Possible involvement of a lanthanide-sensitive protein kinase C substrate in lanthanide promotion of neoplastic transformation.

Abstract

The rare earth elements lanthanum and terbium (0.1-1.0 mM), pharmacological analogs of calcium, induced neoplastic transformation of 12-O-tetradecanoylphorbol-13-acetate (TPA)-sensitive (P+) and to a lesser extent TPA-resistant (P-) preneoplastic mouse JB6 epidermal cells. A maximum of 2500 anchorage-independent colonies per 10(4) cells were induced in P+ lines, a response comparable to that induced by phorbol esters (1.6-16 nM). The maximum lanthanide-induced colony yield in P- lines was 20% of that in P+ lines (approximately 550 colonies), and was observed under conditions where TPA induced less than 30 colonies per 10(4) cells. Lanthanides and TPA produced a synergistic effect on colony size in P+ cells. Lanthanides are not promoting transformation merely by mimicking high calcium: adding exogenous extracellular calcium (up to 50.0 mM) or using calcium ionophore (up to toxic concentrations) to increase intracellular calcium does not promote transformation. Lanthanum will substitute for calcium in activating partially purified protein kinase C (PKC), the calcium-dependent phorbol ester receptor. However, lanthanides must be promoting transformation by a mechanism other than PKC activation because lanthanides failed to activate PKC in intact JB6 cells. Three independent experiments showed a lack of lanthanum effect on PKC-dependent events in intact cells. First, in contrast to TPA, lanthanum pretreatment of JB6 cells did not produce elevated phosphorylation of an 80-kd substrate. Second lanthanum pretreatment did not cause decreased PKC activity after prolonged exposure. Third, lanthanum and TPA affected epidermal growth factor binding with a different magnitude, time course and calcium dependency. We found, however, a PKC substrate in P+, P- and tumorigenic cell lines that is sensitive to lanthanum and increases its migration in sodium dodecylsulfate-polyacrylamide gels from 23 to 21 kd. The above data suggest that: (i) alterations in cation binding may be sufficient for inducing the transformed phenotype; and (ii) lanthanide promotion of neoplastic transformation may be linked to a lanthanide-sensitive PKC substrate, but is not due to a direct PKC activation.