process of colon carcinogenesis in man.