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Co-mitogenic tumor promoters suppress the phosphatidylinositol response in lymphocytes during early mitogenesis
Abstract:
The tumor-promoting agents 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and phorbol-12,13-dibenzoate inhibited the increased accumulation of [32P]phosphatidylinositol (PI) induced in mouse spleen lymphocytes by mitogenic lectins in the presence of [32P]orthophosphate. Similar inhibition of [32P]PI levels by TPA was seen in human tonsil T-lymphocytes stimulated with phytohemagglutinin. Only co-mitogenic phorbol esters prevented the [32P]PI accumulation during early mitogenesis. No increased 32P-labelling due to mitogen or decreases due to TPA was observed when cells were equilibrated with [32P]orthophosphate for 24 h prior to stimulation with mitogen, from which it is concluded that the total concentrations of phosphatidylcholine (PC) and PI are unaffected by mitogen or co-mitogen. The [32P]PI elevation but not the [32P]PC elevation was proportional to T-cell mitogenic potency for the lectins concanavalin A, divalent succinyl concanavalin A and phytohemagglutinin, and was prevented in each case by 5 X 10(-8) M TPA. Escherichia coli lipopolysaccharide did not give increased 32P incorporation into PI or PC, and TPA had no effect on 32P labelled phospholipid levels in the presence of this B-cell mitogen. The results indicate that the phosphatidylinositol response is not an invariable correlate of T-cell mitogenesis by polyclonal mitogens.
Insights
Tumor promoters like TPA inhibit phosphatidylinositol (PI) accumulation during T-cell activation. This suggests the PI response is not always linked to T-cell mitogenesis by polyclonal mitogens.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Mitogenic lectins stimulate lymphocytes, leading to increased phosphatidylinositol (PI) accumulation.
- Tumor-promoting phorbol esters, such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA), are known to affect cellular signaling pathways.
- The role of PI metabolism in T-cell activation and its modulation by external agents requires further investigation.
Purpose of the Study:
- To investigate the effect of tumor-promoting phorbol esters on mitogen-induced phosphatidylinositol (PI) accumulation in lymphocytes.
- To determine if the phosphatidylinositol (PI) response is a consistent indicator of T-cell mitogenesis.
- To differentiate the effects of phorbol esters on T-cell versus B-cell mitogenesis.
Main Methods:
- Mouse spleen lymphocytes and human tonsil T-lymphocytes were stimulated with mitogenic lectins (e.g., phytohemagglutinin, concanavalin A) in the presence of [32P]orthophosphate.
- The accumulation of [32P]phosphatidylinositol (PI) and phosphatidylcholine (PC) was measured.
- The effect of tumor-promoting agents like 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on [32P]PI and [32P]PC levels was assessed.
- Experiments were also conducted using Escherichia coli lipopolysaccharide as a B-cell mitogen.
Main Results:
- Tumor-promoting agents 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and phorbol-12,13-dibenzoate inhibited the mitogen-induced increase in [32P]phosphatidylinositol (PI) levels in both mouse and human lymphocytes.
- This inhibition was specific to T-cell mitogenesis, as TPA did not affect [32P]PI or [32P]PC levels in the presence of the B-cell mitogen Escherichia coli lipopolysaccharide.
- The elevation of [32P]PI, but not [32P]PC, was proportional to the T-cell mitogenic potency of different lectins and was consistently prevented by TPA.
Conclusions:
- The phosphatidylinositol (PI) response is not an invariable correlate of T-cell mitogenesis induced by polyclonal mitogens.
- Tumor-promoting phorbol esters can interfere with the PI signaling pathway during T-cell activation.
- The observed effects highlight the complex interplay between mitogenic signaling and lipid metabolism in lymphocytes.