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N K Ahmed

Publications and source records attributed to N K Ahmed.

6 recordsLinked to original sources
PubMed1977

Metabolic regulation and relationship of endogenous protein kinase activity and steroidogenesis in isolated adrenocortical carcinoma cells of the rat.

In the adrenocortical carcinoma cell, in contrast to normal isolated adrenal cells, 10 to 50 muunits of ACTH do not raise the level of adenosine cyclic 3':5'-monophosphate (cyclic AMP), protein kinase activity, and steroidogenesis. This indicates a lesion in the tumor adenylate cyclase system. Two-tenths to 10 mM cyclic AMP and guanosine cyclic 3':5'-monophosphate (cyclic GMP) which stimulate steroidogenesis in a normal cell, activate protein kinase activity in a concentration-response manner without any detectable rise in steroidogenesis in the adrenocortical carcinoma cell. Cycloheximide and actinomycin D do not inhibit the stimulation of the phosphorylation. These results suggest that the tumor cyclic nucleotide-dependent protein kinase activity is unrelated to steroidogenesis and is also not under the transcriptional or translational control steps. Curiously, muM concentrations of cyclic AMP, in contrast to cyclic GMP, stimulate protein kinase activity. In a normal cell, both cyclic AMP and cyclic GMP, in this concentration range, stimulate protein kinase without an increase in steroidogenesis. It is therefore proposed that, in contrast to the normal cell, there is an additional defect in cyclic GMP-dependent protein kinase.

Adenylyl Cyclases↗
PubMed1977

Partial purification and characterization of the defective cyclic adenosine 3':5'-monophosphate binding protein kinase from adrenocortical carcinoma.

Previous studies with isolated adrenocortical carcinoma 494 cells from this laboratory have indicated that the lack of cyclic adenosine 3':5'-monophosphate (cyclic AMP) control in steroidogenesis in the tumor may be due to the defective cyclic AMP-dependent protein kinase enzyme system. This paper describes the partial purification of such an enzyme. Purification was achieved by precipitation of the tumor homogenate with 30 and 45% ammonium sulfate, adsorption on 3% calcium phosphate gel, and chromatography on DEAE-cellulose. Four major protein peaks were isolated. Peak 2 showed cyclic AMP-binding activity and was investigated further for its kinetic properties. In contrast to the cyclic AMP-dependent protein kinase enzyme found in the normal adrenal gland, the enzyme specifically bound cyclic AMP but failed to phosphorylate exogenous histone. It is postulated that lack of the cyclic nucleotide-dependent kinase activity of the protein kinase enzyme may be responsible for the loss of cyclic AMP-regulated corticosterone synthesis in adrenocortical carcinoma cell. It is further shown that the tumor cyclic AMP-binding enzyme undergoes endogenous phosphorylation, which indicates that it has kinase activity but it is independent of cyclic AMP.

Adenosine Triphosphate↗
PubMed1976

Metabolic regulation of steroidogenesis in isolated adrenal cells of rat. Relationship of adrenocorticotropin-, adenosine 3':5'-monophosphate-and guanosine 3':5'-monophosphate-stimulated steroidogenesis with the activation of protein kinase.

The data presented with the isolated adrenal cells, in the present study, show that adrenocorticotropin in the physiological concentration range stimulates the synthesis of guanosine 3':5'-monophosphate(cyclic GMP), protein kinase activity, and steroidogenesis in a concentration-dependent manner without detectable rise in the levels of adenosine 3':5'-monophosphate (cyclic AMP). Millimolar concentrations of cyclic AMP and cyclic GMP, which stimulate corticosterone synthesis, also activate kinase activity and steroidogenesis in a sigmoid concentration-response manner. The process of phosphorylation activated by corticotropin, cyclic AMP and cyclic GMP is not inhibited by cycloheximide or actinomyin D. It is therefore proposed that the hormonal responses mediated by cyclic GMP and cyclic AMP are via the protein kinase enzymatic steps, and the inhibitory effect of cycloheximide and actinomycin D in corticotropin-stimulated steroidogenesis follows this step. In conjuction with our previous observations that the biosynthetic steps from (20S)-20-hydroxycholesterol to corticosterone are neither inhibited by cycloheximide nor affected by cyclic GMP, it is inferred that the rate-limiting step of adrenal steroidogenesis is the transformation of cholesterol to (20S)-20hydroxycholesterol and this very step is regulated by cyclic GMP and cyclic AMP. Of further significance are the findings that micromolar cincentrations of cyclic AMP and cyclic GMP, which do not stimulate steroidogenesis, effectively stimulate protein kinase activity in a concentration-dependent manner. It is therefore concluded that all cyclic-nucleotide-dependent protein kinase activities of the cell are not necessarily related to steroidogenesis.

Adrenal Glands↗