Protein kinase C: a novel target for inhibiting gastric cancer cell invasion.

Gastric adenocarcinoma is a common neoplasm worldwide. Patients with completely resected disease often have locoregional recurrence, and adjuvant chemotherapy has failed to reduce the common occurrence of metastases. Protein kinase C (PKC) is thought to be important in tumor cell invasion, but its relationship to gastric cancer cell invasion, and thus metastases, remains unexplored. We recently identified and established invasive and noninvasive human gastric adenocarcinoma cell lines, which can now be used to test agents for inhibition of tumor cell invasion by inhibition of PKC activity. The objectives were (a) to test threo-dihydrosphingosine (SPC100221), a specific inhibitor of PKC at its regulatory site, and staurosporine, a potent but nonspecific inhibitor of PKC at its catalytic site, for their effects on gastric cancer cell invasion in vitro and (b) to determine whether the expression of PKC isoforms can distinguish invasive from noninvasive gastric cancer cells. Gastric cancer cell invasion through Matrigel-coated Nuclepore filters in the Boyden chamber assay was analyzed in the presence of graded concentrations of SPC100221 and staurosporine. The invasive SK-GT-1 and SK-GT-5 cell lines and the noninvasive SK-GT-2 and SK-GT-4 cell lines were used. PKC isoform expression was determined by reverse transcription of messenger RNAs to complementary DNA and subsequent amplification by the polymerase chain reaction. The effects of staurosporine and SPC100221 on tumor cell invasion were tested at drug concentrations that did not inhibit cell proliferation, as evidenced by [3H]thymidine uptake. Staurosporine and SPC100221 at subtoxic doses inhibited human gastric cancer cell invasion by 50% at 5 x 10(-9) M and 2 x 10(-7) M, respectively. The expression of PKC beta was observed in the invasive but not the noninvasive gastric cancer cells. Both types of cells, however, expressed the PKC alpha and PKC gamma isoforms. Gastric cancer cell invasion can be inhibited by PKC inhibitors, and expression of PKC beta may be a marker of invasiveness in gastric cancer. PKC appears to represent a new target for inhibition of gastric cancer cell invasion, and SPC100221, in view of its PKC specificity, may provide a model for future drug development in this area. Moreover, PKC beta may have a fundamental role in the development of invasive potential in gastric cancer.