Loss of expression of protein kinase C beta is a common phenomenon in human malignant melanoma: a result of transformation or differentiation?

As with most cancers, the aetiology of human cutaneous melanoma is likely to be multifactorial and to include the accumulation of irreversible alterations in an unknown number of genes. Elucidating this molecular progression necessitates both the identification of genetic perturbations at each clinically relevant stage, and the assessment of their impact on the normal melanocyte. The observation that the epidermal melanocyte, in contrast to metastatic melanoma cells, requires activation of the protein kinase C (PKC) pathway to facilitate growth in vitro indicates that one or more isoforms (or substrates) of this large and complex family of proteins are among those that undergo alteration during the development of malignant melanoma. Consequently, a number of studies have investigated the expression of various PKC family members in both melanocyte and melanoma cell lines, without a consensus of opinion as to which isoforms are of biological significance in melanoma development and progression. The present study involved a comprehensive evaluation of the PKC profile in normal melanocytes and in 16 metastatic melanoma cell lines. The results show that the major difference in isoform expression between epidermal melanocytes and melanoma cells is the loss of PKCbeta protein expression in 90% of melanoma cell lines. Examination of PKCbeta in benign and malignant melanocytic lesions revealed that this protein is either downregulated or absent in both naevi and metastatic melanomas. We conjecture that, although the loss of PKCbeta expression is a common phenomenon in malignant melanocytes, it may be related more to a normal process of melanocytic differentiation than to malignant transformation.

The association between a mutated ras gene and cyclooxygenase-2 expression in human breast cancer cell lines.

Cyclooxygenase (COX) is rate-limiting for arachidonic acid metabolism to the prostanoid family of eicosanoids. Some human breast cancers, notably those which are estrogen receptor (ER)-negative with high metastatic potential, produce high levels of prostaglandin E2 (PGE2). In some cell types, expression of the inducible COX-2 isoform occurred in association with a ras gene mutation. We determined COX-1 and COX-2 expression, and the corresponding PGE2 secretions, in 4 ER-negative human breast cancer cell lines, the MCF10A breast epithelial cell line, and the same non-cancerous line transfected with a mutated ras gene. The highly invasive MDA-MB-231 and Hs578T cancer cell lines, which possess a mutated Ki-ras and H-ras, respectively, expressed constitutive and inducible COX-2, and produced high PGE2 levels; the less invasive MDA-MB-435 and SK-BR-3 lines, without a mutated ras, possessed only low levels of COX-2, and secreted correspondingly low PGE2 levels. Similarly, the ras transfectant, but not parental MCF10A cells, expressed inducible COX-2. Chemosuppression with a selective COX-2 inhibitor may be effective only in that minority of breast cancers which have a mutated ras gene.

Effects of reduced dietary linoleic acid intake, alone or combined with an algal source of docosahexaenoic acid, on MDA-MB-231 breast cancer cell growth and apoptosis in nude mice.

Diets rich in linoleic acid (LA), an n-6 fatty acid, stimulate the progression of human breast cancer cell solid tumors in athymic nude mice, whereas docosahexaenoic acid (DHA) and eicosapentaenoic acid, long-chain n-3 fatty acids, exert suppressive effects. In the present study we used a novel source of DHA, in triglyceride form, to determine the effects of feeding low levels of the fatty acid on the growth of MDA-MB-231 cells injected into the thoracic mammary fat pads of female nude mice. Four different isocaloric diets were used, all of which provided 20% (wt/wt) total fat. The control diets contained 8% (20 mice) or 4% (50 mice) LA; the n-3 fatty acid-supplemented groups of 50 mice were fed 4% LA-containing diets plus 2% or 4% DHA. The tumor growth rates were reduced significantly in mice fed the 4% LA compared with the 8% LA diet; the addition of 4% DHA to the 4% LA-containing diet produced a further reduction in tumor growth rate (p < or = 0.003 at and after Week 6). The final tumor weights were also reduced in the DHA-fed mice compared with the 8% LA dietary group (2% DHA, p = 0.02; 4% DHA, p = 0.01) and in the 4% DHA-fed mice compared with the 4% LA control group (p = 0.02); a similar trend for mice fed the lower level of DHA did not achieve statistical significance. Tumor prostaglandin E2 concentrations were reduced by feeding the lower LA level; further dose-dependent decreases occurred in the DHA dietary groups and were accompanied by reduced levels of 12- and 15-hydroxyeicosatetraenoic acids. These changes in eicosanoid biosynthesis may have been responsible for the observed decreases in cell proliferation, indicated by suppressed Ki-67 expression, and increases in apoptotic activity, as reflected in TdT-mediated dUTP nick end labeling immunohistochemical staining.